Move deferred and out-of-scope items to todo.md

AGENTS.md

@@ -7,3 +7,4 @@
 - One spelling per result: reject the other at `New`, and let the error name the spelling to use.
 - A standing choice a reader would relitigate goes under Decisions in the README, not in a comment.
 - A README example is a `json` block that loads and renders as a category; `readme_test.go` runs every one.
+- Cyclomatic complexity is gated at 14: the table-shaped dispatches (`eachToken`, `calc.factor`, `walkPath`) sit at 13–14 and stay whole; anything else that reaches 14 is decomposed.

Dockerfile

@@ -13,6 +13,7 @@ RUN go mod download
 
 COPY . .
 RUN go vet ./... && \
+	go run github.com/fzipp/gocyclo/cmd/gocyclo@v0.6.0 -over 14 -ignore _test . && \
 	{ unformatted="$(gofmt -l .)"; test -z "$unformatted" || \
 	  { echo "unformatted files:"; echo "$unformatted"; exit 1; }; } && \
 	go test -race ./...

README.md

@@ -1,32 +1,17 @@
 # fejkdata
 
-A Go library and CLI for generating locale-aware fake data from JSON templates.
-Forked from [github.com/Timewave-AB/fakes](https://github.com/Timewave-AB/fakes).
+Locale-aware fake data for tests and fixtures, generated from JSON templates. Use
+it as a Go library or the CLI — no data on disk, no dependencies, and a seed makes output
+reproducible.
 
-## Goals
-
-1. **Valid by construction** — every value passes the check its real consumer
-   applies; facts that belong together come from one draw, within a value and
-   across categories.
-2. **Text means what it says** — a format renders as written; only `{…}` varies,
-   random characters included (`{digits(3)}`). One spelling per result; the wrong
-   one is a load error naming the right one.
-3. **Every mistake is a load error** — `New` rejects; `Fake` on a loaded generator
-   fails only for an unknown path.
-4. **Zero to a value in one command** — `go install`, then `fejkdata sv_SE.person`:
-   no checkout, no flag. Flags are GNU-form (`--seed 42`, `-n 3`) in any position;
-   the first custom template needs no escape and no option.
-5. **Data lives in JSON** — a builtin only for what data can't express.
-6. **Reproducible** — seed in, same stream out; no builtin reads a clock.
-7. **Zero dependencies** — standard library only.
-8. **Docs index the grammar** — every syntax feature is a heading; every example
-   runs under test and shows its output; a rule is stated once.
+```sh
+go install gitea.larvit.se/larvit/fejkdata/cmd/fejkdata@latest
+fejkdata sv_SE.person          # Sara Eriksson
+```
 
 ## CLI
 
 ```sh
-go install gitea.larvit.se/larvit/fejkdata/cmd/fejkdata@latest
-
 fejkdata sv_SE.person                          # Sara Eriksson
 fejkdata sv_SE.person.last                     # Eriksson
 fejkdata --seed 42 sv_SE.address               # the same address every run
@@ -34,25 +19,39 @@ fejkdata -n 3 --separator ', ' sv_SE.word      # nät, barn, sol
 fejkdata --list                                # every path the data offers
 fejkdata --data-path ./mydata sv_SE.word       # layer a directory over the shipped data
 fejkdata --no-shipped-data -d ./mydata --list  # only your data
+fejkdata 'name: {/sv_SE.person.last}'          # name: <a surname> — an inline template
+fejkdata '{"format":"name: {x}","x":["bosse","lina"]}'  # name: bosse or name: lina
 ```
 
 A path names a category, or a field inside one: each dot segment descends one
-level — folders, then the category (a JSON file), then fields.
+level — folders, then the category (a JSON file), then fields. An argument that is
+a JSON object, array or string, or that carries a `{` token, is instead an
+**inline template**: a format string or a JSON value compiled and rendered on the
+spot. Its tokens reach the data by reference from the root —
+`{/sv_SE.person.last}`, so shipped and `--data-path` categories are alike
+available. An inline template sits in no folder, so the folder-relative `{.name}`
+and `{..name}` are rejected naming the root spelling. A path never contains a
+brace, a bracket or a quote, so the two cannot collide (see
+[Decisions](#decisions)).
 
 | Flag | |
 |------|--|
 | `-d`, `--data-path D` | a directory to layer over the shipped data; repeatable, the last wins a name clash |
 | `--no-shipped-data` | load only the `--data-path` directories |
 | `-s`, `--seed N` | reproducible output |
-| `-n`, `--repeat N` | render the path N times, each an independent draw |
+| `-n`, `--repeat N` | render the value N times (up to 1048576), each an independent draw, streamed |
 | `--separator S` | between repeated values (default a newline) |
+| `--format F` | `text` (default), `json`, `ndjson`, `csv` or `sql` — a record's columns, one record per row (json frames them as an array) |
+| `--table T` | the INSERT target for `--format sql` (default: the path's last segment, or `records` for an inline template) |
 | `--list` | print every path, then exit |
 | `--version`, `-h`, `--help` | print, then exit |
 
 `--name value` and `--name=value` both work, a short flag's value attaches or
 follows (`-n3`, `-n 3`) and short flags bundle (`-hn 3`) — see
 [Decisions](#decisions); flags go anywhere, `--` ends them. Exit codes: `0` success, `1` runtime error (missing
-dir, unknown path), `2` misuse. From a checkout: `go run ./cmd/fejkdata …`.
+dir, unknown path), `2` misuse — a bad flag, an argument that names neither a
+template nor a path, or an inline template that does not compile. From a checkout:
+`go run ./cmd/fejkdata …`.
 
 ### Your own data
 
@@ -76,6 +75,67 @@ fejkdata --seed 1 --data-path ./mydata sql
 fejkdata --repeat 100 --data-path ./mydata sql > seed.sql
 ```
 
+That `format` string is the free-form spelling — you hand-write the whole row.
+For structured output a record writes the row for you.
+
+### Records
+
+A record is a template seen as columns: its fields are the columns, its `format`
+the whole. `--format json|ndjson|csv|sql` writes the records; the library's
+`Record` (below) hands back the columns. Every column is a string — typed scalars
+are on the release checklist, see [`todo.md`](todo.md). Save
+`mydata/users.json`:
+
+```json
+{
+  "format": "{first} {last}",
+  "first": ["Ada", "Bo"],
+  "last": ["Lovelace", "Ek"]
+}
+```
+
+```sh
+fejkdata --seed 1 --data-path ./mydata --format json users            # [{"first":"Bo","last":"Lovelace"}]
+fejkdata --seed 1 --data-path ./mydata --format ndjson users          # {"first":"Bo","last":"Lovelace"}
+fejkdata --seed 1 --data-path ./mydata --format csv  users            # first,last  →  Bo,Lovelace
+fejkdata --seed 1 --data-path ./mydata --format sql  users            # INSERT INTO "users" ("first", "last") VALUES ('Bo', 'Lovelace');
+fejkdata --seed 1 --data-path ./mydata --format sql --table people users # INSERT into another table
+```
+
+`--repeat` streams that many records — `json` frames them as one array document,
+`ndjson` writes one object per line, `csv` a row after a header, `sql` one INSERT
+per line. Only a category-level template is a record; a field, choice or folder
+errors, and so does a `repeat` on
+the template itself, which composes the format into one string rather than
+projecting columns — ask for more records with `--repeat`. A `repeat` on a column
+is fine.
+
+A column carrying a newline keeps it inside the quoted CSV field or the SQL string
+literal, so a row can span physical lines: read the stream with a CSV or SQL
+parser rather than splitting it on newlines.
+
+The SQL is ANSI — identifiers in double quotes, a literal quote doubled (`''`),
+backslashes passed through — so a hyphenated field like `postal-code` stays a
+valid identifier. PostgreSQL and SQLite take it as written; MySQL and MariaDB need
+`ANSI_QUOTES` and `NO_BACKSLASH_ESCAPES` set first, or they read `"users"` as a
+string and a backslash as an escape.
+
+A record written only to emit columns still needs a `format` — the grammar's one
+required key — so `"format": ""` carries the fields with an inert format: it
+renders nothing by `Fake`, and is compiled only so the tree's fences still run.
+The columns are the point, and their facts stay together: two columns that read a
+path into one category — `{/currency.code}` and `{/currency.symbol}` — share one
+draw of it, so the record is internally consistent. A bare `{/currency}` names no
+field, so it keeps drawing on its own. That one draw is also why two
+columns may not read overlapping reference *paths* — `{/cat.a}` beside
+`{/cat.a.b}` is refused, naming the fields to write instead, as
+[One draw, one spelling](#one-draw-one-spelling) refuses that pair inside a single
+format. A column may not reference the record it belongs to by any spelling: `{/users.first}`
+or a bare `{/users}` inside `users` describes a draw other than the columns beside
+it, so put a value two columns share in its own category and reference that. A field hold, transform or
+operand ties fields together within one column as always (see
+[Correlated fields](#correlated-fields) and [Decisions](#decisions)).
+
 ## Library
 
 ```sh
@@ -89,6 +149,12 @@ if err != nil {
 }
 v, err := f.Fake("sv_SE.address") // "Kungsvägen 68\n379 17 Stockholm"
 paths := f.List()                  // every path Fake accepts, sorted
+v, err = f.FakeTemplate("name: {/sv_SE.person.last}")      // compile + render in one call
+t, err := f.NewTemplate(`{"format":"name: {x}","x":["bosse","lina"]}`) // compile once
+v = t.Fake()                                              // render many times, no re-parse
+r, err := f.Record("users")               // one record: each field a column
+s := r.JSON()                             // {"first":"Ada","last":"Lovelace"}
+r, err = f.FakeRecord(`{"format":"{x}","x":["a","b"]}`) // compile + render inline
 ```
 
 | Option | |
@@ -98,6 +164,11 @@ paths := f.List()                  // every path Fake accepts, sorted
 | `WithDataFS(fsys)` | layer an `fs.FS`, such as your own `embed.FS` |
 | `WithoutShippedData()` | load only what you give |
 
+A `*Record` carries its columns via `Columns()`, and serializes them with `JSON()`
+(one object), `CSVHeader()`/`CSVLine()`, or `SQLInsert(table)` — the shapes the
+CLI's `--format` writes. `Record` and `FakeRecord` take a record; a path or
+template that is not one — a bare string, a choice, or a folder — errors.
+
 A `*Generator` is safe for concurrent use; a seeded sequence is reproducible only
 when drawn from one goroutine. Changing how a value is composed shifts the seeded
 stream for that value and everything drawn after it.
@@ -110,8 +181,8 @@ work with no data on disk. A directory is a namespace: each JSON file is a
 category named after the file, each subdirectory a dot-path segment, so
 `mydata/sv_SE/person.json` is `sv_SE.person` and replaces the shipped one.
 Sources merge in order; matching folders combine, any other clash is won by the
-last loaded. Names may not use `.`, `|`, `(`, `{`, `}` or `/`; dot-prefixed entries
-are skipped, so a data directory can also be a checkout.
+last loaded. Names may not use `.`, `|`, `(`, `{`, `}`, `[`, `]`, `"` or `/`;
+dot-prefixed entries are skipped, so a data directory can also be a checkout.
 
 Each locale carries `address`, `color`, `company`, `date`, `email`, `ip`,
 `person`, `phone`, `price`, `sentence`, `ssn`, `time`, `url`, `username`,
@@ -180,9 +251,9 @@ many times — each an independent draw — joined by `separator` (default `""`)
 { "format": "{word}", "repeat": 3, "separator": " ", "word": ["foo", "bar", "baz"] }
 ```
 
-Renders e.g. `bar foo baz`. A `separator` without a `repeat` is rejected, and so
-is a `repeat` that multiplies to more than 1 048 576 renders along any path of
-nested repeats.
+Renders e.g. `bar foo baz`. Rejected at load: a `separator` without a `repeat`,
+a `separator` of `""` (the default), and a `repeat` that multiplies to more than
+1 048 576 renders along any path of nested repeats.
 
 ### Options and fields
 
@@ -194,9 +265,11 @@ choice naming its item.
 ### Functions
 
 A `{name(args)}` token calls a builtin. Arguments are checked at `New`: a bad
-count, range, country or expression fails fast, and a length, count or decimal
-place beyond a sane maximum is rejected, so a fat-fingered `hex(2000000000)`
-never tries to allocate gigabytes. Every builtin draws only from the seed — a
+count, range, country or expression fails fast; an integer is written plain
+(`5`, not `+5` or `05`); bounds are finite; a sample that could only ever emit one
+value (`int(5,5)`, `float(1,1,2)`) is rejected naming the text to write instead;
+and a length, count or decimal place beyond a sane maximum is rejected, so a
+fat-fingered `hex(2000000000)` never tries to allocate gigabytes. Every builtin draws only from the seed — a
 time-based id takes its timestamp from the rng, not the clock — so seeded output
 stays reproducible.
 
@@ -243,8 +316,10 @@ hyphenated field can't be an operand.
 ```
 
 Renders e.g. `19.99 x 3 = 59.97`. An operand that can never be a number (`"abc"`,
-or a choice of such) is rejected at load; one that sometimes is not yields `NaN`,
-and a division by zero `Inf` — both print rather than fail.
+or a choice of such) is rejected at load, as is a division by a constant zero
+(`1/0`, or a fixed `"0"` field); an operand that sometimes is not a number yields
+`NaN`, and a division by one that is not constant `Inf` — both print rather than
+fail.
 
 ### Transforms
 
@@ -337,6 +412,29 @@ then costs about what its output costs: an unweighted pick is O(1) whatever the
 list's length, a weighted one O(log n), and long formats, deep nesting and many
 tokens add cost in proportion to the output.
 
+## Goals
+
+1. **Valid by construction** — every value passes the check its real consumer
+   applies; facts that belong together come from one draw, within a value and
+   across categories.
+2. **Text means what it says** — a format renders as written; only `{…}` varies,
+   random characters included (`{digits(3)}`). One spelling per result; the wrong
+   one is a load error naming the right one.
+3. **Every mistake is a load error** — `New` rejects the data and `NewTemplate`
+   the inline template; on a loaded generator `Fake` fails only for an unknown
+   path, and `Template.Fake` cannot fail at all.
+4. **Zero to a value in one command** — `go install`, then `fejkdata sv_SE.person`:
+   no checkout, no flag. Flags are GNU-form (`--seed 42`, `-n 3`) in any position;
+   the first custom template needs no escape and no option.
+5. **Data lives in JSON** — a builtin only for what data can't express.
+6. **Reproducible** — seed in, same stream out; no builtin reads a clock.
+7. **Zero dependencies** — standard library only.
+8. **Docs index the grammar** — every syntax feature is a heading; every example
+   runs under test and shows its output; a rule is stated once.
+9. **Fast enough to be free** — a value renders in about a microsecond and `New`
+   parses and validates the whole set once upfront, so generating fixtures stays
+   noise against a test's own runtime.
+
 ## Decisions
 
 - **Options and fields share one namespace.** `format`, `weight`, `repeat` and
@@ -352,6 +450,33 @@ tokens add cost in proportion to the output.
   naming the double-dash spelling, and `-s=42` is rejected naming both short
   spellings: `=` belongs to the long form, and reading `=42` as the value would
   make `-d=./x` a directory named `=./x`.
+- **An argument is a template by its shape, not by a flag.** A JSON object, array
+  or string, or a string carrying a `{` token, is an inline template; anything else
+  is a path. A name may not contain a brace, a bracket or a quote, so a path can
+  never collide with any of those spellings, and the leading `[` or `"` is gated on
+  valid JSON so a stray copied bracket never swallows an argument — it names
+  nothing, and says so. No `--template` flag is needed. Reserving the characters
+  whole — though only a leading one could collide — keeps one simple name rule
+  instead of a leading-position special case. The JSON string is what makes the
+  library's own advice reachable: the error for an object holding only a format
+  names `"…"`, and that spelling has to work where it is printed.
+- **An inline template skips the cycle fence, and only that one.** `New` proves the
+  loaded tree acyclic, an inline node is a finite tree of its own, and nothing in
+  the tree can reference it, so no render of it reaches itself. Every other fence
+  runs over both, from one `checkScope`.
+- **An inline template that does not compile is misuse (exit 2), including a
+  reference that resolves to nothing** — the whole argument is the spelling under
+  test, and `NewTemplate` compiles, links and validates as one step. An unknown
+  *path* stays a runtime error (exit 1): there the argument is well-formed and only
+  the data is absent.
+- **A padded JSON argument is rejected, not trimmed.** Padding is the one place the
+  two readings disagree — a format string renders it, JSON drops it — so the
+  spelling that renders is named rather than silently chosen.
+- **`FakeTemplate` and `NewTemplate` both stay.** They reach the same value but not
+  at the same cost: `NewTemplate` pays the compile and validation once and renders
+  many times, `FakeTemplate` is the one-shot call, and `--repeat` is exactly the
+  case that needs the first. The pair is `regexp.MustCompile` and `regexp.Match`,
+  not two spellings of one result.
 - **The shipped data is embedded, not discovered.** A directory a machine happens
   to have would make `--seed 42` machine-dependent. Data still lives in `data/`
   as JSON; `--data-path` layers over it.
@@ -364,9 +489,80 @@ tokens add cost in proportion to the output.
   folder, `..` the folder above — what those spellings already mean to anyone who
   has typed a path. A locale's files reach each other without naming the locale,
   so a folder renames and copies without editing its references.
+- **After the first tag, a new fence is a major version.** Data files are the
+  public API, and one spelling per result grows by tightening, so every fence
+  invalidates some file. Each such release names the rejected spelling and its
+  replacement in the changelog and in the load error, and that is the whole
+  migration: a fence rejects one spelling with one replacement, so the fix is
+  local to each site. A fence that would need a non-local rewrite ships a
+  converter with its release instead. Before the first tag there is no
+  compatibility promise.
+- **A `--data-path` override rebinds every reference to the category it
+  replaces.** References bind against the merged tree, so once shipped data uses
+  `{.person}`, a consumer's `sv_SE/person.json` is what every shipped reference
+  into `person` reads, and `New` fails on shipped data the consumer never wrote
+  when that file lacks a field those references read. Accepted: overriding is the
+  point of layering, the error names the reference and the field, and the fix is
+  the consumer's file carrying the fields the shipped tree reads.
+- **The repeat cap bounds renders, not bytes.** A repeat, alone or nested, may
+  ask for at most 1 048 576 renders; how large each render is stays what the data
+  asked for, so `{hex(1048576)}` repeated to the cap is a terabyte, loaded without
+  complaint. A byte estimate would need every builtin to declare a width to fence
+  a shape no data comes near, and the harm lands on the author who wrote it.
+- **64-bit targets only.** The gate builds amd64, and the buffer sizing a render
+  pre-computes (renders × bytes) assumes a 64-bit int; on a 32-bit target it could
+  overflow and panic.
+- **A constant zero divisor is a load error; a divisor that is not constant prints
+  `Inf`.** `1/0` and a fixed `"0"` field are decidable, so they join the
+  never-numeric operand as a load error; the fold stops where an operand varies,
+  so `a/(b*c)` with `b` fixed at `0` and `c` varying loads and prints `Inf` every
+  draw — catching it needs zero-absorbing algebra for a shape nobody writes.
+- **In data, a default written out and a constant spelled as a sample are load
+  errors.** `weight: 1`, `repeat: 1`, `separator: ""`, `int(5,5)`, `float(1,1,2)`,
+  `+5` and `05` each spell what a shorter form already spells, so each is rejected
+  naming that form. The CLI's numbers follow the shell instead: `--seed 007` and
+  `--repeat +3` are 7 and 3, as every command line reads them.
 - **Samples say what they emit, transforms what they do.** `{upper(2)}` is two
   letters, `{uppercase(x)}` is `x` upper-cased; one name for both would turn on
   whether the argument looks like a number.
+- **A record is a template seen as columns, not a second schema format.** A
+  template's `format` composes its fields into one string; `Record` and
+  `--format` project the same fields as columns. Two views of one dataset, so a
+  record author writes the same JSON they already know, and a column is the same
+  field `Fake` renders by dotted path. The `format` is inert to a record — a
+  record-only template writes `"format": ""` — but it is compiled and fenced, so
+  a template that loads renders as whichever shape is asked for.
+- **A record shares one reference draw per category.** Two columns that reference
+  one category — `{/currency.code}` beside `{/currency.symbol}` — read one draw of
+  it, so a record's facts agree the way a template's [correlated
+  fields](#correlated-fields) do. The draw is one per record, so it spans a
+  column's `repeat` and nested templates too (one record is one coherent unit);
+  a bare reference — `{/currency}`, no field — stays an independent draw every
+  time, the rule a format string already follows. Only references share: a sibling
+  field is local to its own column, so a `first` column does not silently bind to
+  a `first` in the column next to it.
+
+  The string view of that same template does not share. `Fake` renders each
+  sibling field as its own expansion, so a `{/currency.code}` field beside a
+  `{/currency.symbol}` field is two draws and may render `EUR $`; writing both
+  references in one `format` holds them together, as
+  [One draw, one spelling](#one-draw-one-spelling) says. The scope is what makes a
+  row coherent when the columns *are* the output, and there the caller cannot fall
+  back on one format string. Widening it to every render would change what `Fake`
+  has emitted since the start, for a correlation a single format already reaches.
+- **A record's column set is fixed before the first draw.** Only a category-level
+  template is a record: a path descending into a field, or naming a folder or a
+  choice, errors. A tail may pass through a choice whose variants carry different
+  fields, so the columns — and with them the CSV header written once ahead of every
+  row — would vary per draw. A fixed column set is what the CSV and `INSERT`
+  contracts rest on, so the restriction holds even where a particular choice would
+  happen to agree.
+- **The performance gate asserts allocations, not wall-clock time.** `AllocsPerRun`
+  is deterministic across machines, so a ±10% ceiling does not flake under CI load,
+  while time varies with the machine and its neighbours. A rendering slowdown
+  almost always costs an allocation too (a lost pre-size, a per-item map, an extra
+  copy). The benchmark suite (see Development) reports time for a human, not as a
+  pass/fail gate.
 
 ## Development
 
@@ -379,6 +575,7 @@ docker compose run --rm cover   # tests with coverage
 docker compose run --rm bench   # benchmarks
 docker compose run --rm build   # compile the library
 docker compose run --rm vet     # go vet
+docker compose run --rm cyclo   # cyclomatic complexity over 14 (test files excluded)
 docker compose run --rm dev     # interactive shell
 ```
 
@@ -391,7 +588,7 @@ docker compose run --rm --user "$(id -u):$(id -g)" tidy  # go mod tidy
 
 Every pull request runs `docker build .` against both the latest and the lowest
 supported Go, and must pass before it can be merged. That build is the whole
-gate — vet, format check and tests — so run it locally before pushing:
+gate — vet, complexity, format check and tests — so run it locally before pushing:
 
 ```sh
 docker build .                                  # latest
@@ -404,9 +601,14 @@ GO_VERSION=1.22.12 docker compose run --rm test # the same tests, without the im
 ```
 fejkdata.go     Generator, New, options, the embedded data set, List
 node.go         the node model and JSON -> node compilation
-render.go          Fake and the recursive renderer (choices, format strings, paths, held draws)
-template.go        the {token} grammar: scanning, arms, operands, validation
-reference.go       {/path} binding across the tree, the render graph, and the walks over it
+path.go         the dotted-path walk, and proving a path resolves
+render.go       Fake and the recursive renderer (choices, format strings, expansions)
+record.go       records: Record, the JSON/CSV/SQL serializers, and their entry points
+inline.go       inline templates: Template, NewTemplate, FakeTemplate, and their compile and link
+template.go     the {token} grammar: scanning, tokens, operands, validation, compiling a format
+hold.go         the hold: one draw per expansion for paths and operands, and its fences
+reference.go    reference sigils, and binding references across the tree
+graph.go        the render graph: edges, cycles, the repeat bound, tree walks
 builtins.go     the {name()} function registry and its implementations
 calc.go         the {calc()} arithmetic evaluator: parser, eval, validation
 data.go         data loading: fs.FS folders/files -> namespace tree, multi-source merge
@@ -416,4 +618,4 @@ data/              shipped data (JSON), embedded at build: locale folders + a mi
 
 ## License
 
-MIT — see [LICENSE](LICENSE).
+MIT — see [LICENSE](LICENSE). Forked from [github.com/Timewave-AB/fakes](https://github.com/Timewave-AB/fakes).

builtins.go

@@ -2,6 +2,7 @@ package fejkdata
 
 import (
 	"encoding/base64"
+	"errors"
 	"fmt"
 	"math"
 	"strconv"
@@ -9,12 +10,12 @@ import (
 	"unicode"
 )
 
-// maxLen caps sample output lengths (hex, nanoid, base64) and the renders a repeat
-// multiplies to along any path; maxDecimals caps float/calc decimal places. So a
-// fat-fingered or overflowing argument fails at New instead of trying to allocate
-// gigabytes — or panicking — at render.
+// maxLen caps sample output lengths (hex, nanoid, base64, digits, upper, lower)
+// and maxDecimals float/calc decimal places, so a fat-fingered or overflowing
+// argument fails at New instead of trying to allocate gigabytes — or panicking —
+// at render.
 const (
-	maxLen      = 1 << 20 // 1,048,576 chars/bytes
+	maxLen      = 1 << 20
 	maxDecimals = 1024
 )
 
@@ -132,7 +133,7 @@ func transformArg(fields map[string]node, a []string) error {
 		_, _, err := refShape(leaf)
 		return err
 	}
-	return checkArm(leaf, fields)
+	return checkArm(leaf, fields, false)
 }
 
 func transformOperand(a []string) []string {
@@ -176,7 +177,7 @@ var asciiFolds = map[rune]string{
 func asciiFold(s string) string {
 	var b strings.Builder
 	for _, r := range s {
-		if r < unicode.MaxASCII {
+		if r <= unicode.MaxASCII {
 			b.WriteRune(r)
 		} else {
 			b.WriteString(asciiFolds[r])
@@ -221,10 +222,31 @@ func randChars(r rng, n int, alphabet string) string {
 	return string(b)
 }
 
+// plainInt parses an integer arg written the one way: no sign, no leading zero.
+func plainInt(s string) (int, error) {
+	n, err := strconv.Atoi(s)
+	if errors.Is(err, strconv.ErrRange) {
+		return 0, fmt.Errorf("%q is past the integer range: %w", s, err)
+	}
+	if err != nil {
+		return 0, fmt.Errorf("%q is not an integer", s)
+	}
+	if strconv.Itoa(n) != s {
+		return 0, fmt.Errorf("%q is not a plain integer; write %d", s, n)
+	}
+	return n, nil
+}
+
 func posIntArg(_ map[string]node, a []string) error {
-	n, err := strconv.Atoi(a[0])
-	if err != nil || n < 1 {
-		return fmt.Errorf("count %q must be a positive integer", a[0])
+	n, err := plainInt(a[0])
+	if errors.Is(err, strconv.ErrRange) {
+		return fmt.Errorf("count %q exceeds the maximum %d", a[0], maxLen)
+	}
+	if err != nil {
+		return fmt.Errorf("count %w", err)
+	}
+	if n < 1 {
+		return fmt.Errorf("count %q must be positive", a[0])
 	}
 	if n > maxLen {
 		return fmt.Errorf("count %d exceeds the maximum %d", n, maxLen)
@@ -233,14 +255,20 @@ func posIntArg(_ map[string]node, a []string) error {
 }
 
 func intRangeArgs(_ map[string]node, a []string) error {
-	lo, e1 := strconv.Atoi(a[0])
-	hi, e2 := strconv.Atoi(a[1])
-	if e1 != nil || e2 != nil {
-		return fmt.Errorf("int(min,max) needs integer args, got %q,%q", a[0], a[1])
+	lo, err := plainInt(a[0])
+	if err != nil {
+		return fmt.Errorf("int(min,max): min %w", err)
+	}
+	hi, err := plainInt(a[1])
+	if err != nil {
+		return fmt.Errorf("int(min,max): max %w", err)
 	}
 	if lo > hi {
 		return fmt.Errorf("int(min,max): min %d > max %d", lo, hi)
 	}
+	if lo == hi {
+		return fmt.Errorf("int(%d,%d) is the constant %d; write it as text", lo, hi, lo)
+	}
 	if uint64(hi)-uint64(lo) >= uint64(math.MaxInt64) { // span hi-lo+1 would overflow int -> IntN panic
 		return fmt.Errorf("int(min,max): range %d..%d is too wide", lo, hi)
 	}
@@ -250,19 +278,28 @@ func intRangeArgs(_ map[string]node, a []string) error {
 func floatArgs(_ map[string]node, a []string) error {
 	lo, e1 := strconv.ParseFloat(a[0], 64)
 	hi, e2 := strconv.ParseFloat(a[1], 64)
-	dp, e3 := strconv.Atoi(a[2])
-	if e1 != nil || e2 != nil || e3 != nil {
-		return fmt.Errorf("float(min,max,dp) needs numeric args, got %q,%q,%q", a[0], a[1], a[2])
+	if e1 != nil || e2 != nil {
+		return fmt.Errorf("float(min,max,dp) needs numeric bounds, got %q,%q", a[0], a[1])
+	}
+	dp, err := plainInt(a[2])
+	if err != nil {
+		return fmt.Errorf("float(min,max,dp): decimals %w", err)
+	}
+	if math.IsNaN(lo) || math.IsNaN(hi) || math.IsInf(lo, 0) || math.IsInf(hi, 0) {
+		return fmt.Errorf("float(min,max,dp) needs finite bounds, got %q,%q", a[0], a[1])
 	}
 	if lo > hi {
 		return fmt.Errorf("float(min,max,dp): min %v > max %v", lo, hi)
 	}
-	if math.IsInf(hi-lo, 0) { // an overflowing span would render as "+Inf"
-		return fmt.Errorf("float(min,max,dp): range %v..%v is too wide", lo, hi)
-	}
 	if dp < 0 || dp > maxDecimals {
 		return fmt.Errorf("float(min,max,dp): decimals %d out of range 0..%d", dp, maxDecimals)
 	}
+	if lo == hi {
+		return fmt.Errorf("float(%s,%s,%d) is the constant %q; write it as text", a[0], a[1], dp, strconv.FormatFloat(lo, 'f', dp, 64))
+	}
+	if math.IsInf(hi-lo, 0) { // an overflowing span would render as "+Inf"
+		return fmt.Errorf("float(min,max,dp): range %v..%v is too wide", lo, hi)
+	}
 	return nil
 }
 

calc.go

@@ -78,14 +78,82 @@ func checkCalc(fields map[string]node, args []string) error {
 			return fmt.Errorf("calc(%q): operand %q is never a number: it renders %q", args[0], name, text)
 		}
 	}
+	if divisor, zero := constantZeroDivisor(expr, fields); zero {
+		return fmt.Errorf("calc(%q) divides by %s, which is always zero", args[0], divisor)
+	}
 	if len(args) == 2 {
-		if dp, err := strconv.Atoi(args[1]); err != nil || dp < 0 || dp > maxDecimals {
-			return fmt.Errorf("calc decimals %q must be an integer in 0..%d", args[1], maxDecimals)
+		dp, err := plainInt(args[1])
+		if err != nil {
+			return fmt.Errorf("calc decimals %w", err)
+		}
+		if dp < 0 || dp > maxDecimals {
+			return fmt.Errorf("calc decimals %d must be in 0..%d", dp, maxDecimals)
 		}
 	}
 	return nil
 }
 
+// constantZeroDivisor finds a division whose right side is a constant zero: number
+// literals and fixed operands folded, anything that varies left unknown.
+func constantZeroDivisor(n calcNode, fields map[string]node) (string, bool) {
+	switch n := n.(type) {
+	case calcNeg:
+		return constantZeroDivisor(n.x, fields)
+	case calcBin:
+		if n.op == '/' {
+			if v, known := constantValue(n.r, fields); known && v == 0 {
+				return calcText(n.r), true
+			}
+		}
+		if d, zero := constantZeroDivisor(n.l, fields); zero {
+			return d, true
+		}
+		return constantZeroDivisor(n.r, fields)
+	}
+	return "", false
+}
+
+// constantValue evaluates an expression whose every operand is fixed.
+func constantValue(n calcNode, fields map[string]node) (float64, bool) {
+	switch n := n.(type) {
+	case calcNum:
+		return float64(n), true
+	case calcVar:
+		t, ok := fields[string(n)].(*template)
+		if !ok || !t.fixed || t.repeat > 1 {
+			return 0, false
+		}
+		v, err := strconv.ParseFloat(strings.TrimSpace(t.lit), 64)
+		return v, err == nil
+	case calcNeg:
+		v, ok := constantValue(n.x, fields)
+		return -v, ok
+	case calcBin:
+		l, lok := constantValue(n.l, fields)
+		r, rok := constantValue(n.r, fields)
+		if !lok || !rok {
+			return 0, false
+		}
+		return calcBin{n.op, calcNum(l), calcNum(r)}.eval(nil), true
+	}
+	return 0, false
+}
+
+// calcText spells an expression node the way an author would read it.
+func calcText(n calcNode) string {
+	switch n := n.(type) {
+	case calcNum:
+		return strconv.FormatFloat(float64(n), 'f', -1, 64)
+	case calcVar:
+		return string(n)
+	case calcNeg:
+		return "-" + calcText(n.x)
+	case calcBin:
+		return "(" + calcText(n.l) + " " + string(n.op) + " " + calcText(n.r) + ")"
+	}
+	return "?"
+}
+
 // neverNumeric reports a node no render of which is a number: fixed text that does
 // not parse, or a choice of only such items. text is one such render.
 func neverNumeric(n node) (text string, never bool) {
@@ -242,6 +310,7 @@ func (p *calcParser) binary(next func() (calcNode, error), ops ...byte) (calcNod
 	}
 }
 
+// factor is a table-shaped scanner, one case per token kind, kept whole on purpose.
 func (p *calcParser) factor() (calcNode, error) {
 	p.space()
 	if p.pos >= len(p.rs) {

calc_test.go

@@ -245,3 +245,28 @@ func TestCalcOverANeverNumericOperandIsRejected(t *testing.T) {
 		}
 	}
 }
+
+func TestCalcConstantZeroDivisorIsRejected(t *testing.T) {
+	for src, want := range map[string]string{
+		`"{calc(1/0)}"`:                                  "divides by 0",
+		`"{calc(2/(1-1))}"`:                              "divides by (1 - 1)",
+		`{"format":"{calc(x/y)}","x":"1","y":"0"}`:       "divides by y",
+		`{"format":"{calc(x/(y*2))}","x":"1","y":" 0 "}`: "divides by (y * 2)",
+		`{"format":"{calc((a/0)+b)}","a":"1","b":"2"}`:   "divides by 0",
+		`{"format":"{calc(a/-0)}","a":"1"}`:              "divides by -0",
+	} {
+		if _, err := compile(parse(t, src)); err == nil || !strings.Contains(err.Error(), want) {
+			t.Errorf("compile(%s) = %v, want the constant zero divisor rejected naming %q", src, err, want)
+		}
+	}
+	if _, err := compile(parse(t, `{"format":"{calc(a/(b*c))}","a":"1","b":"0","c":["1","2"]}`)); err != nil {
+		t.Errorf("compile(a/(b*c)) = %v, want a divisor that varies accepted", err)
+	}
+	f := engine(1)
+	for i := 0; i < 50; i++ {
+		if got := mustRender(t, f, `{"format":"{calc(x/y)}","x":"1","y":["0","1"]}`); got == "+Inf" {
+			return
+		}
+	}
+	t.Fatal("a sometimes-zero divisor never printed +Inf in 50 draws")
+}

cmd/fejkdata/main.go

@@ -8,44 +8,68 @@
 package main
 
 import (
+	"bufio"
+	"encoding/json"
 	"errors"
 	"fmt"
 	"io"
 	"os"
 	"runtime/debug"
+	"sort"
 	"strconv"
 	"strings"
 
 	"gitea.larvit.se/larvit/fejkdata"
 )
 
-const usage = `Usage: fejkdata [flags] <path>
+const usage = `Usage: fejkdata [flags] <path|template>
 
   <path>                 a category, or a dotted path into one (person, person.last)
+  <template>             a format string or JSON value to render inline, e.g.
+                         'name: {/sv_SE.person.last}' or '{"format":"{x}","x":["bosse","lina"]}'
+
+An argument containing a { token, or a JSON object, array or string, is a
+template; any other argument is a path (a path never contains a brace, a bracket
+or a quote). Templates reach the data by reference from the root —
+{/sv_SE.person.last} — whether the data is shipped or layered with --data-path. An
+argument carrying a bracket, a closing brace or a quote but no valid JSON names
+neither.
+
+With --format json, ndjson, csv or sql the argument must name a record — a
+template whose fields are its columns — and the rows are written as one JSON
+array, one JSON object per line, one CSV row (after a header), or one INSERT.
 
   -d, --data-path D      a data directory to layer over the shipped data (repeatable; last wins on a clash)
+      --format F         output form: text (default), json, ndjson, csv or sql
   -h, --help             print this help, then exit
       --list             list the paths the data offers, then exit
       --no-shipped-data  load only the --data-path directories
-  -n, --repeat N         render the path N times (default 1)
+  -n, --repeat N         render the value N times, 1..1048576 (default 1)
   -s, --seed N           seed for reproducible output
       --separator S      string between repeated values (default newline)
+      --table T          the INSERT target for --format sql (default: the path's last segment, or records for an inline template)
       --version          print the version, then exit
 
-Flags may come before or after <path>; -- ends the flags. A short flag's value
-attaches or follows (-n3, -n 3); short flags bundle (-hn 3).
+Flags may come before or after <path|template>; -- ends the flags. A short flag's
+value attaches or follows (-n3, -n 3); short flags bundle (-hn 3).
 `
 
 type invocation struct {
 	dirs         []string
+	format       string
+	formatSet    bool
 	help         bool
 	list         bool
 	noShipped    bool
 	paths        []string
 	repeat       int
+	repeatSet    bool
 	seed         uint64
 	seeded       bool
 	separator    string
+	separatorSet bool
+	table        string
+	tableSet     bool
 	version      bool
 }
 
@@ -58,15 +82,16 @@ type flagDef struct {
 
 var flagDefs = []flagDef{
 	{"data-path", "d", true, func(in *invocation, v string) error { in.dirs = append(in.dirs, v); return nil }},
+	{"format", "", true, func(in *invocation, v string) error { in.format, in.formatSet = v, true; return nil }},
 	{"help", "h", false, func(in *invocation, _ string) error { in.help = true; return nil }},
 	{"list", "", false, func(in *invocation, _ string) error { in.list = true; return nil }},
 	{"no-shipped-data", "", false, func(in *invocation, _ string) error { in.noShipped = true; return nil }},
 	{"repeat", "n", true, func(in *invocation, v string) error {
 		n, err := strconv.Atoi(v)
-		if err != nil || n < 1 {
-			return fmt.Errorf("--repeat needs a positive integer, got %q", v)
+		if err != nil || n < 1 || n > fejkdata.MaxRepeat {
+			return fmt.Errorf("--repeat needs an integer in 1..%d, got %q", fejkdata.MaxRepeat, v)
 		}
-		in.repeat = n
+		in.repeat, in.repeatSet = n, true
 		return nil
 	}},
 	{"seed", "s", true, func(in *invocation, v string) error {
@@ -77,7 +102,8 @@ var flagDefs = []flagDef{
 		in.seed, in.seeded = n, true
 		return nil
 	}},
-	{"separator", "", true, func(in *invocation, v string) error { in.separator = v; return nil }},
+	{"separator", "", true, func(in *invocation, v string) error { in.separator, in.separatorSet = v, true; return nil }},
+	{"table", "", true, func(in *invocation, v string) error { in.table, in.tableSet = v, true; return nil }},
 	{"version", "", false, func(in *invocation, _ string) error { in.version = true; return nil }},
 }
 
@@ -124,8 +150,8 @@ func splitFlags(arg string) ([]flagArg, error) {
 	}
 	var flags []flagArg
 	letters := arg[1:]
-	for i := 0; i < len(letters); i++ {
-		letter := letters[i : i+1]
+	for i, r := range letters {
+		letter := string(r)
 		def := flagByShort(letter)
 		if def == nil {
 			return nil, fmt.Errorf("unknown flag -%s", letter)
@@ -134,7 +160,7 @@ func splitFlags(arg string) ([]flagArg, error) {
 			flags = append(flags, flagArg{def: def})
 			continue
 		}
-		rest := letters[i+1:]
+		rest := letters[i+len(letter):]
 		if strings.HasPrefix(rest, "=") {
 			return nil, fmt.Errorf("-%s takes its value attached (-%s%s) or next (-%s %s); = belongs to --%s=%s",
 				letter, letter, rest[1:], letter, rest[1:], def.long, rest[1:])
@@ -145,7 +171,7 @@ func splitFlags(arg string) ([]flagArg, error) {
 }
 
 func parseArgs(argv []string) (invocation, error) {
-	in := invocation{repeat: 1, separator: "\n"}
+	in := invocation{repeat: 1, separator: "\n", format: "text"}
 	for i := 0; i < len(argv); i++ {
 		arg := argv[i]
 		if arg == "--" {
@@ -183,17 +209,81 @@ func parseArgs(argv []string) (invocation, error) {
 	return in, nil
 }
 
-// check rejects a flag combination that cannot run.
-func (in invocation) check() error {
-	if in.list && len(in.paths) > 0 {
-		return errors.New("--list takes no path")
+// recordFormat is one way to write a record out: the line each record renders,
+// the header that precedes the first one, and the open/close frame plus the
+// between-record separator a document form needs.
+type recordFormat struct {
+	header func(*fejkdata.Record) string
+	line   func(r *fejkdata.Record, table string) string
+	open   string
+	close  string
+	sep    string
+}
+
+// recordFormats is every --format that writes records. json frames the records
+// as one array document; ndjson is the same column, one object per line.
+var recordFormats = map[string]recordFormat{
+	"csv":    {header: (*fejkdata.Record).CSVHeader, line: func(r *fejkdata.Record, _ string) string { return r.CSVLine() }, sep: "\n"},
+	"json":   {line: jsonLine, open: "[", close: "]", sep: ",\n"},
+	"ndjson": {line: jsonLine, sep: "\n"},
+	"sql":    {line: func(r *fejkdata.Record, table string) string { return r.SQLInsert(table) }, sep: "\n"},
+}
+
+func jsonLine(r *fejkdata.Record, _ string) string { return r.JSON() }
+
+// writesRecords reports whether the format writes records rather than plain text.
+func (in invocation) writesRecords() bool {
+	return in.format != "text"
+}
+
+// formatNames lists the --format values, text included, for the misuse error.
+func formatNames() string {
+	names := []string{"text"}
+	for name := range recordFormats {
+		names = append(names, name)
 	}
-	if !in.list && len(in.paths) != 1 {
-		return fmt.Errorf("expected one path, got %d", len(in.paths))
+	sort.Strings(names)
+	return strings.Join(names[:len(names)-1], ", ") + " or " + names[len(names)-1]
+}
+
+// checkFlags rejects a flag value or combination that cannot run.
+func (in invocation) checkFlags() error {
+	if _, ok := recordFormats[in.format]; !ok && in.format != "text" {
+		return fmt.Errorf("--format takes %s, got %q", formatNames(), in.format)
+	}
+	if in.tableSet && in.table == "" {
+		return errors.New("--table names the INSERT target, so it cannot be empty")
+	}
+	if in.tableSet && in.format != "sql" {
+		return errors.New("--table names the INSERT target, so it needs --format sql")
+	}
+	if in.writesRecords() && in.separatorSet {
+		return errors.New("--separator joins text values, so it has no effect with --format " + in.format)
 	}
 	return nil
 }
 
+// check rejects a flag combination or an argument that cannot run, and reports
+// what the argument names, so its shape is settled before any data is read.
+func (in invocation) check() (argKind, error) {
+	if err := in.checkFlags(); err != nil {
+		return argPath, err
+	}
+	if in.list && len(in.paths) > 0 {
+		return argPath, errors.New("--list takes no path")
+	}
+	if in.list && (in.repeatSet || in.separatorSet || in.formatSet || in.tableSet) {
+		return argPath, errors.New("--list takes no --repeat, --separator, --format or --table")
+	}
+	if in.list {
+		return argPath, nil
+	}
+	if len(in.paths) != 1 {
+		return argPath, fmt.Errorf("expected one path or template, got %d", len(in.paths))
+	}
+	return classify(in.paths[0])
+}
+
 func (in invocation) options() []fejkdata.Option {
 	var opts []fejkdata.Option
 	if in.noShipped {
@@ -208,17 +298,137 @@ func (in invocation) options() []fejkdata.Option {
 	return opts
 }
 
-// values renders the path repeat times, joined by the separator.
-func (in invocation) values(f *fejkdata.Generator) (string, error) {
-	vals := make([]string, in.repeat)
-	for i := range vals {
-		v, err := f.Fake(in.paths[0])
+// write streams the argument's renders to w, repeat of them joined by the
+// separator and ended by a newline. A value that renders once renders every time,
+// so a render failure comes before anything is written; a write failure surfaces
+// from Flush, bufio keeping the first one.
+func (in invocation) write(f *fejkdata.Generator, kind argKind, w io.Writer) error {
+	if in.writesRecords() {
+		return in.writeRecords(f, kind, w)
+	}
+	draw, err := in.textDraw(f, kind, in.paths[0])
 	if err != nil {
-			return "", err
+		return err
 	}
-		vals[i] = v
+	out := bufio.NewWriter(w)
+	for i := 0; i < in.repeat; i++ {
+		v, err := draw()
+		if err != nil {
+			return err
 		}
-	return strings.Join(vals, in.separator), nil
+		if i > 0 {
+			out.WriteString(in.separator)
+		}
+		out.WriteString(v)
+	}
+	out.WriteString("\n")
+	return out.Flush()
+}
+
+// textDraw builds what one text render yields: the value, from a path or an
+// inline template.
+func (in invocation) textDraw(f *fejkdata.Generator, kind argKind, arg string) (func() (string, error), error) {
+	if kind != argTemplate {
+		return func() (string, error) { return f.Fake(arg) }, nil
+	}
+	t, err := f.NewTemplate(arg)
+	if err != nil {
+		return nil, templateError{err}
+	}
+	return func() (string, error) { return t.Fake(), nil }, nil
+}
+
+// recordStream builds the record drawer for the argument, plus the INSERT table
+// a sql format names.
+func (in invocation) recordStream(f *fejkdata.Generator, kind argKind, arg string) (func() (*fejkdata.Record, error), string, error) {
+	record := func() (*fejkdata.Record, error) { return f.Record(arg) }
+	if kind == argTemplate {
+		t, err := f.NewRecordTemplate(arg)
+		if err != nil {
+			return nil, "", templateError{err}
+		}
+		record = func() (*fejkdata.Record, error) { return t.Fake(), nil }
+	}
+	table := in.table
+	if table == "" {
+		table = defaultTable(arg, kind)
+	}
+	return record, table, nil
+}
+
+// writeRecords streams a record per line in the chosen format, framing a document
+// form with its open/close brackets and a header preceding the first record.
+func (in invocation) writeRecords(f *fejkdata.Generator, kind argKind, w io.Writer) error {
+	record, table, err := in.recordStream(f, kind, in.paths[0])
+	if err != nil {
+		return err
+	}
+	format := recordFormats[in.format]
+	out := bufio.NewWriter(w)
+	if format.open != "" {
+		out.WriteString(format.open)
+		out.WriteByte('\n')
+	}
+	for i := 0; i < in.repeat; i++ {
+		r, err := record()
+		if err != nil {
+			return err
+		}
+		if i == 0 && format.header != nil {
+			out.WriteString(format.header(r))
+			out.WriteByte('\n')
+		}
+		if i > 0 {
+			out.WriteString(format.sep)
+		}
+		out.WriteString(format.line(r, table))
+	}
+	if format.close != "" {
+		out.WriteByte('\n')
+		out.WriteString(format.close)
+	}
+	out.WriteByte('\n')
+	return out.Flush()
+}
+
+// defaultTable names the INSERT target when --table is absent: the path's last
+// segment, or "records" for an inline template that sits in no folder.
+func defaultTable(arg string, kind argKind) string {
+	if kind == argTemplate {
+		return "records"
+	}
+	segments := strings.Split(arg, ".")
+	return segments[len(segments)-1]
+}
+
+// templateError marks a render failure that is the argument's own fault — an
+// inline template that does not compile. run reports it as misuse (exit 2, with a
+// pointer to --help), unlike an unknown path, which is a runtime error (exit 1).
+type templateError struct{ error }
+
+func (e templateError) Unwrap() error { return e.error }
+
+type argKind int
+
+const (
+	argPath argKind = iota
+	argTemplate
+)
+
+// classify reads what a positional argument names by its shape: a { token, or a
+// JSON object, array or string, is an inline template; anything else is a path.
+func classify(arg string) (argKind, error) {
+	if strings.ContainsRune(arg, '{') || (isJSONStart(strings.TrimSpace(arg)) && json.Valid([]byte(arg))) {
+		return argTemplate, nil
+	}
+	if i := strings.IndexAny(arg, `[]}"`); i >= 0 {
+		return argPath, fmt.Errorf("%q holds a %q, which no path may, and it is not valid JSON, so it names no template either", arg, arg[i:i+1])
+	}
+	return argPath, nil
+}
+
+func isJSONStart(arg string) bool {
+	return strings.HasPrefix(arg, "[") || strings.HasPrefix(arg, `"`)
 }
 
 func main() { os.Exit(run(os.Args[1:], os.Stdout, os.Stderr)) }
@@ -237,7 +447,8 @@ func run(args []string, stdout, stderr io.Writer) int {
 		fmt.Fprintln(stdout, "fejkdata "+buildVersion())
 		return 0
 	}
-	if err := in.check(); err != nil {
+	kind, err := in.check()
+	if err != nil {
 		return misuse(stderr, err)
 	}
 	f, err := fejkdata.New(in.options()...)
@@ -254,17 +465,20 @@ func run(args []string, stdout, stderr io.Writer) int {
 		}
 		return 0
 	}
-	out, err := in.values(f)
-	if err != nil {
+	if err := in.write(f, kind, stdout); err != nil {
+		var te templateError
+		if errors.As(err, &te) {
+			return misuse(stderr, te.error)
+		}
 		fmt.Fprintln(stderr, err)
 		return 1
 	}
-	fmt.Fprintln(stdout, out)
 	return 0
 }
 
 func misuse(stderr io.Writer, err error) int {
-	fmt.Fprintf(stderr, "fejkdata: %v\ntry 'fejkdata --help'\n", err)
+	// A library error already names the program, so the prefix is not doubled.
+	fmt.Fprintf(stderr, "fejkdata: %s\ntry 'fejkdata --help'\n", strings.TrimPrefix(err.Error(), "fejkdata: "))
 	return 2
 }
 

cmd/fejkdata/main_test.go

@@ -2,6 +2,11 @@ package main
 
 import (
 	"bytes"
+	"encoding/csv"
+	"encoding/json"
+	"os"
+	"path/filepath"
+	"regexp"
 	"strings"
 	"testing"
 )
@@ -90,7 +95,7 @@ func TestRunShortFlagValues(t *testing.T) {
 		{[]string{"-s=42", "-d", svSE, "address"}, "-s42"},
 		{[]string{"-s=42", "-d", svSE, "address"}, "--seed=42"},
 		{[]string{"-d=" + svSE, "address"}, "--data-path="},
-		{[]string{"-nd", "3", "-d", svSE, "word"}, `--repeat needs a positive integer, got "d"`},
+		{[]string{"-nd", "3", "-d", svSE, "word"}, `--repeat needs an integer in 1..1048576, got "d"`},
 		{[]string{"--seed=", "-d", svSE, "word"}, `--seed needs an unsigned integer, got ""`},
 	} {
 		code, out, errb := runOut(c.args...)
@@ -299,6 +304,92 @@ func TestRunShippedDataByDefault(t *testing.T) {
 	}
 }
 
+func TestClassify(t *testing.T) {
+	for arg, want := range map[string]argKind{
+		"sv_SE.person":   argPath,
+		"person.last":    argPath,
+		"name: {x}":      argTemplate, // a { token: a path can never carry a brace
+		`{"format":"x"}`: argTemplate,
+		`["a","b"]`:      argTemplate, // a JSON array carries no brace
+		`[1, 2]`:         argTemplate,
+		` ["a","b"]`:     argTemplate, // padding is the template's own error, not a shape verdict
+		`"hello"`:        argTemplate, // a JSON string, the spelling a format-only object names
+	} {
+		got, err := classify(arg)
+		if err != nil || got != want {
+			t.Errorf("classify(%q) = %v, %v; want %v", arg, got, err, want)
+		}
+	}
+	for arg, want := range map[string]string{
+		"[abc]":       `holds a "["`,
+		"[abc].field": `holds a "["`,
+		"x[1]":        `holds a "["`,
+		"a]b":         `holds a "]"`,
+		"a}b":         `holds a "}"`,
+		`"abc`:        `holds a "\""`,
+		`"a]b`:        `holds a "\""`, // the opener the reader typed, not the bracket behind it
+	} {
+		_, err := classify(arg)
+		if err == nil || !strings.Contains(err.Error(), want) {
+			t.Errorf("classify(%q) = %v; want it rejected naming %s", arg, err, want)
+		}
+	}
+}
+
+func TestUsageReferencesResolve(t *testing.T) {
+	for _, token := range regexp.MustCompile(`\{/[^}]+\}`).FindAllString(usage, -1) {
+		code, out, errb := runOut("--seed", "1", token)
+		if code != 0 || strings.TrimSpace(out) == "" {
+			t.Errorf("usage advertises %s: run = %d, %q, stderr %q", token, code, out, errb)
+		}
+	}
+}
+
+func TestRunShapeMisuseBeforeLoad(t *testing.T) {
+	code, _, errb := runOut("--no-shipped-data", "[abc]")
+	if code != 2 || !strings.Contains(errb, "[abc]") || strings.Contains(errb, "--data-path") {
+		t.Fatalf("shape misuse with no data = %d, %q; want the shape error before any load", code, errb)
+	}
+}
+
+func TestRunInlineTemplate(t *testing.T) {
+	code, out, errb := runOut("--seed", "1", "name: {/sv_SE.person.last}")
+	if code != 0 || !strings.HasPrefix(out, "name: ") || strings.Contains(out, "{") {
+		t.Fatalf("inline format string = %d, %q, stderr %q", code, out, errb)
+	}
+	code, out, errb = runOut("--seed", "1", `{"format":"name: {x}","x":["bosse","lina"]}`)
+	if code != 0 || (out != "name: bosse\n" && out != "name: lina\n") {
+		t.Fatalf("inline JSON template = %d, %q, want one name, stderr %q", code, out, errb)
+	}
+	code, out, errb = runOut("--seed", "1", `"name: {/sv_SE.person.last}"`)
+	if code != 0 || !strings.HasPrefix(out, "name: ") || strings.Contains(out, "{") {
+		t.Fatalf("inline JSON string = %d, %q, stderr %q", code, out, errb)
+	}
+	code, out, errb = runOut("--seed", "1", "-n", "2", `{digits(1)}`)
+	if code != 0 || len(strings.Split(strings.TrimRight(out, "\n"), "\n")) != 2 {
+		t.Fatalf("inline template with --repeat = %d, %q, stderr %q", code, out, errb)
+	}
+}
+
+func TestRunTemplateMisuse(t *testing.T) {
+	for arg, want := range map[string]string{
+		"{bad":            "unterminated",
+		"[red,green]":     "names no template either",
+		`{"format":"x"}`:  "is a string",
+		"{/no.such.path}": "no entry",
+		"x[1]":            "names no template either",
+		` ["a","b"] `:     "may not be padded",
+	} {
+		code, out, errb := runOut("--seed", "1", arg)
+		if code != 2 || out != "" || !strings.Contains(errb, "try 'fejkdata --help'") || !strings.Contains(errb, want) {
+			t.Errorf("run(%q) = %d, %q, %q; want misuse naming %q and --help", arg, code, out, errb, want)
+		}
+		if strings.Contains(errb, "fejkdata: fejkdata:") {
+			t.Errorf("run(%q) doubled the program prefix: %q", arg, errb)
+		}
+	}
+}
+
 func TestRunNoShippedData(t *testing.T) {
 	code, list, errb := runOut("--no-shipped-data", "-d", svSE, "--list")
 	if code != 0 {
@@ -316,3 +407,194 @@ func TestRunNoShippedData(t *testing.T) {
 		t.Errorf("--no-shipped-data alone = %d, %q, want misuse naming --data-path", code, errb)
 	}
 }
+
+func TestRunRepeatIsBounded(t *testing.T) {
+	for _, args := range [][]string{
+		{"--repeat", "9223372036854775807", "sv_SE.word"},
+		{"--repeat", "1048577", "sv_SE.word"},
+		{"-n", "99999999999", "sv_SE.word"},
+	} {
+		code, out, errb := runOut(args...)
+		if code != 2 || out != "" || !strings.Contains(errb, "1..1048576") {
+			t.Errorf("run(%v) = %d, %q, %q, want misuse naming the range", args, code, out, errb)
+		}
+	}
+	dir := t.TempDir()
+	if err := os.WriteFile(filepath.Join(dir, "x.json"), []byte(`"x"`), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	code, out, errb := runOut("--no-shipped-data", "-d", dir, "--repeat", "1048576", "--separator", "", "x")
+	if code != 0 || len(out) != 1048576+1 {
+		t.Errorf("repeat at the cap = %d, %d bytes, stderr %q; want every render streamed", code, len(out), errb)
+	}
+}
+
+func TestRunListTakesNoRepeatOrSeparator(t *testing.T) {
+	for _, args := range [][]string{{"--list", "-n", "3"}, {"--list", "--separator", ","}} {
+		code, _, errb := runOut(args...)
+		if code != 2 || !strings.Contains(errb, "--list takes no") {
+			t.Errorf("run(%v) = %d, %q, want misuse", args, code, errb)
+		}
+	}
+}
+
+func TestRunUnknownFlagIsNamedByRune(t *testing.T) {
+	code, _, errb := runOut("-ä", "sv_SE.word")
+	if code != 2 || !strings.Contains(errb, "unknown flag -ä") {
+		t.Errorf("run(-ä) = %d, %q, want the flag named whole", code, errb)
+	}
+}
+
+func TestRunEmptyDataPathIsNamed(t *testing.T) {
+	code, _, errb := runOut("--data-path=", "sv_SE.word")
+	if code != 1 || !strings.Contains(errb, "empty") {
+		t.Errorf("run(--data-path=) = %d, %q, want the empty path named", code, errb)
+	}
+}
+
+func recordDir(t *testing.T) string {
+	t.Helper()
+	dir := t.TempDir()
+	content := `{"format":"{first} {last}","first":["Ada","Bo"],"last":["Lovelace","Ek"]}`
+	if err := os.WriteFile(filepath.Join(dir, "users.json"), []byte(content), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	return dir
+}
+
+func TestRunRecordJSON(t *testing.T) {
+	code, out, errb := runOut("--seed", "1", "--format", "json", "--repeat", "2", "--data-path", recordDir(t), "users")
+	if code != 0 {
+		t.Fatalf("run = %d, stderr=%q", code, errb)
+	}
+	var arr []map[string]string
+	if err := json.Unmarshal([]byte(out), &arr); err != nil {
+		t.Fatalf("json output is not one JSON array: %v\n%q", err, out)
+	}
+	if len(arr) != 2 || len(arr[0]) != 2 || arr[0]["first"] == "" || arr[0]["last"] == "" {
+		t.Fatalf("json output = %q, want an array of two records with first and last columns", out)
+	}
+}
+
+func TestRunRecordNDJSON(t *testing.T) {
+	code, out, errb := runOut("--seed", "1", "--format", "ndjson", "--repeat", "2", "--data-path", recordDir(t), "users")
+	if code != 0 {
+		t.Fatalf("run = %d, stderr=%q", code, errb)
+	}
+	lines := strings.Split(strings.TrimRight(out, "\n"), "\n")
+	if len(lines) != 2 {
+		t.Fatalf("ndjson output = %d lines %q, want one object per line", len(lines), out)
+	}
+	for _, line := range lines {
+		var m map[string]string
+		if err := json.Unmarshal([]byte(line), &m); err != nil || len(m) != 2 {
+			t.Fatalf("ndjson line %q is not one object: %v", line, err)
+		}
+	}
+}
+
+func TestRunRecordCSVRoundTrips(t *testing.T) {
+	code, out, errb := runOut("--seed", "1", "--format", "csv", "--repeat", "3", "--data-path", recordDir(t), "users")
+	if code != 0 {
+		t.Fatalf("run = %d, stderr=%q", code, errb)
+	}
+	lines := strings.Split(strings.TrimRight(out, "\n"), "\n")
+	if len(lines) != 4 {
+		t.Fatalf("csv output = %d lines %q, want a header and 3 rows", len(lines), out)
+	}
+	if lines[0] != "first,last" {
+		t.Fatalf("csv header = %q, want first,last", lines[0])
+	}
+	r, err := csv.NewReader(strings.NewReader(out)).ReadAll()
+	if err != nil {
+		t.Fatalf("csv output is not valid CSV: %v", err)
+	}
+	if len(r) != 4 || len(r[0]) != 2 {
+		t.Fatalf("csv parsed to %v, want 4 records of 2 fields", r)
+	}
+}
+
+func TestRunRecordSQL(t *testing.T) {
+	code, out, errb := runOut("--seed", "1", "--format", "sql", "--repeat", "2", "--table", "people", "--data-path", recordDir(t), "users")
+	if code != 0 {
+		t.Fatalf("run = %d, stderr=%q", code, errb)
+	}
+	lines := strings.Split(strings.TrimRight(out, "\n"), "\n")
+	if len(lines) != 2 || !strings.HasPrefix(lines[0], `INSERT INTO "people" ("first", "last") VALUES (`) {
+		t.Fatalf("sql output = %q, want two INSERT INTO people statements", out)
+	}
+}
+
+func TestRunRecordDefaultTable(t *testing.T) {
+	code, out, errb := runOut("--seed", "1", "--format", "sql", "--data-path", recordDir(t), "users")
+	if code != 0 || !strings.HasPrefix(out, `INSERT INTO "users" (`) {
+		t.Fatalf("default table = %d, %q, want the path's last segment; stderr %q", code, out, errb)
+	}
+}
+
+func TestRunRecordInlineTemplate(t *testing.T) {
+	code, out, errb := runOut("--seed", "1", "--format", "json", `{"format":"{x}","x":["a","b"]}`)
+	if code != 0 {
+		t.Fatalf("inline record = %d, stderr=%q", code, errb)
+	}
+	var arr []map[string]string
+	if err := json.Unmarshal([]byte(out), &arr); err != nil || len(arr) != 1 || (arr[0]["x"] != "a" && arr[0]["x"] != "b") {
+		t.Fatalf("inline record json = %q, want one record with a column x (err %v)", out, err)
+	}
+}
+
+func TestRunRecordMisuse(t *testing.T) {
+	for _, c := range []struct {
+		args []string
+		want string
+	}{
+		{[]string{"--format", "yaml", "users"}, "--format takes csv, json, ndjson, sql or text"},
+		{[]string{"--format", "json", "--separator", ",", "users"}, "--separator joins text values"},
+		{[]string{"--table", "t", "users"}, "--table names the INSERT target"},
+		{[]string{"--format", "json", "--table", "t", "users"}, "--table names the INSERT target"},
+		{[]string{"--format", "sql", "--table", "", "users"}, "--table names the INSERT target"},
+	} {
+		code, out, errb := runOut(c.args...)
+		if code != 2 || out != "" || !strings.Contains(errb, c.want) {
+			t.Errorf("run(%v) = %d, %q, %q; want misuse naming %q", c.args, code, out, errb, c.want)
+		}
+	}
+}
+
+func TestRunRecordOnABareValueIsRuntimeError(t *testing.T) {
+	dir := t.TempDir()
+	if err := os.WriteFile(filepath.Join(dir, "word.json"), []byte(`["a","b"]`), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	code, _, errb := runOut("--format", "json", "--data-path", dir, "word")
+	if code != 1 {
+		t.Fatalf("record on a choice = %d, want exit 1 (runtime error), stderr %q", code, errb)
+	}
+}
+
+func TestRunRecordRejectsATopLevelRepeat(t *testing.T) {
+	dir := t.TempDir()
+	content := `{"format":"{a}-","repeat":3,"separator":"|","a":["x","y"]}`
+	if err := os.WriteFile(filepath.Join(dir, "rep.json"), []byte(content), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	code, _, errb := runOut("--format", "json", "--data-path", dir, "rep")
+	if code != 1 || !strings.Contains(errb, "carries repeat 3") {
+		t.Errorf("record on a repeating path = %d, %q; want exit 1 naming the repeat", code, errb)
+	}
+	code, _, errb = runOut("--format", "json", content)
+	if code != 2 || !strings.Contains(errb, "carries repeat 3") {
+		t.Errorf("record on a repeating inline template = %d, %q; want exit 2 naming the repeat", code, errb)
+	}
+	if code, out, _ := runOut("--seed", "1", "--data-path", dir, "rep"); code != 0 || strings.TrimSpace(out) != "x-|x-|x-" {
+		t.Errorf("text view = %d, %q; want the repeat still composed", code, out)
+	}
+}
+
+func TestRunNumbersFollowTheShell(t *testing.T) {
+	_, want, _ := runOut("--seed", "7", "--repeat", "3", "sv_SE.word")
+	code, got, errb := runOut("--seed", "007", "--repeat", "+3", "sv_SE.word")
+	if code != 0 || got != want {
+		t.Errorf("run(--seed 007 --repeat +3) = %d, %q, stderr %q; want the same as --seed 7 --repeat 3 %q", code, got, errb, want)
+	}
+}

compose.yaml

@@ -42,6 +42,10 @@ services:
     <<: *go
     command: go vet ./...
 
+  cyclo:
+    <<: *go
+    command: go run github.com/fzipp/gocyclo/cmd/gocyclo@v0.6.0 -over 14 -ignore _test .
+
   build:
     <<: *go
     command: go build ./...

data.go

@@ -10,11 +10,12 @@ import (
 )
 
 // dataSource is one tree to load: an fs.FS and the directory in it to start from.
-// label prefixes file names in errors; path, when set, is a directory on disk that
-// must exist.
+// label prefixes file names in errors; onDisk marks path as a directory that must
+// exist.
 type dataSource struct {
 	fsys   fs.FS
 	label  string
+	onDisk bool
 	path   string
 	root   string
 }
@@ -36,10 +37,13 @@ func (s dataSource) name(p string) string {
 func loadData(sources []dataSource) (map[string]node, error) {
 	root := map[string]node{}
 	for _, src := range sources {
-		if src.path != "" {
+		if src.onDisk {
+			if src.path == "" {
+				return nil, fmt.Errorf("a data path is empty")
+			}
 			info, err := os.Stat(src.path)
 			if err != nil {
-				return nil, fmt.Errorf("%s: no such directory", src.path)
+				return nil, err
 			}
 			if !info.IsDir() {
 				return nil, fmt.Errorf("%s is not a directory", src.path)
@@ -64,10 +68,7 @@ func loadData(sources []dataSource) (map[string]node, error) {
 	if err := checkNoCycles(root); err != nil {
 		return nil, err
 	}
-	if err := checkRepeatReach(root); err != nil {
-		return nil, err
-	}
-	if err := checkBoundLevelsHeld(root); err != nil {
+	if err := checkScope(treeScope(root)); err != nil {
 		return nil, err
 	}
 	return root, nil
@@ -87,42 +88,57 @@ func loadDir(src dataSource, dir string) (*group, error) {
 			continue
 		}
 		full := path.Join(dir, e.Name())
+		load := loadFile
 		if e.IsDir() {
-			child, err := loadDir(src, full)
-			if err != nil {
+			load = loadFolder
+		}
+		if err := load(src, g, full, e.Name()); err != nil {
 			return nil, err
 		}
+	}
+	return g, nil
+}
+
+// loadFolder adds a subdirectory as a nested group, unless nothing under it is data.
+func loadFolder(src dataSource, g *group, full, name string) error {
+	child, err := loadDir(src, full)
+	if err != nil {
+		return err
+	}
 	if len(child.children) == 0 {
-				continue
+		return nil
 	}
-			if err := checkName(e.Name()); err != nil {
-				return nil, fmt.Errorf("%s: folder %w", src.name(full), err)
-			}
-			g.children[e.Name()] = child
-			continue
-		}
-		if !strings.HasSuffix(e.Name(), ".json") {
-			continue
-		}
-		name := strings.TrimSuffix(e.Name(), ".json")
 	if err := checkName(name); err != nil {
-			return nil, fmt.Errorf("%s: category %w", src.name(full), err)
+		return fmt.Errorf("%s: folder %w", src.name(full), err)
+	}
+	g.children[name] = child
+	return nil
+}
+
+// loadFile compiles a *.json file into a category named after it; any other file
+// is skipped.
+func loadFile(src dataSource, g *group, full, file string) error {
+	if !strings.HasSuffix(file, ".json") {
+		return nil
+	}
+	name := strings.TrimSuffix(file, ".json")
+	if err := checkName(name); err != nil {
+		return fmt.Errorf("%s: category %w", src.name(full), err)
 	}
 	b, err := fs.ReadFile(src.fsys, full)
 	if err != nil {
-			return nil, fmt.Errorf("%s: %w", src.name(full), err)
+		return fmt.Errorf("%s: %w", src.name(full), err)
 	}
 	var raw any
 	if err := json.Unmarshal(b, &raw); err != nil {
-			return nil, fmt.Errorf("%s: %w", src.name(full), err)
+		return fmt.Errorf("%s: %w", src.name(full), err)
 	}
 	n, err := compile(raw)
 	if err != nil {
-			return nil, fmt.Errorf("%s: %w", src.name(full), err)
+		return fmt.Errorf("%s: %w", src.name(full), err)
 	}
 	g.children[name] = n
-	}
-	return g, nil
+	return nil
 }
 
 // mergeChildren overlays src onto dst. Two groups under the same key merge

data_test.go

@@ -264,3 +264,40 @@ func luhnValid(s string) bool {
 	}
 	return sum%10 == 0
 }
+
+// swedishName matches one or more letter-words, optionally space/hyphen joined
+// ("Storgatan", "Norra Promenaden", "von Flemming"). Used by the composition
+// tests so shipped name lists can grow without re-enumerating them here.
+var swedishName = regexp.MustCompile(`^\p{L}+([ -]\p{L}+)*$`)
+
+func TestShippedStreetComposition(t *testing.T) {
+	// street is a choice of composed {first}{last} templates and literal names.
+	f := newGenerator(t, "data/sv_SE", WithSeed(5))
+	for i := 0; i < 300; i++ {
+		if s := fake(t, f, "address.street"); !swedishName.MatchString(s) {
+			t.Fatalf("street %q is not a Swedish street name", s)
+		}
+	}
+}
+
+func TestShippedLastNameComposition(t *testing.T) {
+	// last is a choice of patronymic {first}sson templates, compound
+	// {first}{last} templates and literal surnames.
+	f := newGenerator(t, "data/sv_SE", WithSeed(6))
+	for i := 0; i < 300; i++ {
+		if s := fake(t, f, "person.last"); !swedishName.MatchString(s) {
+			t.Fatalf("last name %q is not a Swedish surname", s)
+		}
+	}
+}
+
+func TestShippedStreetNumberFormats(t *testing.T) {
+	// Reachable via a hyphenated path; covers all five weighted number variants.
+	f := newGenerator(t, "data/sv_SE", WithSeed(8))
+	re := regexp.MustCompile(`^[1-9]\d{0,2}[A-Z]?$`)
+	for i := 0; i < 300; i++ {
+		if n := fake(t, f, "address.street-number"); !re.MatchString(n) {
+			t.Fatalf("street-number %q does not match %s", n, re)
+		}
+	}
+}

edge_test.go

@@ -1,385 +0,0 @@
-package fejkdata
-
-import (
-	"encoding/json"
-	"fmt"
-	"os"
-	"path/filepath"
-	"regexp"
-	"slices"
-	"strings"
-	"testing"
-)
-
-// --- format string edge cases ---
-
-func TestHashIsLiteral(t *testing.T) {
-	f := engine(1)
-	cases := map[string]string{
-		`{"format":"","x":"v"}`:         "",
-		`{"format":"#","x":"v"}`:        "#",
-		`{"format":"##","x":"v"}`:       "##",
-		`{"format":"#0#1#A#a","x":"v"}`: "#0#1#A#a",
-		`{"format":"#{x}","x":"v"}`:     "#v",
-	}
-	for tmpl, want := range cases {
-		if got := mustRender(t, f, tmpl); got != want {
-			t.Errorf("render(%s) = %q, want %q", tmpl, got, want)
-		}
-	}
-}
-
-func TestMultibyteFormat(t *testing.T) {
-	// Scanning is rune-aware: multibyte literals coexist with class chars and
-	// tokens without corrupting indices.
-	got := mustRender(t, engine(2), `{"format":"Öster{x}-{digits(1)}å","x":"väg"}`)
-	if !regexp.MustCompile(`^Österväg-[0-9]å$`).MatchString(got) {
-		t.Fatalf("multibyte format = %q", got)
-	}
-}
-
-func TestAlternationThreeWay(t *testing.T) {
-	f := engine(4)
-	seen := map[string]bool{}
-	for i := 0; i < 200; i++ {
-		seen[mustRender(t, f, `{"format":"{a|b|c}","a":"A","b":"B","c":"C"}`)] = true
-	}
-	if !seen["A"] || !seen["B"] || !seen["C"] || len(seen) != 3 {
-		t.Fatalf("3-way alternation produced %v, want A, B and C", seen)
-	}
-}
-
-func TestNewErrors(t *testing.T) {
-	// Pointing New at a file (not a directory) fails.
-	file := filepath.Join(t.TempDir(), "xx_XX")
-	if err := os.WriteFile(file, []byte("{}"), 0o644); err != nil {
-		t.Fatal(err)
-	}
-	if _, err := New(WithoutShippedData(), WithDataPath(file)); err == nil {
-		t.Error("New(file) = nil error, want not-a-directory error")
-	}
-	// Invalid JSON in a category file fails.
-	if _, err := New(WithoutShippedData(), WithDataPath(writeData(t, map[string]string{"broken": `{ not json`}))); err == nil {
-		t.Error("New(invalid JSON) = nil error")
-	}
-	// An option that cannot take effect, and a category or folder no dot path can
-	// reach, are mistakes New must name rather than accept and ignore.
-	rejected := map[string]struct {
-		files map[string]string
-		want  string
-	}{
-		"separator without repeat": {
-			map[string]string{"a": `{"format":"{x}","x":"1","separator":","}`},
-			"has no effect without a repeat above 1",
-		},
-		"separator with an explicit repeat of 1": {
-			map[string]string{"a": `{"format":"{x}","x":"1","separator":","}`},
-			"has no effect without a repeat above 1",
-		},
-		"weight outside a choice": {
-			map[string]string{"a": `{"format":"x","weight":5}`},
-			"weight only skews a choice's items",
-		},
-		"non-numeric weight outside a choice": {
-			map[string]string{"a": `{"format":"x","weight":"bad"}`},
-			"weight only skews a choice's items",
-		},
-		"weight used as a field": {
-			map[string]string{"a": `{"format":"{name} {weight}kg","name":"Anvil","weight":["7"]}`},
-			"can never be a field",
-		},
-		"an option name used as a token": {
-			map[string]string{"a": `"{weight}"`},
-			`"weight" is an option and can never be a field`,
-		},
-		"category name with a dot": {
-			map[string]string{"a.b": `"1"`},
-			`category "a.b" contains "."`,
-		},
-		"folder name with a dot": {
-			map[string]string{"a.b/cat": `"1"`},
-			`/a.b: folder "a.b" contains "."`,
-		},
-		// The token grammar reserves three more characters. A name carrying one
-		// still resolves by dot path, but no format can name it, so it is rejected
-		// where it is authored rather than at the token that cannot reach it.
-		"field name with a pipe": {
-			map[string]string{"a": `{"format":"{x}","x":"1","b|c":"2"}`},
-			`field "b|c" contains "|"`,
-		},
-		"field name with a paren": {
-			map[string]string{"a": `{"format":"{x}","x":"1","b(c":"2"}`},
-			`field "b(c" contains "("`,
-		},
-		"field name with a closing brace": {
-			map[string]string{"a": `{"format":"{x}","x":"1","b}c":"2"}`},
-			`field "b}c" contains "}"`,
-		},
-		"category name with a pipe": {
-			map[string]string{"a|b": `"1"`},
-			`category "a|b" contains "|"`,
-		},
-		// An empty name is not a path segment, so List never offered it — while a
-		// bare {}, a trailing dot in Fake("a.") and a {/a.} reference all reached
-		// it. The engine accepted spellings it would never advertise.
-		"empty field name": {
-			map[string]string{"a": `{"format":"[{}]","":"VALUE"}`},
-			`field "" is empty`,
-		},
-		"folder name with a paren": {
-			map[string]string{"a(b/cat": `"1"`},
-			`folder "a(b" contains "("`,
-		},
-		// A repeated arm skews an alternation, which weight is the spelling for.
-		"repeated alternation arm": {
-			map[string]string{"a": `{"format":"{x|x}","x":"1"}`},
-			`arm "x" is repeated`,
-		},
-		"repeated arm among others": {
-			map[string]string{"a": `{"format":"{x|y|x}","x":"1","y":"2"}`},
-			`arm "x" is repeated`,
-		},
-		"repeated path arm": {
-			map[string]string{"a": `{"format":"{p.v|p.v}","p":{"format":"{v}","v":"1"}}`},
-			`arm "p.v" is repeated`,
-		},
-		// A reference arm is the only kind that reaches the repeat check by passing
-		// the per-arm checks rather than falling through them.
-		"repeated reference arm": {
-			map[string]string{"a": `"x"`, "b": `"{/a|/a}"`},
-			`arm "/a" is repeated`,
-		},
-		// An arm that is broken on its own terms is reported as that, not as a
-		// repeat: the repeat is a consequence of the real mistake.
-		"repeated arm with no path": {
-			map[string]string{"a": `"{/|/}"`},
-			"reference has no path",
-		},
-		// No field can be named "", so the token is told that rather than sent to
-		// name one — the fix "no field" points at is itself a load error.
-		"repeated empty arm": {
-			map[string]string{"a": `"{|}"`},
-			"a name is never empty",
-		},
-		"bare empty token": {
-			map[string]string{"a": `{"format":"[{}]","x":"1"}`},
-			"a name is never empty",
-		},
-	}
-	for name, c := range rejected {
-		_, err := New(WithoutShippedData(), WithDataPath(writeData(t, c.files)))
-		if err == nil {
-			t.Errorf("%s: New = nil error, want it rejected at load", name)
-			continue
-		}
-		if !strings.Contains(err.Error(), c.want) {
-			t.Errorf("%s: New = %v, want it to mention %q", name, err, c.want)
-		}
-	}
-	// Data that works today must keep working, and stay reachable.
-	accepted := map[string]struct {
-		files map[string]string
-		path  string
-		want  string
-	}{
-		"option name as a field":     {map[string]string{"a": `{"format":"{name} {Weight}kg","name":"Anvil","Weight":"7"}`}, "a", "Anvil 7kg"},
-		"format spelling as a field": {map[string]string{"a": `{"format":"{Format}","Format":"PDF"}`}, "a", "PDF"},
-		"hyphenated field":           {map[string]string{"a": `{"format":"{x-y}","x-y":"1"}`}, "a.x-y", "1"},
-		"category named Format":      {map[string]string{"Format": `"1"`}, "Format", "1"},
-		"folder named Repeat":        {map[string]string{"Repeat/cat": `"1"`}, "Repeat.cat", "1"},
-		"field with a closing paren": {map[string]string{"a": `{"format":"{b)c}","b)c":"2"}`}, "a", "2"},
-		"repeat without a separator": {map[string]string{"a": `{"format":"{x}","repeat":3,"x":"1"}`}, "a", "111"},
-		// One name in two separate tokens is two independent draws, not a repeated
-		// arm; only a repeat within one alternation is rejected.
-		"one name in two tokens": {map[string]string{"a": `{"format":"{x}{x}","x":"1"}`}, "a", "11"},
-	}
-	for name, c := range accepted {
-		f, err := New(WithoutShippedData(), WithDataPath(writeData(t, c.files)))
-		if err != nil {
-			t.Errorf("%s: New = %v, want it accepted", name, err)
-			continue
-		}
-		if got, err := f.Fake(c.path); err != nil || got != c.want {
-			t.Errorf("%s: Fake(%q) = %q, %v, want %q", name, c.path, got, err, c.want)
-		}
-	}
-}
-
-// --- deep path navigation ---
-
-func TestDeepDottedPath(t *testing.T) {
-	// A 5-segment path descends through alternating object/array nodes; choices
-	// on the path are single-variant, so it resolves deterministically.
-	f := engine(1)
-	f.categories = map[string]node{
-		"deep": compiled(t, `{"format":"{a}","a":{"format":"{b}","b":{"format":"{c}","c":{"format":"{d}","d":"leaf"}}}}`),
-	}
-	if got, err := f.Fake("deep.a.b.c.d"); err != nil || got != "leaf" {
-		t.Fatalf("Fake(deep.a.b.c.d) = %q, %v, want leaf", got, err)
-	}
-	// Rendering the whole tree resolves the same chain.
-	if got, err := f.Fake("deep"); err != nil || got != "leaf" {
-		t.Fatalf("Fake(deep) = %q, %v, want leaf", got, err)
-	}
-}
-
-func TestDescendIntoStringErrors(t *testing.T) {
-	f := engine(1)
-	f.categories = map[string]node{"greeting": compiled(t, `"hej"`)}
-	if _, err := f.Fake("greeting.extra"); err == nil || !strings.Contains(err.Error(), `no field "extra"`) {
-		t.Fatalf("Fake(greeting.extra) = %v, want a no-field error", err)
-	}
-}
-
-// TestPathThroughChoice pins the rule that keeps a dotted path from rendering on
-// one call and failing on the next: every variant must carry the rest of the path.
-func TestPathThroughChoice(t *testing.T) {
-	dir := writeData(t, map[string]string{
-		"every":  `[{"format":"{f}","f":"1"},{"format":"{f}","f":"2"}]`,
-		"notall": `[{"format":"{f}","f":"1"},"plain"]`,
-		"some":   `[{"format":"{f}","f":"1"},{"format":"{f}","f":"2","extra":"x"}]`,
-	})
-	f := newGenerator(t, dir, WithSeed(1))
-	for i := 0; i < 200; i++ {
-		if got := fake(t, f, "every.f"); got != "1" && got != "2" {
-			t.Fatalf("every.f = %q, want 1 or 2", got)
-		}
-	}
-	// A path only some variants carry is reported against what all of them carry.
-	if _, err := f.Fake("some.extra"); err == nil || !strings.Contains(err.Error(), "all carry [f]") {
-		t.Errorf("Fake(some.extra) = %v, want it to name what every variant carries", err)
-	}
-	var first string
-	for i := 0; i < 200; i++ {
-		_, err := f.Fake("notall.f")
-		if err == nil {
-			t.Fatal("notall.f = nil error, want the same failure every call")
-		}
-		if i == 0 {
-			first = err.Error()
-		} else if err.Error() != first {
-			t.Fatalf("notall.f error varies between calls:\n  %s\n  %s", first, err.Error())
-		}
-	}
-}
-
-// TestPathKeyIsUnambiguous pins that the one shape which could collide cannot be
-// written: a field literally named "a.b" and a field "a" holding "b" would both
-// spell "a.b", so a dotted field name is rejected at New and a dot means a path
-// wherever it appears.
-func TestPathKeyIsUnambiguous(t *testing.T) {
-	dir := writeData(t, map[string]string{
-		"cat": `[{"format":"{a.b}","a.b":"1"},{"format":"{a}","a":{"format":"{b}","b":"2"}}]`,
-	})
-	_, err := New(WithoutShippedData(), WithDataPath(dir))
-	if err == nil || !strings.Contains(err.Error(), `field "a.b" contains "."`) {
-		t.Fatalf("New = %v, want the dotted field name rejected", err)
-	}
-	// The same data without the dotted key is fine, and the path resolves.
-	f := newGenerator(t, writeData(t, map[string]string{
-		"cat": `{"format":"{a.b}","a":{"format":"{b}","b":"2"}}`,
-	}), WithSeed(1))
-	if !slices.Contains(f.List(), "cat.a.b") {
-		t.Error("List() omits cat.a.b, which the data carries")
-	}
-	if got := fake(t, f, "cat"); got != "2" {
-		t.Fatalf("cat = %q, want 2", got)
-	}
-}
-
-// TestMissingFieldNamesItself keeps the precise diagnosis for the ordinary typo: a
-// single-variant choice always picks the same item, so it needs no every-variant
-// guard and the error can name the field that is missing.
-func TestMissingFieldNamesItself(t *testing.T) {
-	f := newGenerator(t, "data/sv_SE", WithSeed(1))
-	_, err := f.Fake("person.typo")
-	if err == nil || !strings.Contains(err.Error(), `no field "typo"`) {
-		t.Errorf("Fake(person.typo) = %v, want it to name the missing field", err)
-	}
-}
-
-// --- category root shapes ---
-
-func TestCategoryRootShapes(t *testing.T) {
-	dir := writeData(t, map[string]string{
-		"obj": `"{digits(2)}"`, // object root
-		"lit": `"hello"`,       // bare-string root
-	})
-	f := newGenerator(t, dir, WithSeed(1))
-	if got := fake(t, f, "obj"); !regexp.MustCompile(`^\d\d$`).MatchString(got) {
-		t.Errorf("object-root category = %q, want two digits", got)
-	}
-	if got := fake(t, f, "lit"); got != "hello" {
-		t.Errorf("string-root category = %q, want hello", got)
-	}
-}
-
-// --- very long lists ---
-
-func TestLongStringList(t *testing.T) {
-	const n = 2000
-	names := make([]string, n)
-	for i := range names {
-		names[i] = fmt.Sprintf("name-%04d", i)
-	}
-	list, err := json.Marshal(names)
-	if err != nil {
-		t.Fatal(err)
-	}
-	f := newGenerator(t, writeData(t, map[string]string{"name": string(list)}), WithSeed(1))
-
-	valid := map[string]bool{}
-	for _, v := range names {
-		valid[v] = true
-	}
-	seen := map[string]bool{}
-	for i := 0; i < 20000; i++ {
-		v := fake(t, f, "name")
-		if !valid[v] {
-			t.Fatalf("got %q, not in the list", v)
-		}
-		seen[v] = true
-	}
-	if len(seen) < n*8/10 {
-		t.Fatalf("only %d/%d distinct values seen; selection looks skewed", len(seen), n)
-	}
-}
-
-// --- composition against the shipped sv_SE data ---
-
-// swedishName matches one or more letter-words, optionally space/hyphen joined
-// ("Storgatan", "Norra Promenaden", "von Flemming"). Used by the composition
-// tests so shipped name lists can grow without re-enumerating them here.
-var swedishName = regexp.MustCompile(`^\p{L}+([ -]\p{L}+)*$`)
-
-func TestShippedStreetComposition(t *testing.T) {
-	// street is a choice of composed {first}{last} templates and literal names.
-	f := newGenerator(t, "data/sv_SE", WithSeed(5))
-	for i := 0; i < 300; i++ {
-		if s := fake(t, f, "address.street"); !swedishName.MatchString(s) {
-			t.Fatalf("street %q is not a Swedish street name", s)
-		}
-	}
-}
-
-func TestShippedLastNameComposition(t *testing.T) {
-	// last is a choice of patronymic {first}sson templates, compound
-	// {first}{last} templates and literal surnames.
-	f := newGenerator(t, "data/sv_SE", WithSeed(6))
-	for i := 0; i < 300; i++ {
-		if s := fake(t, f, "person.last"); !swedishName.MatchString(s) {
-			t.Fatalf("last name %q is not a Swedish surname", s)
-		}
-	}
-}
-
-func TestShippedStreetNumberFormats(t *testing.T) {
-	// Reachable via a hyphenated path; covers all five weighted number variants.
-	f := newGenerator(t, "data/sv_SE", WithSeed(8))
-	re := regexp.MustCompile(`^[1-9]\d{0,2}[A-Z]?$`)
-	for i := 0; i < 300; i++ {
-		if n := fake(t, f, "address.street-number"); !re.MatchString(n) {
-			t.Fatalf("street-number %q does not match %s", n, re)
-		}
-	}
-}

fejkdata.go

@@ -28,16 +28,24 @@ import (
 //go:embed data
 var shippedFS embed.FS
 
+// MaxRepeat caps a repeat, and the renders nested repeats multiply to along any
+// path; the CLI's --repeat shares it.
+const MaxRepeat = 1 << 20
+
+var _ [^uint(0)>>63 - 1]struct{} // 64-bit only, per the README's Decisions
+
 // ErrNoData is returned by New when no source is loaded at all.
 var ErrNoData = errors.New("no data: WithoutShippedData needs at least one WithDataPath or WithDataFS")
 
 // Generator generates fake data from a loaded namespace tree. Create one with [New].
 // It is safe for concurrent use; a seeded sequence is reproducible only when drawn
-// from one goroutine.
+// from one goroutine. The compiled tree is immutable after [New], and Fake,
+// NewTemplate and List read it concurrently without a lock.
 type Generator struct {
 	mu         sync.Mutex
 	rand       *session
 	categories map[string]node
+	records    map[node]recordShape
 }
 
 // session is one generator's mutable render state: the seeded rng plus the {seq()}
@@ -72,7 +80,7 @@ func WithSeed(seed uint64) Option {
 // layer several; the last wins a name clash.
 func WithDataPath(dir string) Option {
 	return func(c *config) {
-		c.sources = append(c.sources, dataSource{fsys: os.DirFS(dir), label: dir, path: dir})
+		c.sources = append(c.sources, dataSource{fsys: os.DirFS(dir), label: dir, onDisk: true, path: dir})
 	}
 }
 
@@ -109,7 +117,11 @@ func New(opts ...Option) (*Generator, error) {
 	if err != nil {
 		return nil, fmt.Errorf("fejkdata: %w", err)
 	}
-	return &Generator{rand: newRand(c.seed, c.seeded), categories: cats}, nil
+	rng, err := newRand(c.seed, c.seeded)
+	if err != nil {
+		return nil, fmt.Errorf("fejkdata: %w", err)
+	}
+	return &Generator{rand: rng, categories: cats}, nil
 }
 
 // List returns the sorted dotted paths Fake can render: every category, the dotted
@@ -200,12 +212,19 @@ func join(prefix, name string) string {
 	return prefix + "." + name
 }
 
-func newRand(seed uint64, seeded bool) *session {
-	r := rand.New(rand.NewPCG(seed, seed^0x9e3779b97f4a7c15))
-	if !seeded {
+// randomBytes seeds an unseeded generator.
+var randomBytes = crand.Read
+
+func newRand(seed uint64, seeded bool) (*session, error) {
+	var r *rand.Rand
+	if seeded {
+		r = rand.New(rand.NewPCG(seed, seed^0x9e3779b97f4a7c15))
+	} else {
 		var b [16]byte
-		_, _ = crand.Read(b[:])
+		if _, err := randomBytes(b[:]); err != nil {
+			return nil, fmt.Errorf("seeding from crypto/rand: %w", err)
+		}
 		r = rand.New(rand.NewPCG(binary.LittleEndian.Uint64(b[:8]), binary.LittleEndian.Uint64(b[8:])))
 	}
-	return &session{Rand: r, counters: map[string]uint64{}}
+	return &session{Rand: r, counters: map[string]uint64{}}, nil
 }

fejkdata_test.go

@@ -1,8 +1,11 @@
 package fejkdata
 
 import (
+	"errors"
+	"io/fs"
 	"strings"
 	"testing"
+	"testing/fstest"
 )
 
 // newGenerator creates a generator over a single data directory, failing on
@@ -38,8 +41,25 @@ func fake(t *testing.T, f *Generator, path string) string {
 
 func TestNewMissingDirectory(t *testing.T) {
 	_, err := New(WithoutShippedData(), WithDataPath("data/de_DE"))
-	if err == nil || !strings.Contains(err.Error(), "de_DE") {
-		t.Fatalf("New(missing) error = %v, want it to name the path", err)
+	if err == nil || !strings.Contains(err.Error(), "de_DE") || !errors.Is(err, fs.ErrNotExist) {
+		t.Fatalf("New(missing) error = %v, want the real error, naming the path", err)
+	}
+}
+
+func TestNewRejectsAnEmptyDataPath(t *testing.T) {
+	_, err := New(WithoutShippedData(), WithDataPath(""))
+	if err == nil || !strings.Contains(err.Error(), "empty") {
+		t.Fatalf("New(WithDataPath(\"\")) = %v, want the empty path named", err)
+	}
+}
+
+func TestNewReportsAnEntropyFailure(t *testing.T) {
+	saved := randomBytes
+	randomBytes = func([]byte) (int, error) { return 0, errors.New("no entropy") }
+	defer func() { randomBytes = saved }()
+	_, err := New(WithoutShippedData(), WithDataFS(fstest.MapFS{"w.json": {Data: []byte(`"x"`)}}))
+	if err == nil || !strings.Contains(err.Error(), "no entropy") {
+		t.Fatalf("New() without entropy = %v, want the failure reported", err)
 	}
 }
 

graph.go

@@ -0,0 +1,251 @@
+package fejkdata
+
+import (
+	"fmt"
+	"sort"
+	"strings"
+)
+
+// walkNodes calls fn once per contained node, passing the dot path that reaches it,
+// visiting keys in sorted order so which of several broken nodes gets reported does
+// not depend on map iteration.
+func walkNodes(root map[string]node, fn func(path string, n node) error) error {
+	for _, name := range sortedNames(root) {
+		if err := eachNode(root[name], name, fn); err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+// eachNode visits n and every node contained within it once, passing the dot path
+// that reaches each. It never crosses a reference edge — a bound {/path} field is
+// skipped — so a single inline node is walked on its own.
+func eachNode(n node, path string, fn func(path string, n node) error) error {
+	seen := map[node]bool{}
+	var visit func(string, node) error
+	visit = func(path string, m node) error {
+		if m == nil || seen[m] {
+			return nil
+		}
+		seen[m] = true
+		if err := fn(path, m); err != nil {
+			return err
+		}
+		for _, c := range contained(m) {
+			if err := visit(join(path, c.name), c.node); err != nil {
+				return err
+			}
+		}
+		return nil
+	}
+	return visit(path, n)
+}
+
+// namedNode is a contained child and the segment reaching it; a choice's items carry
+// no segment, matching how a dot path steps over a choice.
+type namedNode struct {
+	name string
+	node node
+}
+
+func contained(n node) []namedNode {
+	switch n := n.(type) {
+	case *group:
+		return named(n.children)
+	case *choice:
+		out := make([]namedNode, len(n.items))
+		for i, it := range n.items {
+			out[i] = namedNode{node: it}
+		}
+		return out
+	case *template:
+		return named(n.fields)
+	default:
+		return nil
+	}
+}
+
+// named skips a bound {/path} key: it is a render edge, not containment, so using
+// it as a path segment would report a node under a path that does not reach it. Only
+// a template's fields hold bindings — loadDir skips a dot-prefixed entry, so a
+// group's children never carry the prefix — so this one skip serves both.
+func named(m map[string]node) []namedNode {
+	out := make([]namedNode, 0, len(m))
+	for _, name := range sortedNames(m) {
+		if isRef(name) {
+			continue
+		}
+		out = append(out, namedNode{name: name, node: m[name]})
+	}
+	return out
+}
+
+func sortedNames(m map[string]node) []string {
+	names := make([]string, 0, len(m))
+	for name := range m {
+		names = append(names, name)
+	}
+	sort.Strings(names)
+	return names
+}
+
+// renderEdge is a child a node renders into, labelled by what reaches it (a field
+// name, reference, or choice index) for a readable cycle report. operand names
+// the builtin when the label is its operand rather than a token, so an error can
+// name it the way the author wrote it.
+type renderEdge struct {
+	to      node
+	label   string
+	operand string
+}
+
+// reached names an edge as the author spelled it, the vocabulary boundReaders uses
+// for the sibling fence.
+func (e renderEdge) reached() string {
+	if e.operand != "" {
+		return fmt.Sprintf("%s operand %q", e.operand, e.label)
+	}
+	return "{" + e.label + "}"
+}
+
+// renderEdges lists the children rendering n recurses into, mirroring expand: a
+// choice's items, and a template's field/reference tokens plus its operands. A
+// group renders nothing, so it has no edges.
+func renderEdges(n node) []renderEdge {
+	switch n := n.(type) {
+	case *choice:
+		es := make([]renderEdge, len(n.items))
+		for i, it := range n.items {
+			es[i] = renderEdge{to: it, label: fmt.Sprintf("[%d]", i)}
+		}
+		return es
+	case *template:
+		var es []renderEdge
+		add := func(name, operand string) {
+			a := splitArm(name, n.refs)
+			c, ok := n.fields[a.key]
+			if !ok {
+				return
+			}
+			for _, leaf := range pathLeaves(c, a.tail) {
+				es = append(es, renderEdge{leaf, name, operand})
+			}
+		}
+		_ = eachToken(n.format, func(t ftoken) error {
+			if t.kind != 'b' {
+				return nil
+			}
+			if fn, _, isFunc := funcCall(t.body); isFunc {
+				for _, operand := range tokenOperands(t.body) {
+					add(operand, fn)
+				}
+				return nil
+			}
+			for _, name := range strings.Split(t.body, "|") {
+				add(name, "")
+			}
+			return nil
+		})
+		return es
+	default:
+		return nil
+	}
+}
+
+// pathLeaves lists what a token's dotted tail renders: a choice on the way
+// contributes every variant, since any of them may be the one drawn. checkPath has
+// already proved the tail resolves in every variant.
+func pathLeaves(n node, tail []string) []node {
+	var out []node
+	_ = walkPath(n, tail, pathWalk{
+		choice: func(c *choice, _ []string) ([]node, error) { return c.items, nil },
+		leaf:   func(n node) error { out = append(out, n); return nil },
+	})
+	return out
+}
+
+// nodeScope is the set of nodes one validation pass covers: a whole loaded tree,
+// or a single inline node.
+type nodeScope func(fn func(path string, n node) error) error
+
+func treeScope(root map[string]node) nodeScope {
+	return func(fn func(path string, n node) error) error { return walkNodes(root, fn) }
+}
+
+func inlineScope(n node) nodeScope {
+	return func(fn func(path string, m node) error) error { return eachNode(n, "template", fn) }
+}
+
+// checkScope runs the per-node fences over a scope, each over the whole scope
+// before the next, so which of several broken nodes is reported does not depend on
+// the walk. It runs after checkNoCycles, whose guarantee is what lets the walks
+// terminate.
+func checkScope(s nodeScope) error {
+	mem := reachMemo{}
+	if err := s(func(path string, n node) error { return repeatCheck(path, n, mem) }); err != nil {
+		return err
+	}
+	return s(heldCheck)
+}
+
+type reachMemo map[node]int
+
+func (m reachMemo) of(n node) int {
+	if r, done := m[n]; done {
+		return r
+	}
+	r := 1
+	for _, e := range renderEdges(n) {
+		if c := m.of(e.to); c > r {
+			r = c
+		}
+	}
+	if t, ok := n.(*template); ok {
+		r *= t.repeat
+	}
+	m[n] = r
+	return r
+}
+
+// repeatCheck bounds the renders a repeat multiplies to along any root-to-leaf
+// path, so nested repeats cannot build what one repeat may not.
+func repeatCheck(path string, n node, mem reachMemo) error {
+	if t, ok := n.(*template); ok && t.repeat > 1 && mem.of(n) > MaxRepeat {
+		return fmt.Errorf("%s: repeat %d multiplies to %d renders along one path, above the maximum %d", path, t.repeat, mem.of(n), MaxRepeat)
+	}
+	return nil
+}
+
+// checkNoCycles rejects a reference cycle: a node whose rendering can reach itself
+// — directly, mutually, or through a chain — never terminates, so it must fail at
+// New rather than stack-overflow at render. It is a depth-first walk of the render
+// graph (renderEdges); grey marks nodes on the current path so a back-edge to one
+// is the cycle, while black lets a shared node (a DAG, not a cycle) be skipped.
+// Every node is a root: a field its parent's format never renders is still reachable
+// by dot path, so a cycle in one would otherwise reach render and be fatal there.
+func checkNoCycles(root map[string]node) error {
+	const (
+		grey  = 1
+		black = 2
+	)
+	color := map[node]int{}
+	var visit func(n node, path string) error
+	visit = func(n node, path string) error {
+		switch color[n] {
+		case grey:
+			return fmt.Errorf("reference cycle: %s", path)
+		case black:
+			return nil
+		}
+		color[n] = grey
+		for _, e := range renderEdges(n) {
+			if err := visit(e.to, path+" -> "+e.label); err != nil {
+				return err
+			}
+		}
+		color[n] = black
+		return nil
+	}
+	return walkNodes(root, func(path string, n node) error { return visit(n, path) })
+}

hold.go

@@ -0,0 +1,316 @@
+package fejkdata
+
+import (
+	"fmt"
+	"sort"
+	"strings"
+)
+
+// heldCheck rejects every route to a held name except the ones that read its draw.
+// An expansion holds one draw of that name; anything else that renders it draws
+// again, and the two disagree. checkNoOverlap settles the spellings within one
+// format (a token, an operand); this settles the rest — a reference, whether it
+// sits in that format or in anything the format renders, however deep.
+func heldCheck(path string, n node) error {
+	t, ok := n.(*template)
+	if !ok || len(t.held) == 0 {
+		return nil
+	}
+	readers := boundReaders(t.format, t.bound, t.refs)
+	for _, head := range heldHeads(t) {
+		if err := checkHeadHeld(t, head, readers); err != nil {
+			return fmt.Errorf("%s: %w", path, err)
+		}
+	}
+	return nil
+}
+
+// heldHeads lists a template's held names, operand heads first, then paths, each
+// in name order: a level read both ways is reported by the operand's fence, and
+// which overlap is reported does not vary.
+func heldHeads(t *template) []string {
+	heads := make([]string, 0, len(t.held))
+	for head := range t.held {
+		heads = append(heads, head)
+	}
+	sort.Slice(heads, func(i, j int) bool {
+		_, pi := t.bound[heads[i]]
+		_, pj := t.bound[heads[j]]
+		if pi != pj {
+			return !pi
+		}
+		return heads[i] < heads[j]
+	})
+	return heads
+}
+
+// pinned collects what the hold of head answers for: a path pins the levels it
+// passes through and the leaf it lands on, an operand exactly the value its render
+// produces.
+func pinned(t *template, head string, readers []reader) map[node]bool {
+	held := map[node]bool{}
+	if _, isPath := t.bound[head]; !isPath {
+		operandDraw(t.fields[head], held)
+		return held
+	}
+	for _, r := range readers {
+		if a := splitArm(r.name, t.refs); a.key == head {
+			coverPath(t.fields[head], a.tail, held)
+		}
+	}
+	return held
+}
+
+// checkHeadHeld rejects every route to what head's hold pins except the readers
+// holding it. One seen set across the edges: a node that cannot reach the level
+// cannot reach it by another route either, so it is walked once.
+func checkHeadHeld(t *template, head string, readers []reader) error {
+	held := pinned(t, head, readers)
+	if len(held) == 0 {
+		return nil // a fixed head holds nothing to reach
+	}
+	reader, isPath := t.bound[head]
+	seen := map[node]bool{}
+	for _, e := range renderEdges(t) {
+		if splitArm(e.label, t.refs).key == head || !renders(e.to, held, seen) {
+			continue
+		}
+		if isPath {
+			return fmt.Errorf("%s renders %q, which {%s} reads a path into; name the fields you want instead", e.reached(), head, reader)
+		}
+		return fmt.Errorf("%s renders %q, which a {%s()} also reads; reach it one way so it is drawn once", e.reached(), head, operandReader(t, head))
+	}
+	return nil
+}
+
+// operandReader names the builtin whose operand holds head.
+func operandReader(t *template, head string) string {
+	fn := ""
+	_ = eachToken(t.format, func(tok ftoken) error {
+		if tok.kind != 'b' || fn != "" {
+			return nil
+		}
+		if name, _, isFunc := funcCall(tok.body); isFunc {
+			for _, operand := range tokenOperands(tok.body) {
+				if splitArm(operand, t.refs).key == head {
+					fn = name
+				}
+			}
+		}
+		return nil
+	})
+	return fn
+}
+
+// coverPath collects what holding one path pins: every choice level the path
+// passes through, whole, and the leaf it renders.
+func coverPath(n node, tail []string, into map[node]bool) {
+	_ = walkPath(n, tail, pathWalk{
+		choice: func(c *choice, _ []string) ([]node, error) { cover(c, into, false); return nil, nil },
+		leaf:   func(n node) error { cover(n, into, false); return nil },
+	})
+}
+
+// cover collects a level and everything contained in it. A fixed string outside a
+// choice is left out — it cannot disagree with itself — but inside one each
+// variant carries its own, so there it counts.
+func cover(n node, into map[node]bool, inChoice bool) {
+	if isFixed(n) && !inChoice {
+		return
+	}
+	into[n] = true
+	_, isChoice := n.(*choice)
+	for _, c := range contained(n) {
+		cover(c.node, into, inChoice || isChoice)
+	}
+}
+
+// operandDraw collects what one held draw of an operand answers for: the operand
+// and what rendering it settles inside itself. The builtin renders its operand
+// whole, so that draw fixes every value the render produced, and a second route to
+// any of them disagrees with it.
+//
+// The walk stops at a reference edge, which is where the operand's own value ends
+// and a shared source begins: two names referencing one category are two draws, the
+// same rule {word} {word} follows.
+func operandDraw(n node, into map[node]bool) {
+	if isFixed(n) {
+		return
+	}
+	if into[n] {
+		return
+	}
+	into[n] = true
+	for _, e := range renderEdges(n) {
+		if isRef(e.label) {
+			continue
+		}
+		operandDraw(e.to, into)
+	}
+}
+
+// isFixed is a string that varies nothing: fixed text with no fields to read into.
+func isFixed(n node) bool {
+	t, ok := n.(*template)
+	return ok && t.fixed && len(t.fields) == 0
+}
+
+// renders reports whether rendering n can reach anything in want, following the
+// same edges expand does. seen keeps a node shared by several routes from being
+// walked twice; checkNoCycles has already proved the graph is a DAG, so the walk
+// ends.
+func renders(n node, want, seen map[node]bool) bool {
+	if want[n] {
+		return true
+	}
+	if seen[n] {
+		return false
+	}
+	seen[n] = true
+	for _, e := range renderEdges(n) {
+		if renders(e.to, want, seen) {
+			return true
+		}
+	}
+	return false
+}
+
+// checkNoOverlap rejects a format that both renders a level and reads a path into
+// it — {p} beside {p.first}, {p.addr} beside {p.addr.city}, {.p} beside
+// {/sv_SE.p.first}. The path reads the level's held draw while rendering the level
+// expands it afresh, so their values would disagree. Reads are compared by their
+// one spelling, in sorted order, so which pair is reported depends neither on how
+// a reference was written nor on where the tokens sit.
+func checkNoOverlap(format string, bound map[string]string, refs map[string]refBinding) error {
+	names := boundReaders(format, bound, refs)
+	// Stable over one format-order scan, so two readers of one name (a token and a
+	// calc operand both naming "p") are reported as the format writes them.
+	sort.SliceStable(names, func(i, j int) bool { return names[i].path < names[j].path })
+	for i, level := range names {
+		for _, path := range names[i+1:] {
+			if strings.HasPrefix(path.path, level.path+".") {
+				return fmt.Errorf("%s renders a level that {%s} reads a path into; name the fields you want instead", level.label, path.name)
+			}
+		}
+	}
+	return nil
+}
+
+// reader is one way a format reaches a bound field: as written, by its one
+// spelling, and how to name it.
+type reader struct{ name, path, label string }
+
+// boundReaders lists every way a format reaches a bound field, in the order the
+// format writes them. An operand renders its field, so it names a level exactly
+// as a token does; one scan finds both, which is what puts them in one order.
+func boundReaders(format string, bound map[string]string, refs map[string]refBinding) []reader {
+	var names []reader
+	_ = eachToken(format, func(t ftoken) error {
+		if t.kind != 'b' {
+			return nil
+		}
+		if fn, _, isFunc := funcCall(t.body); isFunc {
+			for _, operand := range tokenOperands(t.body) {
+				a := splitArm(operand, refs)
+				if _, isBound := bound[a.key]; isBound {
+					names = append(names, reader{a.name, a.path, fmt.Sprintf("%s operand %q", fn, operand)})
+				}
+			}
+			return nil
+		}
+		for _, a := range splitArms(t.body, refs) {
+			if _, isBound := bound[a.key]; isBound {
+				names = append(names, reader{a.name, a.path, "token {" + a.name + "}"})
+			}
+		}
+		return nil
+	})
+	return names
+}
+
+// checkNoRepeatedRead rejects a bare token repeated on a held name: {w} {w} beside
+// {uppercase(w)} would read one draw twice, where {w} {w} alone draws twice. The
+// error names the single-token spelling.
+func checkNoRepeatedRead(format string, c formatOps, refs map[string]refBinding) error {
+	count := map[string]int{}
+	return eachToken(format, func(t ftoken) error {
+		if t.kind != 'b' {
+			return nil
+		}
+		if _, _, isFunc := funcCall(t.body); isFunc {
+			return nil
+		}
+		for _, a := range splitArms(t.body, refs) {
+			if len(a.tail) > 0 || !c.held[a.key] {
+				continue
+			}
+			if count[a.key]++; count[a.key] > 1 {
+				return fmt.Errorf("token {%s} is repeated, and %s holds %q to one draw per expansion; write {%s} once", a.name, c.holder[a.key], a.key, a.name)
+			}
+		}
+		return nil
+	})
+}
+
+// draws is what an expansion has already drawn for its held names: the variant each
+// was drawn as, so every path under it reads one row, and the value each read, by
+// its one spelling, so the same read written twice reads one value.
+type draws struct {
+	variant map[string]node
+	value   map[string]string
+}
+
+// readField renders one arm of a token. An arm's key is a sibling field or a
+// reference linkRefs bound into fields. A name the expansion holds — a level some
+// token addresses by dotted path, or a field an operand reads — is drawn once and
+// kept, so {place.postal-code} and {place.locality} read one row, either read twice
+// gives one value, and a shown operand is the operand computed. Every other name is
+// drawn afresh, so {word} {word} still draws twice. checkTokens, checkPath and
+// linkRefs prove every step, so the walk cannot fail.
+func readField(s *session, t *template, held, refScope *draws, a arm) string {
+	if !t.held[a.key] {
+		if len(a.tail) > 0 {
+			panic(fmt.Sprintf("fejkdata: %q reads a path into %q, which the expansion does not hold", a.name, a.key))
+		}
+		return render(s, t.fields[a.key], refScope)
+	}
+	// A reference that reads a path reads the caller's scope, so its draw outlives
+	// this expansion; a sibling, and a reference read whole, stay local to it.
+	d := held
+	if isRef(a.key) && refScope != nil && len(a.tail) > 0 {
+		d = refScope
+	}
+	if v, read := d.value[a.path]; read {
+		return v
+	}
+	var v string
+	_ = walkPath(t.fields[a.key], a.tail, pathWalk{
+		// Hold the draw at every level passed through, so two paths sharing a
+		// prefix share it.
+		choice: func(c *choice, rest []string) ([]node, error) {
+			key := a.key
+			if consumed := len(a.tail) - len(rest); consumed > 0 {
+				key = a.steps[consumed-1]
+			}
+			n, drew := d.variant[key]
+			if !drew {
+				n = drawn(s, c)
+				d.variant[key] = n
+			}
+			return []node{n}, nil
+		},
+		leaf: func(n node) error { v = render(s, n, refScope); return nil },
+	})
+	d.value[a.path] = v
+	return v
+}
+
+// drawn resolves a choice to one variant, so a bound head is a concrete node the
+// rest of the expansion shares. Nested choices unwrap too: a draw is one value, not
+// another set to pick from.
+func drawn(s *session, n node) node {
+	for c, ok := n.(*choice); ok; c, ok = n.(*choice) {
+		n = pick(s, c)
+	}
+	return n
+}

hold_test.go

@@ -711,3 +711,13 @@ func TestRepeatedBareTokenOfAHeldNameIsRejected(t *testing.T) {
 		}
 	}
 }
+
+func TestReadFieldPanicsOnAnUnheldPath(t *testing.T) {
+	tm, ok := compiled(t, `{"format":"{w}","w":{"format":"{x}","x":"1"}}`).(*template)
+	if !ok {
+		t.Fatal("not a template")
+	}
+	mustPanic(t, "unheld arm with a path", func() {
+		readField(engine(1).rand, tm, nil, nil, arm{name: "w.x", key: "w", tail: []string{"x"}, path: "w.x"})
+	})
+}

inline.go

@@ -0,0 +1,88 @@
+package fejkdata
+
+import (
+	"encoding/json"
+	"fmt"
+	"strings"
+)
+
+// Template is an inline template compiled, referenced and validated against a
+// generator's loaded data once, ready to render many times with [Template.Fake].
+// It is safe for concurrent use: Fake serializes on its generator's lock, so a
+// seeded sequence is reproducible only when a generator — and its templates — are
+// drawn from one goroutine.
+type Template struct {
+	g *Generator
+	n node
+}
+
+// Fake renders the template with one draw.
+func (t *Template) Fake() string {
+	t.g.mu.Lock()
+	defer t.g.mu.Unlock()
+	return render(t.g.rand, t.n, nil)
+}
+
+// NewTemplate compiles an inline template — a format string or a JSON value — and
+// binds its references against the loaded tree, so repeated renders pay the
+// compile and validation once. It shares [New]'s guarantees: a bad template errors
+// here, and rendering cannot fail.
+func (f *Generator) NewTemplate(input string) (*Template, error) {
+	n, err := compileInput(input)
+	if err != nil {
+		return nil, fmt.Errorf("fejkdata: %w", err)
+	}
+	scope := inlineScope(n)
+	if err := linkNodeRefs(scope, f.categories); err != nil {
+		return nil, fmt.Errorf("fejkdata: %w", err)
+	}
+	if err := checkScope(scope); err != nil {
+		return nil, fmt.Errorf("fejkdata: %w", err)
+	}
+	return &Template{g: f, n: n}, nil
+}
+
+// FakeTemplate compiles and renders an inline template in one call. It is
+// [NewTemplate] then [Template.Fake]; to render the same template many times, hold
+// the *Template and call its Fake.
+func (f *Generator) FakeTemplate(input string) (string, error) {
+	t, err := f.NewTemplate(input)
+	if err != nil {
+		return "", err
+	}
+	return t.Fake(), nil
+}
+
+// compileInput compiles an inline template: a JSON value, or a bare format string
+// when the input is not JSON.
+func compileInput(input string) (node, error) {
+	var raw any
+	if err := json.Unmarshal([]byte(input), &raw); err != nil {
+		return compile(input)
+	}
+	if trimmed := strings.TrimSpace(input); trimmed != input {
+		return nil, fmt.Errorf("a JSON template may not be padded with spaces, which a format string would render; write %s", trimmed)
+	}
+	return compile(raw)
+}
+
+// linkNodeRefs binds the references in an inline node's templates against the
+// loaded tree.
+func linkNodeRefs(scope nodeScope, root map[string]node) error {
+	return scope(func(path string, m node) error {
+		t, ok := m.(*template)
+		if !ok {
+			return nil
+		}
+		for _, name := range refTokens(t.format) {
+			sigil, rest, err := refShape(name)
+			if err != nil {
+				return fmt.Errorf("%s: reference {%s}: %w", path, name, err)
+			}
+			if sigil != "/" {
+				return fmt.Errorf("%s: reference {%s}: an inline template has no folder; write {/%s}", path, name, rest)
+			}
+		}
+		return linkTemplateRefs(nil, path, t, root)
+	})
+}

inline_test.go

@@ -0,0 +1,159 @@
+package fejkdata
+
+import (
+	"strings"
+	"testing"
+)
+
+func tmpl(t *testing.T, f *Generator, input string) string {
+	t.Helper()
+	s, err := f.FakeTemplate(input)
+	if err != nil {
+		t.Fatalf("FakeTemplate(%q): %v", input, err)
+	}
+	return s
+}
+
+func shipped(t *testing.T, opts ...Option) *Generator {
+	t.Helper()
+	f, err := New(append([]Option{WithSeed(1)}, opts...)...)
+	if err != nil {
+		t.Fatal(err)
+	}
+	return f
+}
+
+func TestFakeTemplateFormatString(t *testing.T) {
+	f := shipped(t)
+	got := tmpl(t, f, "name: {/sv_SE.person.last}")
+	if !strings.HasPrefix(got, "name: ") || strings.HasSuffix(got, " ") || strings.Contains(got, "{") {
+		t.Fatalf("FakeTemplate = %q, want a rendered last name after the prefix", got)
+	}
+}
+
+func TestFakeTemplateJSONObject(t *testing.T) {
+	f := shipped(t)
+	seen := map[string]bool{}
+	for i := 0; i < 50; i++ {
+		seen[tmpl(t, f, `{"format":"name: {x}","x":["bosse","lina"]}`)] = true
+	}
+	if !seen["name: bosse"] || !seen["name: lina"] || len(seen) != 2 {
+		t.Fatalf("JSON template produced %v, want both names", seen)
+	}
+}
+
+func TestFakeTemplateJSONArray(t *testing.T) {
+	f := shipped(t)
+	seen := map[string]bool{}
+	for i := 0; i < 50; i++ {
+		seen[tmpl(t, f, `["foo","bar","baz"]`)] = true
+	}
+	if len(seen) != 3 {
+		t.Fatalf("JSON array choice produced %v, want three items", seen)
+	}
+}
+
+func TestFakeTemplateCorrelatedReferences(t *testing.T) {
+	dir := writeData(t, map[string]string{
+		"person": `[{"format":"{first} {last}","first":"Ada","last":"Lovelace"},{"format":"{first} {last}","first":"Bo","last":"Ek"}]`,
+	})
+	f := newGenerator(t, dir, WithSeed(1))
+	for i := 0; i < 100; i++ {
+		got := tmpl(t, f, "{/person.first} {/person.last}")
+		if got != "Ada Lovelace" && got != "Bo Ek" {
+			t.Fatalf("correlated references = %q, want one person's first and last", got)
+		}
+	}
+}
+
+func TestFakeTemplateDeterministic(t *testing.T) {
+	a, b := shipped(t), shipped(t)
+	for i := 0; i < 20; i++ {
+		in := "row: {/misc.uuid} {digits(3)}"
+		if x, y := tmpl(t, a, in), tmpl(t, b, in); x != y {
+			t.Fatalf("same seed diverged: %q != %q", x, y)
+		}
+	}
+}
+
+func TestFieldlessTokenHint(t *testing.T) {
+	f := shipped(t)
+	_, err := f.FakeTemplate(`{sv_SE.person.last}`)
+	if err == nil || !strings.Contains(err.Error(), "write {/sv_SE.person.last}") {
+		t.Fatalf("FakeTemplate(bare token) = %v, want a hint naming {/sv_SE.person.last}", err)
+	}
+	_, err = f.FakeTemplate(`{"format":"{x}","repeat":2}`)
+	if err == nil || !strings.Contains(err.Error(), `this template has none — write {/x}`) {
+		t.Errorf("FakeTemplate(fieldless object) = %v, want the hint without calling it a bare string", err)
+	}
+	// A hint is only a drop-in where the name is the whole token: {/x} inside a
+	// transform or an alternation renders a different value, so none is offered.
+	for _, input := range []string{`{ /sv_SE.person.last }`, "{lowercase(x)}", "{x|y}"} {
+		_, err := f.FakeTemplate(input)
+		if err == nil || strings.Contains(err.Error(), "write {") {
+			t.Errorf("FakeTemplate(%q) = %v, want no hint naming a spelling that means something else", input, err)
+		}
+	}
+}
+
+func TestFakeTemplateErrors(t *testing.T) {
+	f := shipped(t)
+	for _, c := range []struct {
+		input string
+		want  string
+	}{
+		{`{"x":"Q"}`, "missing string \"format\""},
+		{`"{x}"`, `no field "x"`},
+		{`"{digits(0)}"`, "must be positive"},
+		{`name: {/no.such.path}`, "no entry"},
+		{`name: {..nope}`, "write {/nope}"},
+		{`{"format":"x"}`, "is a string"},
+		{`{/misc.country} {/misc.country.alpha2}`, "renders a level"},
+		{`{"format":"{/misc.country.alpha2} {x}","x":"{/misc.country}"}`, "reads a path into"},
+	} {
+		_, err := f.FakeTemplate(c.input)
+		if err == nil || !strings.Contains(err.Error(), c.want) {
+			t.Errorf("FakeTemplate(%q) = %v, want an error containing %q", c.input, err, c.want)
+		}
+	}
+}
+
+func TestFakeTemplateJSONString(t *testing.T) {
+	f := shipped(t)
+	got := tmpl(t, f, `"name: {/sv_SE.person.last}"`)
+	if !strings.HasPrefix(got, "name: ") || strings.Contains(got, "{") {
+		t.Fatalf("FakeTemplate(JSON string) = %q, want a rendered last name after the prefix", got)
+	}
+}
+
+func TestPaddedJSONIsRejected(t *testing.T) {
+	f := shipped(t)
+	in := `{"format":"{x}","x":["a","b"]}`
+	_, err := f.NewTemplate("  " + in + "  ")
+	if err == nil || !strings.Contains(err.Error(), "write "+in) {
+		t.Fatalf("NewTemplate(padded JSON) = %v, want an error naming the unpadded spelling", err)
+	}
+}
+
+func TestNewTemplateReusable(t *testing.T) {
+	f := shipped(t)
+	reusable, err := f.NewTemplate(`{digits(2)}`)
+	if err != nil {
+		t.Fatalf("NewTemplate: %v", err)
+	}
+	seen := map[string]bool{}
+	for i := 0; i < 50; i++ {
+		seen[reusable.Fake()] = true
+	}
+	if len(seen) < 2 {
+		t.Fatalf("Template.Fake() repeated %v, want varied draws from one compile", seen)
+	}
+}
+
+func TestFakeTemplateRepeatBound(t *testing.T) {
+	f := shipped(t)
+	_, err := f.FakeTemplate(`{"format":"{x}","repeat":200,"x":{"format":"{y}","repeat":200,"y":{"format":"z","repeat":200}}}`)
+	if err == nil || !strings.Contains(err.Error(), "maximum") {
+		t.Errorf("nested repeat over the cap = %v, want it rejected naming the maximum", err)
+	}
+}

loading_test.go

@@ -1,9 +1,12 @@
 package fejkdata
 
 import (
+	"encoding/json"
+	"fmt"
 	"os"
 	"path/filepath"
 	"reflect"
+	"regexp"
 	"slices"
 	"strings"
 	"testing"
@@ -201,3 +204,214 @@ func TestRepeatProductAlongAPathIsCapped(t *testing.T) {
 		t.Errorf("New = %v, want 1024 x 1024 along one path accepted", err)
 	}
 }
+
+func TestNewErrors(t *testing.T) {
+	// Pointing New at a file (not a directory) fails.
+	file := filepath.Join(t.TempDir(), "xx_XX")
+	if err := os.WriteFile(file, []byte("{}"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	if _, err := New(WithoutShippedData(), WithDataPath(file)); err == nil {
+		t.Error("New(file) = nil error, want not-a-directory error")
+	}
+	// Invalid JSON in a category file fails.
+	if _, err := New(WithoutShippedData(), WithDataPath(writeData(t, map[string]string{"broken": `{ not json`}))); err == nil {
+		t.Error("New(invalid JSON) = nil error")
+	}
+	// An option that cannot take effect, and a category or folder no dot path can
+	// reach, are mistakes New must name rather than accept and ignore.
+	rejected := map[string]struct {
+		files map[string]string
+		want  string
+	}{
+		"separator without repeat": {
+			map[string]string{"a": `{"format":"{x}","x":"1","separator":","}`},
+			"has no effect without a repeat above 1",
+		},
+		"separator with an explicit repeat of 1": {
+			map[string]string{"a": `{"format":"{x}","x":"1","separator":","}`},
+			"has no effect without a repeat above 1",
+		},
+		"weight outside a choice": {
+			map[string]string{"a": `{"format":"x","weight":5}`},
+			"weight only skews a choice's items",
+		},
+		"non-numeric weight outside a choice": {
+			map[string]string{"a": `{"format":"x","weight":"bad"}`},
+			"weight only skews a choice's items",
+		},
+		"weight used as a field": {
+			map[string]string{"a": `{"format":"{name} {weight}kg","name":"Anvil","weight":["7"]}`},
+			"can never be a field",
+		},
+		"an option name used as a token": {
+			map[string]string{"a": `"{weight}"`},
+			`"weight" is an option and can never be a field`,
+		},
+		"category name with a dot": {
+			map[string]string{"a.b": `"1"`},
+			`category "a.b" contains "."`,
+		},
+		"folder name with a dot": {
+			map[string]string{"a.b/cat": `"1"`},
+			`/a.b: folder "a.b" contains "."`,
+		},
+		// The token grammar reserves three more characters. A name carrying one
+		// still resolves by dot path, but no format can name it, so it is rejected
+		// where it is authored rather than at the token that cannot reach it.
+		"field name with a pipe": {
+			map[string]string{"a": `{"format":"{x}","x":"1","b|c":"2"}`},
+			`field "b|c" contains "|"`,
+		},
+		"field name with a paren": {
+			map[string]string{"a": `{"format":"{x}","x":"1","b(c":"2"}`},
+			`field "b(c" contains "("`,
+		},
+		"field name with a closing brace": {
+			map[string]string{"a": `{"format":"{x}","x":"1","b}c":"2"}`},
+			`field "b}c" contains "}"`,
+		},
+		"category name with a pipe": {
+			map[string]string{"a|b": `"1"`},
+			`category "a|b" contains "|"`,
+		},
+		"field name with a bracket": {
+			map[string]string{"a": `{"format":"{x}","x":"1","b[c":"2"}`},
+			`field "b[c" contains "["`,
+		},
+		"category name with a bracket": {
+			map[string]string{"[abc]": `"1"`},
+			`category "[abc]" contains "["`,
+		},
+		"field name with a quote": {
+			map[string]string{"a": `{"format":"{x}","x":"1","b\"c":"2"}`},
+			`field "b\"c" contains "\""`,
+		},
+		// An empty name is not a path segment, so List never offered it — while a
+		// bare {}, a trailing dot in Fake("a.") and a {/a.} reference all reached
+		// it. The engine accepted spellings it would never advertise.
+		"empty field name": {
+			map[string]string{"a": `{"format":"[{}]","":"VALUE"}`},
+			`field "" is empty`,
+		},
+		"folder name with a paren": {
+			map[string]string{"a(b/cat": `"1"`},
+			`folder "a(b" contains "("`,
+		},
+		// A repeated arm skews an alternation, which weight is the spelling for.
+		"repeated alternation arm": {
+			map[string]string{"a": `{"format":"{x|x}","x":"1"}`},
+			`arm "x" is repeated`,
+		},
+		"repeated arm among others": {
+			map[string]string{"a": `{"format":"{x|y|x}","x":"1","y":"2"}`},
+			`arm "x" is repeated`,
+		},
+		"repeated path arm": {
+			map[string]string{"a": `{"format":"{p.v|p.v}","p":{"format":"{v}","v":"1"}}`},
+			`arm "p.v" is repeated`,
+		},
+		// A reference arm is the only kind that reaches the repeat check by passing
+		// the per-arm checks rather than falling through them.
+		"repeated reference arm": {
+			map[string]string{"a": `"x"`, "b": `"{/a|/a}"`},
+			`arm "/a" is repeated`,
+		},
+		// An arm that is broken on its own terms is reported as that, not as a
+		// repeat: the repeat is a consequence of the real mistake.
+		"repeated arm with no path": {
+			map[string]string{"a": `"{/|/}"`},
+			"reference has no path",
+		},
+		// No field can be named "", so the token is told that rather than sent to
+		// name one — the fix "no field" points at is itself a load error.
+		"repeated empty arm": {
+			map[string]string{"a": `"{|}"`},
+			"a name is never empty",
+		},
+		"bare empty token": {
+			map[string]string{"a": `{"format":"[{}]","x":"1"}`},
+			"a name is never empty",
+		},
+	}
+	for name, c := range rejected {
+		_, err := New(WithoutShippedData(), WithDataPath(writeData(t, c.files)))
+		if err == nil {
+			t.Errorf("%s: New = nil error, want it rejected at load", name)
+			continue
+		}
+		if !strings.Contains(err.Error(), c.want) {
+			t.Errorf("%s: New = %v, want it to mention %q", name, err, c.want)
+		}
+	}
+	// Data that works today must keep working, and stay reachable.
+	accepted := map[string]struct {
+		files map[string]string
+		path  string
+		want  string
+	}{
+		"option name as a field":     {map[string]string{"a": `{"format":"{name} {Weight}kg","name":"Anvil","Weight":"7"}`}, "a", "Anvil 7kg"},
+		"format spelling as a field": {map[string]string{"a": `{"format":"{Format}","Format":"PDF"}`}, "a", "PDF"},
+		"hyphenated field":           {map[string]string{"a": `{"format":"{x-y}","x-y":"1"}`}, "a.x-y", "1"},
+		"category named Format":      {map[string]string{"Format": `"1"`}, "Format", "1"},
+		"folder named Repeat":        {map[string]string{"Repeat/cat": `"1"`}, "Repeat.cat", "1"},
+		"field with a closing paren": {map[string]string{"a": `{"format":"{b)c}","b)c":"2"}`}, "a", "2"},
+		"repeat without a separator": {map[string]string{"a": `{"format":"{x}","repeat":3,"x":"1"}`}, "a", "111"},
+		// One name in two separate tokens is two independent draws, not a repeated
+		// arm; only a repeat within one alternation is rejected.
+		"one name in two tokens": {map[string]string{"a": `{"format":"{x}{x}","x":"1"}`}, "a", "11"},
+	}
+	for name, c := range accepted {
+		f, err := New(WithoutShippedData(), WithDataPath(writeData(t, c.files)))
+		if err != nil {
+			t.Errorf("%s: New = %v, want it accepted", name, err)
+			continue
+		}
+		if got, err := f.Fake(c.path); err != nil || got != c.want {
+			t.Errorf("%s: Fake(%q) = %q, %v, want %q", name, c.path, got, err, c.want)
+		}
+	}
+}
+
+func TestCategoryRootShapes(t *testing.T) {
+	dir := writeData(t, map[string]string{
+		"obj": `"{digits(2)}"`, // object root
+		"lit": `"hello"`,       // bare-string root
+	})
+	f := newGenerator(t, dir, WithSeed(1))
+	if got := fake(t, f, "obj"); !regexp.MustCompile(`^\d\d$`).MatchString(got) {
+		t.Errorf("object-root category = %q, want two digits", got)
+	}
+	if got := fake(t, f, "lit"); got != "hello" {
+		t.Errorf("string-root category = %q, want hello", got)
+	}
+}
+
+func TestLongStringList(t *testing.T) {
+	const n = 2000
+	names := make([]string, n)
+	for i := range names {
+		names[i] = fmt.Sprintf("name-%04d", i)
+	}
+	list, err := json.Marshal(names)
+	if err != nil {
+		t.Fatal(err)
+	}
+	f := newGenerator(t, writeData(t, map[string]string{"name": string(list)}), WithSeed(1))
+
+	valid := map[string]bool{}
+	for _, v := range names {
+		valid[v] = true
+	}
+	seen := map[string]bool{}
+	for i := 0; i < 20000; i++ {
+		v := fake(t, f, "name")
+		if !valid[v] {
+			t.Fatalf("got %q, not in the list", v)
+		}
+		seen[v] = true
+	}
+	if len(seen) < n*8/10 {
+		t.Fatalf("only %d/%d distinct values seen; selection looks skewed", len(seen), n)
+	}
+}

node.go

@@ -57,6 +57,15 @@ type template struct {
 
 func (*template) isNode() {}
 
+// field is the node a path segment names; a binding is a render edge, not a field.
+func (t *template) field(seg string) (node, bool) {
+	if isRef(seg) {
+		return nil, false
+	}
+	n, ok := t.fields[seg]
+	return n, ok
+}
+
 // compile converts parsed JSON into a node tree, validating structure up front.
 // Only a choice's items carry a weight, so one here would be inert whatever its type.
 func compile(v any) (node, error) {
@@ -78,10 +87,24 @@ func compileItem(v any) (node, error) {
 	case map[string]any:
 		return compileTemplate(v)
 	default:
-		return nil, fmt.Errorf("unsupported node type %T", v)
+		return nil, fmt.Errorf("a template value must be a string, a list or an object, not %s", jsonKind(v))
 	}
 }
 
+// jsonKind names a JSON value a template cannot hold, in the data format's own
+// terms rather than the decoding library's.
+func jsonKind(v any) string {
+	switch v.(type) {
+	case float64:
+		return "a number"
+	case bool:
+		return "a boolean"
+	case nil:
+		return "null"
+	}
+	return fmt.Sprintf("%T", v)
+}
+
 func compileString(s string) (node, error) {
 	if err := checkTokens(s, nil); err != nil {
 		return nil, err
@@ -149,8 +172,8 @@ func compileChoice(items []any) (node, error) {
 		}
 		c.cum = cum
 	}
-	// Safe to precompute: a choice's items come from one file, so no group can
-	// appear inside one, and neither mergeChildren nor linkRefs can reach in.
+	// Computed before linkRefs binds references into the items: a binding is keyed
+	// by a reference sigil, which paths skips, so the set is the same after.
 	c.shared = sharedPaths(c.items)
 	return c, nil
 }
@@ -181,29 +204,71 @@ func checkNoRepeatedItem(items []any) error {
 }
 
 func compileTemplate(m map[string]any) (node, error) {
-	format, ok := m["format"].(string)
-	if !ok {
-		return nil, fmt.Errorf("template object missing string \"format\"")
-	}
-	repeat, err := repeatOf(m)
+	o, err := readOptions(m)
 	if err != nil {
 		return nil, err
 	}
-	sep := ""
+	fields, err := compileFields(m)
+	if err != nil {
+		return nil, err
+	}
+	if len(fields) == 0 && o.repeat == 1 && !o.weighted {
+		return nil, fmt.Errorf("an object holding only a format is a string; write %q", o.format)
+	}
+	if err := checkTokens(o.format, fields); err != nil {
+		return nil, err
+	}
+	t := &template{format: o.format, fields: fields, repeat: o.repeat, separator: o.separator}
+	if err := t.compileFormat(); err != nil {
+		return nil, err
+	}
+	return t, nil
+}
+
+// templateOptions is what a template object's option keys say.
+type templateOptions struct {
+	format    string
+	repeat    int
+	separator string
+	weighted  bool
+}
+
+func readOptions(m map[string]any) (templateOptions, error) {
+	var o templateOptions
+	format, ok := m["format"].(string)
+	if !ok {
+		return o, fmt.Errorf("template object missing string \"format\"")
+	}
+	o.format = format
+	repeat, err := repeatOf(m)
+	if err != nil {
+		return o, err
+	}
+	o.repeat = repeat
 	if sv, ok := m["separator"]; ok {
-		if sep, ok = sv.(string); !ok {
-			return nil, fmt.Errorf("separator must be a string, got %T", sv)
+		if o.separator, ok = sv.(string); !ok {
+			return o, fmt.Errorf("separator must be a string, got %T", sv)
 		}
 		if repeat == 1 {
-			return nil, fmt.Errorf("separator joins repeated renders, so it has no effect without a repeat above 1")
+			return o, fmt.Errorf("separator joins repeated renders, so it has no effect without a repeat above 1")
+		}
+		if o.separator == "" {
+			return o, fmt.Errorf("separator \"\" is the default, so it has no effect; drop it")
 		}
 	}
-	t := &template{format: format, fields: make(map[string]node, len(m)), repeat: repeat, separator: sep}
+	_, o.weighted = m["weight"]
+	return o, nil
+}
+
+// compileFields compiles every non-option key of a template object, in name order
+// so which of several bad fields is reported does not vary.
+func compileFields(m map[string]any) (map[string]node, error) {
+	fields := make(map[string]node, len(m))
 	keys := make([]string, 0, len(m))
 	for k := range m {
 		keys = append(keys, k)
 	}
-	sort.Strings(keys) // so which of several bad fields is reported does not vary
+	sort.Strings(keys)
 	for _, k := range keys {
 		if isOption(k) {
 			continue
@@ -215,55 +280,9 @@ func compileTemplate(m map[string]any) (node, error) {
 		if err != nil {
 			return nil, fmt.Errorf("field %q: %w", k, err)
 		}
-		t.fields[k] = n
-	}
-	if _, weighted := m["weight"]; len(t.fields) == 0 && repeat == 1 && !weighted {
-		return nil, fmt.Errorf("an object holding only a format is a string; write %q", format)
-	}
-	if err := checkTokens(format, t.fields); err != nil {
-		return nil, err
-	}
-	if err := t.compileFormat(); err != nil {
-		return nil, err
-	}
-	return t, nil
-}
-
-// checkPath reports whether a token's dotted tail can address a node whichever way
-// the draw goes, by the reachability rule descend applies — a choice must carry
-// the whole remaining path in the set every variant shares — plus the
-// rules a held draw adds, which descend has no need of: a level a path reads may
-// not carry a repeat, and each variant answers for that itself. So a path that
-// validates here resolves on every render, and a typo is a New-time error.
-func checkPath(n node, tail []string, level string) error {
-	if len(tail) == 0 {
-		return nil
-	}
-	switch n := n.(type) {
-	case *template:
-		if n.repeat > 1 {
-			return fmt.Errorf("the level %q carries a repeat, which a path reading one draw of it cannot apply", level)
-		}
-		child, ok := n.fields[tail[0]]
-		if !ok {
-			return fmt.Errorf("no field %q", tail[0])
-		}
-		return checkPath(child, tail[1:], level+"."+tail[0])
-	case *choice:
-		if want := strings.Join(tail, "."); !n.shared[want] {
-			return unreachableInChoice(n, want)
-		}
-		// Reachability is settled; each variant still answers for itself, so a
-		// rule about the level (its repeat) holds behind a choice as in front.
-		for _, item := range n.items {
-			if err := checkPath(item, tail, level); err != nil {
-				return err
-			}
-		}
-		return nil
-	default:
-		return fmt.Errorf("cannot descend into %T at %q", n, tail[0])
+		fields[k] = n
 	}
+	return fields, nil
 }
 
 // repeatOf reads a template's "repeat" (default 1): how many times its format
@@ -283,8 +302,8 @@ func repeatOf(m map[string]any) (int, error) {
 	if r == 1 {
 		return 0, fmt.Errorf("repeat 1 is the default, so it has no effect; drop it")
 	}
-	if r > maxLen { // cap so a fat-fingered repeat can't build a multi-GB string
-		return 0, fmt.Errorf("repeat %v exceeds the maximum %d", rv, maxLen)
+	if r > MaxRepeat { // caps the renders one repeat asks for; repeatCheck bounds what nested ones multiply to
+		return 0, fmt.Errorf("repeat %v exceeds the maximum %d", rv, MaxRepeat)
 	}
 	return int(r), nil
 }
@@ -318,23 +337,23 @@ func weightOf(raw any) (float64, error) {
 
 // reservedInName is what a category, folder or field name may not contain: a dot
 // separates the segments of a path, '|' the arms of a token, '(' opens a function
-// call, braces delimit the token and '/' starts a reference. A name carrying one is
-// reachable by no format, so it is rejected where it is authored rather than at
-// the token that cannot reach it.
-const reservedInName = ".|({}/"
+// call, braces delimit the token, '/' starts a reference, and brackets and a quote
+// open a JSON value. A name carrying one is rejected where it is authored rather
+// than where it would be unreachable.
+const reservedInName = ".|({}/[]\""
 
 // reservedList spells reservedInName for an error message, so the two cannot drift.
 var reservedList = strings.Join(strings.Split(reservedInName, ""), " ")
 
-// checkName rejects a name the dot path and {token} grammars cannot spell. Both a
-// category or folder and a field go through it, so there is one answer to what a
-// name may contain.
+// checkName rejects a name the dot path, {token} and JSON grammars cannot spell.
+// Both a category or folder and a field go through it, so there is one answer to
+// what a name may contain.
 func checkName(name string) error {
 	if name == "" {
 		return fmt.Errorf("%q is empty, which is not a path segment, so List never offers it", name)
 	}
 	if i := strings.IndexAny(name, reservedInName); i >= 0 {
-		return fmt.Errorf("%q contains %q; a name may not use %s, which the dot path and {token} grammars reserve",
+		return fmt.Errorf("%q contains %q; a name may not use %s, which the dot path, {token} and JSON grammars reserve",
 			name, name[i:i+1], reservedList)
 	}
 	return nil

one_spelling_test.go

@@ -35,6 +35,7 @@ func TestInertObjectIsRejected(t *testing.T) {
 		`[{"format":"a","weight":1},"b"]`:                    "weight 1",
 		`{"format":"{x}","x":"v","repeat":1}`:                "repeat 1",
 		`{"format":"{x}","x":"v","separator":","}`:           "separator",
+		`{"format":"{x}","x":"v","repeat":2,"separator":""}`: "default",
 	} {
 		if _, err := compile(parse(t, src)); err == nil || !strings.Contains(err.Error(), want) {
 			t.Errorf("compile(%s) = %v, want an error mentioning %s", src, err, want)
@@ -46,3 +47,13 @@ func TestInertObjectIsRejected(t *testing.T) {
 		}
 	}
 }
+
+func TestInlineFolderSigilsAreRejected(t *testing.T) {
+	f := shipped(t)
+	for _, input := range []string{"{.sv_SE.person.last}", "{..sv_SE.person.last}"} {
+		_, err := f.NewTemplate(input)
+		if err == nil || !strings.Contains(err.Error(), "write {/sv_SE.person.last}") {
+			t.Errorf("NewTemplate(%q) = %v, want an error naming the root spelling", input, err)
+		}
+	}
+}

path.go

@@ -0,0 +1,110 @@
+package fejkdata
+
+import (
+	"fmt"
+	"sort"
+	"strings"
+)
+
+// pathWalk is what one walk of a dotted path does at each kind of level: choice
+// returns the variants to continue into (none stops the walk); level runs at each
+// template a segment descends into; leaf runs where the tail ends. A nil action
+// is skipped.
+type pathWalk struct {
+	choice func(c *choice, rest []string) ([]node, error)
+	level  func(t *template, rest []string) error
+	leaf   func(n node) error
+}
+
+// walkPath descends tail from n: a group or template by its next segment, a
+// choice by w.choice, which consumes no segment. A missing segment is an error,
+// so no walk reaches past what the data holds. A table-shaped dispatch, one case
+// per node kind, kept whole on purpose.
+func walkPath(n node, tail []string, w pathWalk) error {
+	if len(tail) == 0 {
+		if w.leaf != nil {
+			return w.leaf(n)
+		}
+		return nil
+	}
+	switch n := n.(type) {
+	case *group:
+		child, ok := n.children[tail[0]]
+		if !ok {
+			return fmt.Errorf("no entry %q", tail[0])
+		}
+		return walkPath(child, tail[1:], w)
+	case *template:
+		if w.level != nil {
+			if err := w.level(n, tail); err != nil {
+				return err
+			}
+		}
+		child, ok := n.field(tail[0])
+		if !ok {
+			return fmt.Errorf("no field %q", tail[0])
+		}
+		return walkPath(child, tail[1:], w)
+	case *choice:
+		if w.choice == nil {
+			return nil
+		}
+		next, err := w.choice(n, tail)
+		if err != nil {
+			return err
+		}
+		for _, item := range next {
+			if err := walkPath(item, tail, w); err != nil {
+				return err
+			}
+		}
+		return nil
+	}
+	return fmt.Errorf("cannot descend into %T at %q", n, tail[0])
+}
+
+// carriedByAll is the choice rule a path that must resolve on every call obeys:
+// the rest of the tail must be one every variant carries.
+func carriedByAll(c *choice, rest []string) error {
+	if want := strings.Join(rest, "."); !c.shared[want] {
+		return unreachableInChoice(c, want)
+	}
+	return nil
+}
+
+// unreachableInChoice reports that a path cannot step through this choice, listing
+// what every variant does carry. It reads the precomputed set, so a failing path
+// costs no more than a rendering one.
+func unreachableInChoice(c *choice, want string) error {
+	if len(c.shared) == 0 {
+		return fmt.Errorf("no variant of this %d-way choice carries %q", len(c.items), want)
+	}
+	offered := make([]string, 0, len(c.shared))
+	for p := range c.shared {
+		offered = append(offered, p)
+	}
+	sort.Strings(offered)
+	return fmt.Errorf("not every variant of this %d-way choice carries %q; all carry %v", len(c.items), want, offered)
+}
+
+// checkPath proves a dotted tail resolves whichever way the draws go — a choice
+// must carry the rest of the path in the set every variant shares — and that no
+// level a path reads carries a repeat, which one draw of it could not apply. So a
+// path that validates here resolves on every render, and a typo is a New-time
+// error.
+func checkPath(n node, tail []string, level string) error {
+	return walkPath(n, tail, pathWalk{
+		choice: func(c *choice, rest []string) ([]node, error) {
+			if err := carriedByAll(c, rest); err != nil {
+				return nil, err
+			}
+			return c.items, nil
+		},
+		level: func(t *template, rest []string) error {
+			if t.repeat > 1 {
+				return fmt.Errorf("the level %q carries a repeat, which a path reading one draw of it cannot apply", join(level, strings.Join(tail[:len(tail)-len(rest)], ".")))
+			}
+			return nil
+		},
+	})
+}

path_test.go

@@ -0,0 +1,152 @@
+package fejkdata
+
+import (
+	"slices"
+	"strings"
+	"testing"
+)
+
+func TestWalkPathStopsAtAMissingSegment(t *testing.T) {
+	n := compiled(t, `{"format":"{a}","a":{"format":"{b}","b":"leaf"}}`)
+	var seen []string
+	walk := pathWalk{
+		level: func(tm *template, rest []string) error { seen = append(seen, "level:"+rest[0]); return nil },
+		leaf:  func(n node) error { seen = append(seen, "leaf"); return nil },
+	}
+	if err := walkPath(n, []string{"a", "b"}, walk); err != nil {
+		t.Fatalf("walkPath(a.b) = %v", err)
+	}
+	if want := []string{"level:a", "level:b", "leaf"}; !slices.Equal(seen, want) {
+		t.Errorf("walk visited %v, want %v", seen, want)
+	}
+	seen = nil
+	err := walkPath(n, []string{"a", "nope", "deeper"}, walk)
+	if err == nil || !strings.Contains(err.Error(), `no field "nope"`) {
+		t.Errorf("walkPath(a.nope.deeper) = %v, want the missing segment named", err)
+	}
+	if slices.Contains(seen, "leaf") {
+		t.Errorf("walk reached a leaf past a missing segment: %v", seen)
+	}
+}
+
+func TestWalkPathChoiceConsumesNoSegment(t *testing.T) {
+	n := compiled(t, `[{"format":"{f}","f":"1"},{"format":"{f}","f":"2"}]`)
+	var leaves []node
+	err := walkPath(n, []string{"f"}, pathWalk{
+		choice: func(c *choice, rest []string) ([]node, error) {
+			if len(rest) != 1 || rest[0] != "f" {
+				t.Errorf("choice saw rest %v, want [f]", rest)
+			}
+			return c.items, nil
+		},
+		leaf: func(n node) error { leaves = append(leaves, n); return nil },
+	})
+	if err != nil || len(leaves) != 2 {
+		t.Fatalf("walkPath through a choice = %v, %d leaves, want both variants' f", err, len(leaves))
+	}
+}
+
+func TestDeepDottedPath(t *testing.T) {
+	// A 5-segment path descends through alternating object/array nodes; choices
+	// on the path are single-variant, so it resolves deterministically.
+	f := engine(1)
+	f.categories = map[string]node{
+		"deep": compiled(t, `{"format":"{a}","a":{"format":"{b}","b":{"format":"{c}","c":{"format":"{d}","d":"leaf"}}}}`),
+	}
+	if got, err := f.Fake("deep.a.b.c.d"); err != nil || got != "leaf" {
+		t.Fatalf("Fake(deep.a.b.c.d) = %q, %v, want leaf", got, err)
+	}
+	// Rendering the whole tree resolves the same chain.
+	if got, err := f.Fake("deep"); err != nil || got != "leaf" {
+		t.Fatalf("Fake(deep) = %q, %v, want leaf", got, err)
+	}
+}
+
+func TestDescendIntoStringErrors(t *testing.T) {
+	f := engine(1)
+	f.categories = map[string]node{"greeting": compiled(t, `"hej"`)}
+	if _, err := f.Fake("greeting.extra"); err == nil || !strings.Contains(err.Error(), `no field "extra"`) {
+		t.Fatalf("Fake(greeting.extra) = %v, want a no-field error", err)
+	}
+}
+
+// TestPathThroughChoice pins the rule that keeps a dotted path from rendering on
+// one call and failing on the next: every variant must carry the rest of the path.
+func TestPathThroughChoice(t *testing.T) {
+	dir := writeData(t, map[string]string{
+		"every":  `[{"format":"{f}","f":"1"},{"format":"{f}","f":"2"}]`,
+		"notall": `[{"format":"{f}","f":"1"},"plain"]`,
+		"some":   `[{"format":"{f}","f":"1"},{"format":"{f}","f":"2","extra":"x"}]`,
+	})
+	f := newGenerator(t, dir, WithSeed(1))
+	for i := 0; i < 200; i++ {
+		if got := fake(t, f, "every.f"); got != "1" && got != "2" {
+			t.Fatalf("every.f = %q, want 1 or 2", got)
+		}
+	}
+	// A path only some variants carry is reported against what all of them carry.
+	if _, err := f.Fake("some.extra"); err == nil || !strings.Contains(err.Error(), "all carry [f]") {
+		t.Errorf("Fake(some.extra) = %v, want it to name what every variant carries", err)
+	}
+	var first string
+	for i := 0; i < 200; i++ {
+		_, err := f.Fake("notall.f")
+		if err == nil {
+			t.Fatal("notall.f = nil error, want the same failure every call")
+		}
+		if i == 0 {
+			first = err.Error()
+		} else if err.Error() != first {
+			t.Fatalf("notall.f error varies between calls:\n  %s\n  %s", first, err.Error())
+		}
+	}
+}
+
+// TestPathKeyIsUnambiguous pins that the one shape which could collide cannot be
+// written: a field literally named "a.b" and a field "a" holding "b" would both
+// spell "a.b", so a dotted field name is rejected at New and a dot means a path
+// wherever it appears.
+func TestPathKeyIsUnambiguous(t *testing.T) {
+	dir := writeData(t, map[string]string{
+		"cat": `[{"format":"{a.b}","a.b":"1"},{"format":"{a}","a":{"format":"{b}","b":"2"}}]`,
+	})
+	_, err := New(WithoutShippedData(), WithDataPath(dir))
+	if err == nil || !strings.Contains(err.Error(), `field "a.b" contains "."`) {
+		t.Fatalf("New = %v, want the dotted field name rejected", err)
+	}
+	// The same data without the dotted key is fine, and the path resolves.
+	f := newGenerator(t, writeData(t, map[string]string{
+		"cat": `{"format":"{a.b}","a":{"format":"{b}","b":"2"}}`,
+	}), WithSeed(1))
+	if !slices.Contains(f.List(), "cat.a.b") {
+		t.Error("List() omits cat.a.b, which the data carries")
+	}
+	if got := fake(t, f, "cat"); got != "2" {
+		t.Fatalf("cat = %q, want 2", got)
+	}
+}
+
+// TestMissingFieldNamesItself keeps the precise diagnosis for the ordinary typo: a
+// single-variant choice always picks the same item, so it needs no every-variant
+// guard and the error can name the field that is missing.
+func TestMissingFieldNamesItself(t *testing.T) {
+	f := newGenerator(t, "data/sv_SE", WithSeed(1))
+	_, err := f.Fake("person.typo")
+	if err == nil || !strings.Contains(err.Error(), `no field "typo"`) {
+		t.Errorf("Fake(person.typo) = %v, want it to name the missing field", err)
+	}
+}
+
+func TestBindingKeyIsNotAPathSegment(t *testing.T) {
+	dir := writeData(t, map[string]string{
+		"color": `["red","blue"]`,
+		"name":  `{"format":"{/color} {w}","w":"x"}`,
+	})
+	f := newGenerator(t, dir, WithSeed(1))
+	if slices.Contains(f.List(), "name./color") {
+		t.Error("List() advertises a binding key")
+	}
+	if _, err := f.Fake("name./color"); err == nil || !strings.Contains(err.Error(), `no field "/color"`) {
+		t.Errorf("Fake(name./color) = %v, want a no-field error", err)
+	}
+}

perf_test.go

@@ -0,0 +1,81 @@
+package fejkdata
+
+import (
+	"fmt"
+	"strings"
+	"testing"
+	"testing/fstest"
+)
+
+// One Fake call descends depth levels: each level's "a" is the next template down.
+func nestedJSON(depth int) string {
+	s := `"leaf"`
+	for i := 0; i < depth; i++ {
+		s = fmt.Sprintf(`{"format":"{a}","a":%s}`, s)
+	}
+	return s
+}
+
+// One Fake call expands n sibling tokens.
+func wideTokenJSON(n int) string {
+	var toks, fields strings.Builder
+	for i := 0; i < n; i++ {
+		fmt.Fprintf(&toks, "{f%d}", i)
+		if i > 0 {
+			fields.WriteByte(',')
+		}
+		fmt.Fprintf(&fields, `"f%d":"x"`, i)
+	}
+	return fmt.Sprintf(`{"format":"%s",%s}`, toks.String(), fields.String())
+}
+
+func TestNoRenderAllocRegression(t *testing.T) {
+	shapes := []struct {
+		name string
+		json string
+		base float64
+	}{
+		{"nested depth 25", nestedJSON(25), 27},
+		{"nested depth 100", nestedJSON(100), 102},
+		{"wide 500 tokens", wideTokenJSON(500), 9},
+	}
+	for _, s := range shapes {
+		f, err := New(WithoutShippedData(), WithDataFS(fstest.MapFS{"x.json": {Data: []byte(s.json)}}))
+		if err != nil {
+			t.Fatalf("New(%s): %v", s.name, err)
+		}
+		allocs := testing.AllocsPerRun(10000, func() { f.Fake("x") })
+		if allocs > s.base*1.10 {
+			t.Errorf("%s: %.1f allocs/op regressed past %.1f (baseline %.1f + 10%%); bump the baseline only as a deliberate change", s.name, allocs, s.base*1.10, s.base)
+		}
+	}
+}
+
+// A record's fences read the compiled tree, so they belong to New, not to a draw.
+func TestNoRecordAllocRegression(t *testing.T) {
+	for _, s := range []struct{ name, json string }{
+		{"record 3 columns", `{"format":"","a":"x","b":"y","c":"z"}`},
+		{"record 50 columns", wideTokenJSON(50)},
+	} {
+		f, err := New(WithoutShippedData(), WithDataFS(fstest.MapFS{"x.json": {Data: []byte(s.json)}}))
+		if err != nil {
+			t.Fatalf("New(%s): %v", s.name, err)
+		}
+		if _, err := f.Record("x"); err != nil {
+			t.Fatalf("Record(%s): %v", s.name, err) // else the gate would measure the error path
+		}
+		const base = 4.0
+		if allocs := testing.AllocsPerRun(10000, func() { f.Record("x") }); allocs > base*1.10 {
+			t.Errorf("%s: %.1f allocs/op regressed past %.1f (baseline %.1f + 10%%); a record fence running per draw is the usual cause", s.name, allocs, base*1.10, base)
+		}
+	}
+}
+
+func BenchmarkNestedDepth25(b *testing.B)  { benchPath(b, tmpData(b, "deep", nestedJSON(25)), "deep") }
+func BenchmarkNestedDepth100(b *testing.B) { benchPath(b, tmpData(b, "deep", nestedJSON(100)), "deep") }
+func BenchmarkWideTokens100(b *testing.B) {
+	benchPath(b, tmpData(b, "wide", wideTokenJSON(100)), "wide")
+}
+func BenchmarkWideTokens500(b *testing.B) {
+	benchPath(b, tmpData(b, "wide", wideTokenJSON(500)), "wide")
+}

readme_test.go

@@ -43,6 +43,34 @@ func TestReadmeExamplesLoadAndRender(t *testing.T) {
 	}
 }
 
+func TestReadmeRecordExample(t *testing.T) {
+	src := readme(t)
+	section := src[strings.Index(src, "### Records"):]
+	section = section[:strings.Index(section, "## Library")]
+	block := jsonBlock.FindStringSubmatch(section)
+	if block == nil {
+		t.Fatal("README lost the Records example")
+	}
+	f, err := New(WithDataPath(writeData(t, map[string]string{"users": block[1]})), WithSeed(1))
+	if err != nil {
+		t.Fatal(err)
+	}
+	r, err := f.Record("users")
+	if err != nil {
+		t.Fatal(err)
+	}
+	got := r.Columns()
+	if len(got) != 2 || got[0].Name != "first" || got[0].Value != "Bo" || got[1].Name != "last" || got[1].Value != "Lovelace" {
+		t.Fatalf("record columns = %v, want first=Bo, last=Lovelace with seed 1", got)
+	}
+	if r.CSVHeader() != "first,last" || r.CSVLine() != "Bo,Lovelace" {
+		t.Fatalf("csv = %q, %q, want first,last / Bo,Lovelace", r.CSVHeader(), r.CSVLine())
+	}
+	if r.SQLInsert("users") != `INSERT INTO "users" ("first", "last") VALUES ('Bo', 'Lovelace');` {
+		t.Fatalf("sql = %q, want the seeded INSERT", r.SQLInsert("users"))
+	}
+}
+
 func TestReadmeSQLExampleOutput(t *testing.T) {
 	src := readme(t)
 	src = src[strings.Index(src, "### Your own data"):]

record.go

@@ -0,0 +1,285 @@
+package fejkdata
+
+import (
+	"encoding/csv"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"sort"
+	"strings"
+)
+
+// Column is one rendered column of a record.
+type Column struct {
+	Name  string
+	Value string
+}
+
+// Record is one record rendered from a template: every direct field is a column,
+// listed in name order. Each column is its own expansion, so a sibling field is
+// local to it, while a reference that reads a path is drawn once for the whole
+// record.
+type Record struct {
+	columns []Column
+}
+
+// Columns returns the record's columns in name order.
+func (r *Record) Columns() []Column {
+	return append([]Column(nil), r.columns...)
+}
+
+// JSON renders the record as one JSON object, every column a string.
+func (r *Record) JSON() string {
+	m := make(map[string]string, len(r.columns))
+	for _, c := range r.columns {
+		m[c.Name] = c.Value
+	}
+	b, _ := json.Marshal(m)
+	return string(b)
+}
+
+// CSVHeader renders the column names as one CSV header line.
+func (r *Record) CSVHeader() string {
+	return csvLine(r.names())
+}
+
+// CSVLine renders the column values as one CSV row.
+func (r *Record) CSVLine() string {
+	return csvLine(r.values())
+}
+
+func (r *Record) names() []string {
+	out := make([]string, len(r.columns))
+	for i, c := range r.columns {
+		out[i] = c.Name
+	}
+	return out
+}
+
+func (r *Record) values() []string {
+	out := make([]string, len(r.columns))
+	for i, c := range r.columns {
+		out[i] = c.Value
+	}
+	return out
+}
+
+func csvLine(cols []string) string {
+	var b strings.Builder
+	w := csv.NewWriter(&b)
+	_ = w.Write(cols)
+	w.Flush()
+	line := strings.TrimSuffix(b.String(), "\n")
+	if line == "" {
+		return `""` // a blank line is a row every CSV reader drops
+	}
+	return line
+}
+
+// SQLInsert renders the record as one INSERT statement into table: identifiers in
+// ANSI double quotes, every value a single-quoted string literal.
+func (r *Record) SQLInsert(table string) string {
+	cols := make([]string, len(r.columns))
+	vals := make([]string, len(r.columns))
+	for i, c := range r.columns {
+		cols[i] = quoteIdent(c.Name)
+		vals[i] = "'" + strings.ReplaceAll(c.Value, "'", "''") + "'"
+	}
+	return fmt.Sprintf("INSERT INTO %s (%s) VALUES (%s);", quoteIdent(table), strings.Join(cols, ", "), strings.Join(vals, ", "))
+}
+
+func quoteIdent(s string) string {
+	return `"` + strings.ReplaceAll(s, `"`, `""`) + `"`
+}
+
+// Record renders a path as one record: the template it names, with each direct
+// field drawn as a column. Only a category-level template is a record — a path
+// that descends into a field, or that names a folder or a choice, is an error.
+func (f *Generator) Record(path string) (*Record, error) {
+	f.mu.Lock()
+	defer f.mu.Unlock()
+	_, n, tail, err := resolveCategory(f.categories, strings.Split(path, "."))
+	if err != nil {
+		return nil, fmt.Errorf("fejkdata: %s: %w", path, err)
+	}
+	if len(tail) > 0 {
+		return nil, fmt.Errorf("fejkdata: %s descends into %q, a field; only a category-level template is a record", path, tail[0])
+	}
+	shape := f.recordShapeOf(n)
+	if shape.err != nil {
+		return nil, fmt.Errorf("fejkdata: %s %w", path, shape.err)
+	}
+	return renderRecord(f.rand, shape.t, shape.columns), nil
+}
+
+// recordShape is what recordOf settled about a node: the template to project, its
+// columns, or why it is not a record.
+type recordShape struct {
+	t       *template
+	columns []string
+	err     error
+}
+
+// recordShapeOf fences a node once and remembers the answer. Callers hold the
+// generator's lock.
+func (f *Generator) recordShapeOf(n node) recordShape {
+	if shape, done := f.records[n]; done {
+		return shape
+	}
+	t, columns, err := recordOf(n)
+	shape := recordShape{t: t, columns: columns, err: err}
+	if f.records == nil {
+		f.records = map[node]recordShape{}
+	}
+	f.records[n] = shape
+	return shape
+}
+
+// RecordTemplate is an inline record compiled, referenced and validated once,
+// ready to render many times with [RecordTemplate.Fake].
+type RecordTemplate struct {
+	g       *Generator
+	t       *template
+	columns []string
+}
+
+// Fake renders the record with one draw.
+func (t *RecordTemplate) Fake() *Record {
+	t.g.mu.Lock()
+	defer t.g.mu.Unlock()
+	return renderRecord(t.g.rand, t.t, t.columns)
+}
+
+// NewRecordTemplate compiles an inline record — a JSON object with a format and
+// fields — and binds its references against the loaded tree.
+func (f *Generator) NewRecordTemplate(input string) (*RecordTemplate, error) {
+	t, err := f.NewTemplate(input)
+	if err != nil {
+		return nil, err
+	}
+	tm, columns, err := recordOf(t.n)
+	if err != nil {
+		return nil, fmt.Errorf("fejkdata: an inline record %w", err)
+	}
+	return &RecordTemplate{g: f, t: tm, columns: columns}, nil
+}
+
+// FakeRecord compiles and renders an inline record in one call.
+func (f *Generator) FakeRecord(input string) (*Record, error) {
+	t, err := f.NewRecordTemplate(input)
+	if err != nil {
+		return nil, err
+	}
+	return t.Fake(), nil
+}
+
+// recordOf is the fence both record entry points pass. The columns come back with
+// the template, fixed for every draw the caller goes on to make.
+func recordOf(n node) (*template, []string, error) {
+	t, ok := n.(*template)
+	if !ok {
+		return nil, nil, errors.New("names a choice, not a template; a record is a template whose fields are its columns")
+	}
+	if t.repeat != 1 {
+		return nil, nil, fmt.Errorf("carries repeat %d, which composes its format into one string; a record projects columns instead — drop the repeat and render the record again for more rows", t.repeat)
+	}
+	columns := recordColumns(t)
+	if len(columns) == 0 {
+		return nil, nil, errors.New("has no fields, so no columns")
+	}
+	if err := checkColumnRefs(t, columns); err != nil {
+		return nil, nil, err
+	}
+	return t, columns, nil
+}
+
+// checkColumnRefs rejects the reference reads a record's shared draw cannot answer
+// for: one column rendering a level another reads a path into, and a column
+// reading the record back through its own path.
+func checkColumnRefs(t *template, columns []string) error {
+	reads, err := columnRefs(t, columns)
+	if err != nil {
+		return err
+	}
+	sort.Slice(reads, func(i, j int) bool {
+		if reads[i].a.path != reads[j].a.path {
+			return reads[i].a.path < reads[j].a.path
+		}
+		return reads[i].column < reads[j].column
+	})
+	for i, level := range reads {
+		for _, into := range reads[i+1:] {
+			if strings.HasPrefix(into.a.path, level.a.path+".") {
+				return fmt.Errorf("column %q renders {%s}, a level column %q reads a path into with {%s}; name the fields you want instead",
+					level.column, level.a.name, into.column, into.a.name)
+			}
+		}
+	}
+	return nil
+}
+
+// columnRef is one held reference read, and the column whose render reaches it.
+type columnRef struct {
+	column string
+	a      arm
+}
+
+// columnRefs lists every reference that reads a path, anywhere a column renders,
+// following the same edges expand does. A reference landing back on the record
+// itself is reported rather than collected, whether it reads a path or the record
+// whole: either way the column describes a draw other than its neighbours'.
+func columnRefs(t *template, columns []string) ([]columnRef, error) {
+	var out []columnRef
+	var err error
+	for _, name := range columns {
+		seen := map[node]bool{}
+		var walk func(n node)
+		walk = func(n node) {
+			if n == nil || seen[n] || err != nil {
+				return
+			}
+			seen[n] = true
+			if tm, ok := n.(*template); ok {
+				for _, ref := range refTokens(tm.format) {
+					a := splitArm(ref, tm.refs)
+					if tm.fields[a.key] == node(t) {
+						err = fmt.Errorf("column %q reads {%s}, which points back at this record; a column cannot read another column — move the shared value into its own category and reference that", name, a.name)
+						return
+					}
+					if len(a.tail) > 0 {
+						out = append(out, columnRef{name, a})
+					}
+				}
+			}
+			for _, e := range renderEdges(n) {
+				walk(e.to)
+			}
+		}
+		walk(t.fields[name])
+	}
+	return out, err
+}
+
+// renderRecord draws each column once, in the name order recordOf fixed, over one
+// reference scope shared across them.
+func renderRecord(s *session, t *template, columns []string) *Record {
+	scope := &draws{variant: map[string]node{}, value: map[string]string{}}
+	r := &Record{columns: make([]Column, len(columns))}
+	for i, name := range columns {
+		r.columns[i] = Column{Name: name, Value: render(s, t.fields[name], scope)}
+	}
+	return r
+}
+
+// recordColumns is the sorted non-reference field names — the columns a record
+// projects. A {/path} binding is carried in fields under its root path, so only a
+// name that is not a reference is a column.
+func recordColumns(t *template) []string {
+	var names []string
+	for _, name := range sortedNames(t.fields) {
+		if !isRef(name) {
+			names = append(names, name)
+		}
+	}
+	return names
+}

record_test.go

@@ -0,0 +1,408 @@
+package fejkdata
+
+import (
+	"encoding/csv"
+	"encoding/json"
+	"strings"
+	"testing"
+)
+
+func recordCat(t *testing.T) *Generator {
+	t.Helper()
+	dir := writeData(t, map[string]string{
+		"users": `{
+			"format": "{first} {last}",
+			"first": ["Ada", "Bo"],
+			"last": ["Lovelace", "Ek"]
+		}`,
+	})
+	return newGenerator(t, dir, WithSeed(1))
+}
+
+func TestRecordProjectsFieldsAsColumns(t *testing.T) {
+	f := recordCat(t)
+	r, err := f.Record("users")
+	if err != nil {
+		t.Fatal(err)
+	}
+	got := r.Columns()
+	if len(got) != 2 || got[0].Name != "first" || got[1].Name != "last" {
+		t.Fatalf("Record.Columns() = %v, want columns first, last in name order", got)
+	}
+	if (got[0].Value != "Ada" && got[0].Value != "Bo") || (got[1].Value != "Lovelace" && got[1].Value != "Ek") {
+		t.Fatalf("columns = %v, want the field values", got)
+	}
+}
+
+func TestRecordIsDeterministic(t *testing.T) {
+	a, b := recordCat(t), recordCat(t)
+	for i := 0; i < 20; i++ {
+		x, _ := a.Record("users")
+		y, _ := b.Record("users")
+		if x.JSON() != y.JSON() {
+			t.Fatalf("same seed diverged: %s != %s", x.JSON(), y.JSON())
+		}
+	}
+}
+
+func TestRecordSkipsReferenceBindings(t *testing.T) {
+	dir := writeData(t, map[string]string{
+		"name": `{"format": "{first} {last}", "first": "Ada", "last": "Lovelace"}`,
+		"user": `{"format": "they are {/name}", "id": "1"}`,
+	})
+	f := newGenerator(t, dir, WithSeed(1))
+	r, err := f.Record("user")
+	if err != nil {
+		t.Fatal(err)
+	}
+	got := r.Columns()
+	if len(got) != 1 || got[0].Name != "id" {
+		t.Fatalf("Record.Columns() = %v, want only the id column (a {/path} is not a column)", got)
+	}
+}
+
+func TestRecordErrors(t *testing.T) {
+	dir := writeData(t, map[string]string{
+		"bare":      `"hello"`,
+		"pick":      `["a", "b"]`,
+		"group/cat": `"x"`,
+	})
+	f := newGenerator(t, dir, WithSeed(1))
+	for _, c := range []struct {
+		path string
+		want string
+	}{
+		{"bare", "has no fields, so no columns"},
+		{"pick", "names a choice"},
+		{"group", "names a folder"},
+		{"nope", "no entry"},
+	} {
+		if _, err := f.Record(c.path); err == nil || !strings.Contains(err.Error(), c.want) {
+			t.Errorf("Record(%q) = %v, want an error containing %q", c.path, err, c.want)
+		}
+	}
+}
+
+func TestRecordJSON(t *testing.T) {
+	f := recordCat(t)
+	r, err := f.Record("users")
+	if err != nil {
+		t.Fatal(err)
+	}
+	var m map[string]string
+	if err := json.Unmarshal([]byte(r.JSON()), &m); err != nil {
+		t.Fatalf("Record.JSON() is not valid JSON: %v\n%s", err, r.JSON())
+	}
+	if len(m) != 2 || (m["first"] != "Ada" && m["first"] != "Bo") {
+		t.Fatalf("Record.JSON() = %s, want two addressable columns", r.JSON())
+	}
+	for _, f := range r.Columns() {
+		if m[f.Name] != f.Value {
+			t.Fatalf("JSON column %q = %q, want %q", f.Name, m[f.Name], f.Value)
+		}
+	}
+}
+
+func TestRecordCSV(t *testing.T) {
+	dir := writeData(t, map[string]string{
+		"note": `{"format": "{word}, {word}", "word": ["a", "b,c", "d\"e", "f\n"]}`,
+	})
+	f := newGenerator(t, dir, WithSeed(1))
+	seen := map[string]bool{}
+	for i := 0; i < 200 && len(seen) < 4; i++ {
+		r, err := f.Record("note")
+		if err != nil {
+			t.Fatal(err)
+		}
+		if got := r.CSVHeader(); got != "word" {
+			t.Fatalf("CSVHeader() = %q, want word", got)
+		}
+		rec, err := csv.NewReader(strings.NewReader(r.CSVLine() + "\n")).Read()
+		if err != nil {
+			t.Fatalf("CSVLine() is not valid CSV: %v\n%q", err, r.CSVLine())
+		}
+		want := r.Columns()[0].Value
+		if len(rec) != 1 || rec[0] != want {
+			t.Fatalf("CSVLine() = %v, want the field value %q round-tripped", rec, want)
+		}
+		seen[want] = true
+	}
+	if len(seen) != 4 {
+		t.Fatalf("round-tripped %d of the 4 values; the comma, quote and newline shapes must each survive", len(seen))
+	}
+}
+
+func TestRecordCSVEmptyValueStaysARow(t *testing.T) {
+	dir := writeData(t, map[string]string{"blank": `{"format": "", "note": ""}`})
+	f := newGenerator(t, dir, WithSeed(1))
+	r, err := f.Record("blank")
+	if err != nil {
+		t.Fatal(err)
+	}
+	rows, err := csv.NewReader(strings.NewReader(r.CSVHeader() + "\n" + r.CSVLine() + "\n")).ReadAll()
+	if err != nil {
+		t.Fatalf("csv: %v", err)
+	}
+	if len(rows) != 2 || len(rows[1]) != 1 || rows[1][0] != "" {
+		t.Fatalf("one empty column parsed to %v, want a header and one row of one empty field", rows)
+	}
+}
+
+func TestRecordRejectsOverlappingReferenceColumns(t *testing.T) {
+	cat := `{"format":"","a":[{"format":"A={b}","b":"1"},{"format":"A={b}","b":"2"}]}`
+	for _, c := range []struct{ name, row string }{
+		{"sibling columns", `{"format":"","whole":"{/cat.a}","inner":"{/cat.a.b}"}`},
+		{"through a nested template", `{"format":"","whole":"{/cat.a}","inner":{"format":"{/cat.a.b} {x}","x":"1"}}`},
+		{"through a column repeat", `{"format":"","whole":"{/cat.a}","inner":{"format":"{/cat.a.b}","repeat":2,"separator":"-"}}`},
+		{"through a choice variant", `{"format":"","whole":"{/cat.a}","inner":[{"format":"{/cat.a.b} {x}","x":"1"},{"format":"{/cat.a.b}! {x}","x":"2"}]}`},
+		{"as a builtin operand", `{"format":"","whole":"{uppercase(/cat.a)}","inner":"{/cat.a.b}"}`},
+	} {
+		f := newGenerator(t, writeData(t, map[string]string{"cat": cat, "row": c.row}), WithSeed(1))
+		_, err := f.Record("row")
+		if err == nil || !strings.Contains(err.Error(), "reads a path into") {
+			t.Errorf("%s: Record = %v, want the overlap rejected the way one format is", c.name, err)
+			continue
+		}
+		if !strings.Contains(err.Error(), `"whole"`) || !strings.Contains(err.Error(), `"inner"`) {
+			t.Errorf("%s: error %q names neither column; it must name both", c.name, err)
+		}
+		if _, err := f.Fake("row"); err != nil {
+			t.Errorf("%s: Fake(row) = %v, want the string view untouched", c.name, err)
+		}
+	}
+}
+
+func TestInlineRecordRejectsOverlappingColumns(t *testing.T) {
+	dir := writeData(t, map[string]string{
+		"cat": `{"format":"","a":[{"format":"A={b}","b":"1"},{"format":"A={b}","b":"2"}]}`,
+	})
+	f := newGenerator(t, dir, WithSeed(1))
+	_, err := f.FakeRecord(`{"format":"","whole":"{/cat.a}","inner":"{/cat.a.b}"}`)
+	if err == nil || !strings.Contains(err.Error(), "reads a path into") {
+		t.Fatalf("inline record over an overlapping pair = %v, want the inline entry point to refuse it too", err)
+	}
+}
+
+func TestRecordRejectsAColumnReadingItsOwnRecord(t *testing.T) {
+	for _, c := range []struct{ name, column string }{
+		{"a path into itself", `"full":"{/person.first} {/person.last}"`},
+		{"the record read whole", `"whole":"{/person}"`},
+		{"the record as an operand", `"up":"{uppercase(/person)}"`},
+	} {
+		person := `{"format":"{first} {last}","first":["Ada","Bo"],"last":["Lovelace","Ek"],` + c.column + `}`
+		f := newGenerator(t, writeData(t, map[string]string{"person": person}), WithSeed(1))
+		if _, err := f.Record("person"); err == nil || !strings.Contains(err.Error(), "points back at this record") {
+			t.Errorf("%s: Record = %v, want it refused; the column would contradict the columns beside it", c.name, err)
+		}
+	}
+}
+
+func TestRecordBareReferenceStaysIndependentAsAnOperand(t *testing.T) {
+	dir := writeData(t, map[string]string{
+		"cur": `[{"format":"{code}","code":"aud"},{"format":"{code}","code":"eur"}]`,
+		"row": `{"format":"","up":"{uppercase(/cur)}","low":"{lowercase(/cur)}"}`,
+	})
+	f := newGenerator(t, dir, WithSeed(1))
+	sawMismatch := false
+	for i := 0; i < 200 && !sawMismatch; i++ {
+		r, err := f.Record("row")
+		if err != nil {
+			t.Fatal(err)
+		}
+		m := map[string]string{}
+		for _, c := range r.Columns() {
+			m[c.Name] = c.Value
+		}
+		sawMismatch = !strings.EqualFold(m["up"], m["low"])
+	}
+	if !sawMismatch {
+		t.Error("two bare-reference operand columns never disagreed; a bare reference draws on its own, as the plain spelling does")
+	}
+	for i := 0; i < 50; i++ {
+		v, err := f.FakeTemplate("{/cur}|{uppercase(/cur)}")
+		if err != nil {
+			t.Fatal(err)
+		}
+		if parts := strings.Split(v, "|"); !strings.EqualFold(parts[0], parts[1]) {
+			t.Fatalf("one format rendered %q; within an expansion a bare reference is still one draw", v)
+		}
+	}
+}
+
+func TestRecordSQLInsert(t *testing.T) {
+	dir := writeData(t, map[string]string{
+		"person": `{"format": "{last}", "last": "O'Brien"}`,
+	})
+	f := newGenerator(t, dir, WithSeed(1))
+	r, err := f.Record("person")
+	if err != nil {
+		t.Fatal(err)
+	}
+	got := r.SQLInsert("people")
+	if got != `INSERT INTO "people" ("last") VALUES ('O''Brien');` {
+		t.Fatalf("SQLInsert() = %q, want quoted identifiers and the single quote doubled", got)
+	}
+}
+
+func TestRecordRejectsFieldDescent(t *testing.T) {
+	dir := writeData(t, map[string]string{
+		"cat": `{"format":"{sub}","sub":{"format":"{x}","x":"1"}}`,
+		"row": `[{"format":"{x}","x":"1"},{"format":"{x}","x":"2"}]`,
+	})
+	f := newGenerator(t, dir, WithSeed(1))
+	for _, path := range []string{"cat.sub", "row.x"} {
+		if _, err := f.Record(path); err == nil || !strings.Contains(err.Error(), "field") {
+			t.Errorf("Record(%q) = %v, want a 'descends into a field' error", path, err)
+		}
+	}
+}
+
+func TestRecordSharesAReferenceAcrossColumns(t *testing.T) {
+	dir := writeData(t, map[string]string{
+		"currency": `[{"format":"{code}","code":"AUD","symbol":"$"},{"format":"{code}","code":"EUR","symbol":"€"}]`,
+		"price":    `{"format":"","code":"{/currency.code}","symbol":"{/currency.symbol}"}`,
+	})
+	f := newGenerator(t, dir, WithSeed(1))
+	for i := 0; i < 100; i++ {
+		r, err := f.Record("price")
+		if err != nil {
+			t.Fatal(err)
+		}
+		m := map[string]string{}
+		for _, c := range r.Columns() {
+			m[c.Name] = c.Value
+		}
+		switch m["code"] {
+		case "AUD":
+			if m["symbol"] != "$" {
+				t.Fatalf("record %q: code AUD but symbol %q, want one currency draw across columns", r.JSON(), m["symbol"])
+			}
+		case "EUR":
+			if m["symbol"] != "€" {
+				t.Fatalf("record %q: code EUR but symbol %q, want one currency draw across columns", r.JSON(), m["symbol"])
+			}
+		default:
+			t.Fatalf("record %q has unexpected code %q", r.JSON(), m["code"])
+		}
+	}
+}
+
+func TestRecordSharesAReferenceIntoAColumnRepeat(t *testing.T) {
+	dir := writeData(t, map[string]string{
+		"currency": `[{"format":"{code}","code":"AUD"},{"format":"{code}","code":"EUR"}]`,
+		"order":    `{"format":"","codes":{"format":"{/currency.code}","repeat":3,"separator":"-"}}`,
+	})
+	f := newGenerator(t, dir, WithSeed(1))
+	for i := 0; i < 50; i++ {
+		r, err := f.Record("order")
+		if err != nil {
+			t.Fatal(err)
+		}
+		parts := strings.Split(r.Columns()[0].Value, "-")
+		if len(parts) != 3 || parts[0] != parts[1] || parts[1] != parts[2] {
+			t.Fatalf("codes column = %q, want one shared draw across its repeat", r.Columns()[0].Value)
+		}
+	}
+}
+
+func TestRecordBareReferenceStaysIndependent(t *testing.T) {
+	dir := writeData(t, map[string]string{
+		"currency": `[{"format":"{code}","code":"AUD"},{"format":"{code}","code":"EUR"}]`,
+		"order":    `{"format":"","whole":"{/currency}","code":"{/currency.code}"}`,
+	})
+	f := newGenerator(t, dir, WithSeed(1))
+	sawMismatch := false
+	for i := 0; i < 100; i++ {
+		r, err := f.Record("order")
+		if err != nil {
+			t.Fatal(err)
+		}
+		m := map[string]string{}
+		for _, c := range r.Columns() {
+			m[c.Name] = c.Value
+		}
+		if m["whole"] != m["code"] {
+			sawMismatch = true
+			break
+		}
+	}
+	if !sawMismatch {
+		t.Fatal("a bare {/currency} column never disagreed with a tailed {/currency.code} column; a bare reference should draw independently")
+	}
+}
+
+func TestRecordSQLQuotesIdentifiers(t *testing.T) {
+	dir := writeData(t, map[string]string{
+		"row": `{"format": "", "postal-code": "1", "street-number": "2"}`,
+	})
+	f := newGenerator(t, dir, WithSeed(1))
+	r, err := f.Record("row")
+	if err != nil {
+		t.Fatal(err)
+	}
+	got := r.SQLInsert("my-table")
+	if got != `INSERT INTO "my-table" ("postal-code", "street-number") VALUES ('1', '2');` {
+		t.Fatalf("SQLInsert() = %q, want hyphenated identifiers and table quoted", got)
+	}
+}
+
+func TestFakeRecordAndTemplate(t *testing.T) {
+	f := recordCat(t)
+	in := `{"format":"{x} {y}","x":["1","2"],"y":["3","4"]}`
+	want, err := f.FakeRecord(in)
+	if err != nil {
+		t.Fatalf("FakeRecord: %v", err)
+	}
+	if len(want.Columns()) != 2 {
+		t.Fatalf("FakeRecord columns = %v, want two columns", want.Columns())
+	}
+	reusable, err := f.NewRecordTemplate(in)
+	if err != nil {
+		t.Fatalf("NewRecordTemplate: %v", err)
+	}
+	for i := 0; i < 20; i++ {
+		if got := reusable.Fake(); len(got.Columns()) != 2 {
+			t.Fatalf("RecordTemplate.Fake() = %v, want two columns", got.Columns())
+		}
+	}
+}
+
+func TestInlineRecordErrors(t *testing.T) {
+	f := recordCat(t)
+	for _, c := range []struct {
+		input string
+		want  string
+	}{
+		{`"hello"`, "has no fields, so no columns"},
+		{`["a","b"]`, "a record is a template whose fields are its columns"},
+		{`{"format":"{a}-","repeat":3,"separator":"|","a":["x","y"]}`, "carries repeat 3"},
+	} {
+		if _, err := f.FakeRecord(c.input); err == nil || !strings.Contains(err.Error(), c.want) {
+			t.Errorf("FakeRecord(%q) = %v, want an error naming %q", c.input, err, c.want)
+		}
+	}
+}
+
+func TestRecordRejectsATopLevelRepeat(t *testing.T) {
+	dir := writeData(t, map[string]string{
+		"rep": `{"format":"{a}-","repeat":3,"separator":"|","a":["x","y"]}`,
+	})
+	f := newGenerator(t, dir, WithSeed(1))
+	if _, err := f.Record("rep"); err == nil || !strings.Contains(err.Error(), "carries repeat 3") {
+		t.Errorf("Record on a repeating template = %v, want an error naming the repeat", err)
+	}
+	if v, err := f.Fake("rep"); err != nil || v != "x-|x-|x-" {
+		t.Errorf("Fake(rep) = %q, %v, want the repeat still composed for the string view", v, err)
+	}
+}
+
+func TestRecordTemplateRejectsATopLevelRepeat(t *testing.T) {
+	f := recordCat(t)
+	in := `{"format":"{a}-","repeat":3,"separator":"|","a":["x","y"]}`
+	if _, err := f.NewRecordTemplate(in); err == nil || !strings.Contains(err.Error(), "carries repeat 3") {
+		t.Errorf("NewRecordTemplate on a repeating template = %v, want an error naming the repeat", err)
+	}
+	if _, err := f.NewTemplate(in); err != nil {
+		t.Errorf("NewTemplate on the same input = %v, want the string view to still compile", err)
+	}
+}

reference.go

@@ -2,7 +2,6 @@ package fejkdata
 
 import (
 	"fmt"
-	"sort"
 	"strings"
 )
 
@@ -30,6 +29,9 @@ func refShape(name string) (sigil, rest string, err error) {
 	if rest == "" {
 		return "", "", fmt.Errorf("reference has no path")
 	}
+	if strings.HasPrefix(rest, "/") {
+		return "", "", fmt.Errorf("the path after %s starts at a name, not a /; write {%s%s}", sigil, sigil, rest[1:])
+	}
 	if strings.HasPrefix(rest, ".") {
 		return "", "", fmt.Errorf("a reference starts with / (the root), . (this folder) or .. (the folder above)")
 	}
@@ -70,6 +72,14 @@ func refSegments(name string, folder []string) ([]string, error) {
 // error, never a random render-time one.
 func linkRefs(root map[string]node) error {
 	return eachTemplate(root, func(folder []string, path string, t *template) error {
+		return linkTemplateRefs(folder, path, t, root)
+	})
+}
+
+// linkTemplateRefs binds one template's references against root. A template with
+// none is left untouched, so an inline format that references nothing costs only
+// the refTokens scan.
+func linkTemplateRefs(folder []string, path string, t *template, root map[string]node) error {
 	names := refTokens(t.format)
 	if len(names) == 0 {
 		return nil
@@ -83,7 +93,7 @@ func linkRefs(root map[string]node) error {
 		if err != nil {
 			return fmt.Errorf("%s: reference {%s}: %w", path, name, err)
 		}
-			head, target, tail, err := resolveRef(root, segments)
+		head, target, tail, err := resolveCategory(root, segments)
 		if err != nil {
 			return fmt.Errorf("%s: reference {%s}: %w", path, name, err)
 		}
@@ -98,7 +108,6 @@ func linkRefs(root map[string]node) error {
 		return fmt.Errorf("%s: %w", path, err)
 	}
 	return nil
-	})
 }
 
 // eachTemplate calls fn once per template, with the folder its category sits in
@@ -137,251 +146,11 @@ func eachTemplate(root map[string]node, fn func(folder []string, path string, t
 	return inFolder(nil, root)
 }
 
-// checkBoundLevelsHeld rejects every route to a held name except the ones that read
-// its draw. An expansion holds one draw of that name; anything else that renders it
-// draws again, and the two disagree. checkNoOverlap settles the spellings within one
-// format (a token, an operand); this settles the rest — a reference, whether it
-// sits in that format or in anything the format renders, however deep.
-//
-// It runs after checkNoCycles, whose guarantee is what lets the walk terminate.
-func checkBoundLevelsHeld(root map[string]node) error {
-	return walkNodes(root, func(path string, n node) error {
-		t, ok := n.(*template)
-		if !ok || len(t.held) == 0 {
-			return nil
-		}
-		heads := make([]string, 0, len(t.held))
-		for head := range t.held {
-			heads = append(heads, head)
-		}
-		// Operand heads first, then paths, each in name order: a level read both
-		// ways is reported by the operand's fence, and which overlap is reported
-		// does not vary.
-		sort.Slice(heads, func(i, j int) bool {
-			_, pi := t.bound[heads[i]]
-			_, pj := t.bound[heads[j]]
-			if pi != pj {
-				return !pi
-			}
-			return heads[i] < heads[j]
-		})
-		readers := boundReaders(t.format, t.bound, t.refs)
-		for _, head := range heads {
-			// What one draw answers for depends on how the draw is read: a path pins
-			// the levels it passes through and the leaf it lands on, an operand
-			// exactly the value its render produces.
-			held := map[node]bool{}
-			reader, isPath := t.bound[head]
-			if isPath {
-				for _, r := range readers {
-					if a := splitArm(r.name, t.refs); a.key == head {
-						coverPath(t.fields[head], a.tail, held)
-					}
-				}
-			} else {
-				operandDraw(t.fields[head], held)
-			}
-			if len(held) == 0 {
-				continue // an early out: a fixed head holds nothing to reach
-			}
-			// One seen set across the edges: a node that cannot reach the level
-			// cannot reach it by another route either, so it is walked once here.
-			seen := map[node]bool{}
-			for _, e := range renderEdges(t) {
-				if splitArm(e.label, t.refs).key == head {
-					continue // a token or operand reading this draw, the routes allowed
-				}
-				if renders(e.to, held, seen) {
-					if isPath {
-						return fmt.Errorf("%s: %s renders %q, which {%s} reads a path into; name the fields you want instead", path, e.reached(), head, reader)
-					}
-					return fmt.Errorf("%s: %s renders %q, which a {%s()} also reads; reach it one way so it is drawn once", path, e.reached(), head, operandReader(t, head))
-				}
-			}
-		}
-		return nil
-	})
-}
-
-// operandReader names the builtin whose operand holds head.
-func operandReader(t *template, head string) string {
-	fn := ""
-	_ = eachToken(t.format, func(tok ftoken) error {
-		if tok.kind != 'b' || fn != "" {
-			return nil
-		}
-		if name, _, isFunc := funcCall(tok.body); isFunc {
-			for _, operand := range tokenOperands(tok.body) {
-				if splitArm(operand, t.refs).key == head {
-					fn = name
-				}
-			}
-		}
-		return nil
-	})
-	return fn
-}
-
-// coverPath collects what holding one path pins: every choice level the path
-// passes through, whole, and the leaf it renders.
-func coverPath(n node, tail []string, into map[node]bool) {
-	if _, isChoice := n.(*choice); isChoice || len(tail) == 0 {
-		cover(n, into, false)
-		return
-	}
-	t, ok := n.(*template)
-	if !ok {
-		return
-	}
-	if child, ok := t.fields[tail[0]]; ok {
-		coverPath(child, tail[1:], into)
-	}
-}
-
-// cover collects a level and everything contained in it. A fixed string outside a
-// choice is left out — it cannot disagree with itself — but inside one each
-// variant carries its own, so there it counts.
-func cover(n node, into map[node]bool, inChoice bool) {
-	if isFixed(n) && !inChoice {
-		return
-	}
-	into[n] = true
-	_, isChoice := n.(*choice)
-	for _, c := range contained(n) {
-		cover(c.node, into, inChoice || isChoice)
-	}
-}
-
-// operandDraw collects what one held draw of an operand answers for: the operand
-// and what rendering it settles inside itself. The builtin renders its operand
-// whole, so that draw fixes every value the render produced, and a second route to
-// any of them disagrees with it.
-//
-// The walk stops at a reference edge, which is where the operand's own value ends
-// and a shared source begins: two names referencing one category are two draws, the
-// same rule {word} {word} follows.
-func operandDraw(n node, into map[node]bool) {
-	if isFixed(n) {
-		return
-	}
-	if into[n] {
-		return
-	}
-	into[n] = true
-	for _, e := range renderEdges(n) {
-		if isRef(e.label) {
-			continue
-		}
-		operandDraw(e.to, into)
-	}
-}
-
-// isFixed is a string that varies nothing: fixed text with no fields to read into.
-func isFixed(n node) bool {
-	t, ok := n.(*template)
-	return ok && t.fixed && len(t.fields) == 0
-}
-
-// renders reports whether rendering n can reach anything in want, following the
-// same edges expand does. seen keeps a node shared by several routes from being
-// walked twice; checkNoCycles has already proved the graph is a DAG, so the walk
-// ends.
-func renders(n node, want, seen map[node]bool) bool {
-	if want[n] {
-		return true
-	}
-	if seen[n] {
-		return false
-	}
-	seen[n] = true
-	for _, e := range renderEdges(n) {
-		if renders(e.to, want, seen) {
-			return true
-		}
-	}
-	return false
-}
-
-// walkNodes calls fn once per contained node, passing the dot path that reaches it,
-// visiting keys in sorted order so which of several broken nodes gets reported does
-// not depend on map iteration.
-func walkNodes(root map[string]node, fn func(path string, n node) error) error {
-	seen := map[node]bool{}
-	var visit func(string, node) error
-	visit = func(path string, n node) error {
-		if n == nil || seen[n] {
-			return nil
-		}
-		seen[n] = true
-		if err := fn(path, n); err != nil {
-			return err
-		}
-		for _, c := range contained(n) {
-			if err := visit(join(path, c.name), c.node); err != nil {
-				return err
-			}
-		}
-		return nil
-	}
-	for _, name := range sortedNames(root) {
-		if err := visit(name, root[name]); err != nil {
-			return err
-		}
-	}
-	return nil
-}
-
-// namedNode is a contained child and the segment reaching it; a choice's items carry
-// no segment, matching how a dot path steps over a choice.
-type namedNode struct {
-	name string
-	node node
-}
-
-func contained(n node) []namedNode {
-	switch n := n.(type) {
-	case *group:
-		return named(n.children)
-	case *choice:
-		out := make([]namedNode, len(n.items))
-		for i, it := range n.items {
-			out[i] = namedNode{node: it}
-		}
-		return out
-	case *template:
-		return named(n.fields)
-	default:
-		return nil
-	}
-}
-
-// named skips a bound {/path} key: it is a render edge, not containment, so using
-// it as a path segment would report a node under a path that does not reach it. Only
-// a template's fields hold bindings — loadDir skips a dot-prefixed entry, so a
-// group's children never carry the prefix — so this one skip serves both.
-func named(m map[string]node) []namedNode {
-	out := make([]namedNode, 0, len(m))
-	for _, name := range sortedNames(m) {
-		if isRef(name) {
-			continue
-		}
-		out = append(out, namedNode{name: name, node: m[name]})
-	}
-	return out
-}
-
-func sortedNames(m map[string]node) []string {
-	names := make([]string, 0, len(m))
-	for name := range m {
-		names = append(names, name)
-	}
-	sort.Strings(names)
-	return names
-}
-
-// resolveRef walks a reference path through the folders to the category it names,
-// returning that head, the node, and the tail left to read into it.
-func resolveRef(root map[string]node, segments []string) (head []string, target node, tail []string, err error) {
+// resolveCategory walks a dotted path through the folders to the category it
+// names, returning that head, the node, and the tail left to read into it. A
+// descent of its own rather than a walkPath: it walks groups only and returns
+// where they end, not a leaf.
+func resolveCategory(root map[string]node, segments []string) (head []string, target node, tail []string, err error) {
 	var n node = &group{children: root}
 	i := 0
 	for ; i < len(segments); i++ {
@@ -413,148 +182,3 @@ func refTokens(format string) []string {
 	}
 	return refs
 }
-
-// renderEdge is a child a node renders into, labelled by what reaches it (a field
-// name, reference, or choice index) for a readable cycle report. operand names
-// the builtin when the label is its operand rather than a token, so an error can
-// name it the way the author wrote it.
-type renderEdge struct {
-	to      node
-	label   string
-	operand string
-}
-
-// reached names an edge as the author spelled it, the vocabulary boundReaders uses
-// for the sibling fence.
-func (e renderEdge) reached() string {
-	if e.operand != "" {
-		return fmt.Sprintf("%s operand %q", e.operand, e.label)
-	}
-	return "{" + e.label + "}"
-}
-
-// renderEdges lists the children rendering n recurses into, mirroring expand: a
-// choice's items, and a template's field/reference tokens plus its operands. A
-// group renders nothing, so it has no edges.
-func renderEdges(n node) []renderEdge {
-	switch n := n.(type) {
-	case *choice:
-		es := make([]renderEdge, len(n.items))
-		for i, it := range n.items {
-			es[i] = renderEdge{to: it, label: fmt.Sprintf("[%d]", i)}
-		}
-		return es
-	case *template:
-		var es []renderEdge
-		add := func(name, operand string) {
-			a := splitArm(name, n.refs)
-			c, ok := n.fields[a.key]
-			if !ok {
-				return
-			}
-			for _, leaf := range pathLeaves(c, a.tail) {
-				es = append(es, renderEdge{leaf, name, operand})
-			}
-		}
-		_ = eachToken(n.format, func(t ftoken) error {
-			if t.kind != 'b' {
-				return nil
-			}
-			if fn, _, isFunc := funcCall(t.body); isFunc {
-				for _, operand := range tokenOperands(t.body) {
-					add(operand, fn)
-				}
-				return nil
-			}
-			for _, name := range strings.Split(t.body, "|") {
-				add(name, "")
-			}
-			return nil
-		})
-		return es
-	default:
-		return nil
-	}
-}
-
-// pathLeaves lists what a token's dotted tail renders. A path draws the levels it
-// passes through but renders only what it lands on, so the leaf is the edge — a
-// bare token, whose tail is empty, lands on the field itself. A choice on the way
-// contributes every variant, since any of them may be the one drawn. checkPath has
-// already proved the tail resolves in every variant, so the walk drops nothing.
-func pathLeaves(n node, tail []string) []node {
-	if len(tail) == 0 {
-		return []node{n}
-	}
-	if c, ok := n.(*choice); ok {
-		var out []node
-		for _, it := range c.items {
-			out = append(out, pathLeaves(it, tail)...)
-		}
-		return out
-	}
-	return pathLeaves(child(n, tail[0]), tail[1:])
-}
-
-// checkRepeatReach bounds the renders a repeat multiplies to along any root-to-leaf
-// path, so nested repeats cannot build what one repeat may not. It runs after
-// checkNoCycles, whose guarantee is what lets the walk terminate.
-func checkRepeatReach(root map[string]node) error {
-	reach := map[node]int{}
-	var of func(n node) int
-	of = func(n node) int {
-		if r, done := reach[n]; done {
-			return r
-		}
-		r := 1
-		for _, e := range renderEdges(n) {
-			if c := of(e.to); c > r {
-				r = c
-			}
-		}
-		if t, ok := n.(*template); ok {
-			r *= t.repeat
-		}
-		reach[n] = r
-		return r
-	}
-	return walkNodes(root, func(path string, n node) error {
-		if t, ok := n.(*template); ok && t.repeat > 1 && of(n) > maxLen {
-			return fmt.Errorf("%s: repeat %d multiplies to %d renders along one path, above the maximum %d", path, t.repeat, of(n), maxLen)
-		}
-		return nil
-	})
-}
-
-// checkNoCycles rejects a reference cycle: a node whose rendering can reach itself
-// — directly, mutually, or through a chain — never terminates, so it must fail at
-// New rather than stack-overflow at render. It is a depth-first walk of the render
-// graph (renderEdges); grey marks nodes on the current path so a back-edge to one
-// is the cycle, while black lets a shared node (a DAG, not a cycle) be skipped.
-// Every node is a root: a field its parent's format never renders is still reachable
-// by dot path, so a cycle in one would otherwise reach render and be fatal there.
-func checkNoCycles(root map[string]node) error {
-	const (
-		grey  = 1
-		black = 2
-	)
-	color := map[node]int{}
-	var visit func(n node, path string) error
-	visit = func(n node, path string) error {
-		switch color[n] {
-		case grey:
-			return fmt.Errorf("reference cycle: %s", path)
-		case black:
-			return nil
-		}
-		color[n] = grey
-		for _, e := range renderEdges(n) {
-			if err := visit(e.to, path+" -> "+e.label); err != nil {
-				return err
-			}
-		}
-		color[n] = black
-		return nil
-	}
-	return walkNodes(root, func(path string, n node) error { return visit(n, path) })
-}

reference_test.go

@@ -312,3 +312,37 @@ func TestReferenceSigilErrors(t *testing.T) {
 		}
 	}
 }
+
+func TestSpellingsOfOneReferenceAreOneLevel(t *testing.T) {
+	person := `[{"format":"{first} {last}","first":"Ada","last":"Byron"},{"format":"{first} {last}","first":"Bo","last":"Ek"}]`
+	_, err := New(WithoutShippedData(), WithDataPath(writeData(t, map[string]string{
+		"sv_SE/person": person,
+		"sv_SE/mail":   `"{.person} <{/sv_SE.person.first}>"`,
+	})))
+	if err == nil || !strings.Contains(err.Error(), "reads a path into") || !strings.Contains(err.Error(), "{.person}") {
+		t.Errorf("New = %v, want the bare spelling rejected beside the path spelling", err)
+	}
+	dir := writeData(t, map[string]string{
+		"sv_SE/word": `["alpha","beta","gamma"]`,
+		"sv_SE/loud": `"{/sv_SE.word} {uppercase(.word)}"`,
+	})
+	f := newGenerator(t, dir, WithSeed(2))
+	for i := 0; i < 50; i++ {
+		got := strings.Fields(fake(t, f, "sv_SE.loud"))
+		if len(got) != 2 || strings.ToUpper(got[0]) != got[1] {
+			t.Fatalf("loud = %q, want one draw under both spellings", got)
+		}
+	}
+}
+
+func TestSlashAfterAReferenceSigilIsRejected(t *testing.T) {
+	for name, files := range map[string]map[string]string{
+		"after ..": {"sv_SE/person": `"Ada"`, "sv_SE/deep/a": `"{../person}"`},
+		"after .":  {"sv_SE/person": `"Ada"`, "sv_SE/a": `"{./person}"`},
+	} {
+		_, err := New(WithoutShippedData(), WithDataPath(writeData(t, files)))
+		if err == nil || !strings.Contains(err.Error(), "person}") || !strings.Contains(err.Error(), "write {") {
+			t.Errorf("%s: New = %v, want the slash rejected naming the spelling", name, err)
+		}
+	}
+}

render.go

@@ -27,68 +27,41 @@ func (f *Generator) Fake(path string) (string, error) {
 	if _, ok := n.(*group); ok {
 		return "", fmt.Errorf("fejkdata: %s names a folder, not a value", path)
 	}
-	return render(f.rand, n), nil
+	return render(f.rand, n, nil), nil
 }
 
 // descend walks named fields to the node a path names. It is the one render-side
 // step that can fail, because the path comes from the caller and may name a field
 // that does not exist. A choice consumes no segment, so the rest of the path must
-// be one every variant carries (the set compile stored) before a variant is picked
-// — a path that resolves at all resolves on every call.
-func descend(s *session, n node, segments []string) (node, error) {
-	if len(segments) == 0 {
-		return n, nil
+// be one every variant carries before a variant is picked — a path that resolves
+// at all resolves on every call.
+func descend(s *session, root node, segments []string) (node, error) {
+	var found node
+	err := walkPath(root, segments, pathWalk{
+		choice: func(c *choice, rest []string) ([]node, error) {
+			if err := carriedByAll(c, rest); err != nil {
+				return nil, err
 			}
-	switch n := n.(type) {
-	case *group:
-		child, ok := n.children[segments[0]]
-		if !ok {
-			return nil, fmt.Errorf("no entry %q", segments[0])
-		}
-		return descend(s, child, segments[1:])
-	case *template:
-		child, ok := n.fields[segments[0]]
-		if !ok {
-			return nil, fmt.Errorf("no field %q", segments[0])
-		}
-		return descend(s, child, segments[1:])
-	case *choice:
-		if want := strings.Join(segments, "."); !n.shared[want] {
-			return nil, unreachableInChoice(n, want)
-		}
-		return descend(s, pick(s, n), segments)
-	default:
-		return nil, fmt.Errorf("cannot descend into %T at %q", n, segments[0])
-	}
-}
-
-// unreachableInChoice reports that a path cannot step through this choice, listing
-// what every variant does carry. It reads the precomputed set, so a failing path
-// costs no more than a rendering one.
-func unreachableInChoice(c *choice, want string) error {
-	if len(c.shared) == 0 {
-		return fmt.Errorf("no variant of this %d-way choice carries %q", len(c.items), want)
-	}
-	offered := make([]string, 0, len(c.shared))
-	for p := range c.shared {
-		offered = append(offered, p)
-	}
-	sort.Strings(offered)
-	return fmt.Errorf("not every variant of this %d-way choice carries %q; all carry %v", len(c.items), want, offered)
+			return []node{pick(s, c)}, nil
+		},
+		leaf: func(n node) error { found = n; return nil },
+	})
+	return found, err
 }
 
 // render evaluates a compiled node to a string. compile validates every node up
-// front, so rendering a compiled tree cannot fail.
-func render(s *session, n node) string {
+// front, so rendering a compiled tree cannot fail. refScope carries the draws a
+// reference shares beyond its own expansion; nil keeps every reference local.
+func render(s *session, n node, refScope *draws) string {
 	switch n := n.(type) {
 	case *choice:
-		return render(s, pick(s, n))
+		return render(s, pick(s, n), refScope)
 	case *template:
 		if n.repeat == 1 {
 			if n.fixed {
 				return n.lit
 			}
-			return expand(s, n)
+			return expand(s, n, refScope)
 		}
 		var b strings.Builder
 		b.Grow(n.repeat * (n.grow + len(n.separator)))
@@ -96,7 +69,7 @@ func render(s *session, n node) string {
 			if i > 0 {
 				b.WriteString(n.separator)
 			}
-			b.WriteString(expand(s, n))
+			b.WriteString(expand(s, n, refScope))
 		}
 		return b.String()
 	default:
@@ -118,11 +91,12 @@ func pick(r rng, c *choice) node {
 
 // expand renders a template's compiled ops. compile validated every token, so this
 // cannot fail.
-func expand(s *session, t *template) string {
+func expand(s *session, t *template, refScope *draws) string {
 	var b strings.Builder
 	b.Grow(t.grow)
 	// One draw per held name, for this expansion only: a nested template and each
-	// repeat iteration get their own, since each is its own expansion.
+	// repeat iteration get their own, since each is its own expansion. A reference
+	// reads the caller's scope instead, whenever one was supplied.
 	var held *draws
 	if len(t.held) > 0 {
 		held = &draws{
@@ -136,7 +110,7 @@ func expand(s *session, t *template) string {
 		case 'l':
 			b.WriteString(o.lit)
 		case 'f':
-			b.WriteString(readField(s, t, held, o.arms[s.IntN(len(o.arms))]))
+			b.WriteString(readField(s, t, held, refScope, o.arms[s.IntN(len(o.arms))]))
 		case 'b':
 			// Read before the call, so the value a calc computes is the value the
 			// format showed. calcVars fixed the order op.operands holds.
@@ -144,7 +118,7 @@ func expand(s *session, t *template) string {
 			if len(o.operands) > 0 {
 				operands = make([]string, len(o.operands))
 				for j, a := range o.operands {
-					operands[j] = readField(s, t, held, a)
+					operands[j] = readField(s, t, held, refScope, a)
 				}
 			}
 			b.WriteString(o.call(s, b.String(), operands)) // b.String() is the output so far
@@ -152,76 +126,3 @@ func expand(s *session, t *template) string {
 	}
 	return b.String()
 }
-
-// draws is what an expansion has already drawn for its held names: the variant each
-// was drawn as, so every path under it reads one row, and the value each read, so
-// the same name read twice reads one value.
-type draws struct {
-	variant map[string]node
-	value   map[string]string
-}
-
-// readField renders one arm of a token. An arm's key is a sibling field or a
-// {/path} reference, which linkRefs bound into fields too. A name the expansion
-// holds — a level some token addresses by dotted path, or a sibling a {calc()}
-// reads — is drawn once and kept, so {place.postal-code} and {place.locality} read
-// one row, either read twice gives one value, and a shown operand is the operand
-// computed. Every other name is drawn afresh, so {word} {word} still draws twice.
-// checkTokens, checkPath and linkRefs prove every step, so this cannot fail.
-func readField(s *session, t *template, held *draws, a arm) string {
-	if !t.held[a.key] {
-		return render(s, t.fields[a.key])
-	}
-	if v, read := held.value[a.name]; read {
-		return v
-	}
-	n, drew := held.variant[a.key]
-	if !drew {
-		n = drawn(s, t.fields[a.key])
-		held.variant[a.key] = n
-	}
-	// Hold the draw at every level passed through, so two paths sharing a prefix
-	// share it.
-	for i, seg := range a.tail {
-		if i < len(a.steps) {
-			step, drew := held.variant[a.steps[i]]
-			if !drew {
-				step = drawn(s, child(n, seg))
-				held.variant[a.steps[i]] = step
-			}
-			n = step
-			continue
-		}
-		n = child(n, seg)
-	}
-	v := render(s, n)
-	held.value[a.name] = v
-	return v
-}
-
-// child is the node one path segment names below an already-drawn node. It holds
-// while checkPath and the set a choice shares (see sharedPaths) agree with this
-// walk: both prove the segment exists and that drawn leaves a template here. Each
-// way that can break panics naming the segment, so a slip in that agreement
-// reports where it happened rather than surfacing a nil node a level later.
-func child(n node, seg string) node {
-	t, ok := n.(*template)
-	if !ok {
-		panic(fmt.Sprintf("fejkdata: %q under %T, which carries no fields", seg, n))
-	}
-	c, ok := t.fields[seg]
-	if !ok {
-		panic(fmt.Sprintf("fejkdata: no field %q under a drawn level", seg))
-	}
-	return c
-}
-
-// drawn resolves a choice to one variant, so a bound head is a concrete node the
-// rest of the expansion shares. Nested choices unwrap too: a draw is one value, not
-// another set to pick from.
-func drawn(s *session, n node) node {
-	for c, ok := n.(*choice); ok; c, ok = n.(*choice) {
-		n = pick(s, c)
-	}
-	return n
-}

shipped_data_test.go

@@ -108,6 +108,16 @@ func TestFakeIsSafeForConcurrentUse(t *testing.T) {
 					return
 				}
 				f.List()
+				if _, err := f.Record("sv_SE.person"); err != nil {
+					t.Error(err)
+					return
+				}
+				tmpl, err := f.NewTemplate("{/sv_SE.person.last}")
+				if err != nil {
+					t.Error(err)
+					return
+				}
+				tmpl.Fake()
 			}
 		}()
 	}

template.go

@@ -2,7 +2,6 @@ package fejkdata
 
 import (
 	"fmt"
-	"sort"
 	"strings"
 )
 
@@ -16,7 +15,8 @@ type ftoken struct {
 
 // eachToken scans a format string once and calls fn for each unit, the single
 // source of truth for how braces are read: "{{" and "}}" are literal braces, a "{"
-// opens a token that must reach its "}", and a lone "}" is an error.
+// opens a token that must reach its "}", and a lone "}" is an error. A table-shaped
+// scanner, one case per rune kind, kept whole on purpose.
 func eachToken(format string, fn func(ftoken) error) error {
 	rs := []rune(format)
 	for i := 0; i < len(rs); i++ {
@@ -140,7 +140,7 @@ func checkTokens(format string, fields map[string]node) error {
 				}
 				continue // its target is checked at New (see linkRefs)
 			}
-			if err := checkArm(name, fields); err != nil {
+			if err := checkArm(name, fields, len(names) == 1); err != nil {
 				return fmt.Errorf("token {%s}: %w", t.body, err)
 			}
 		}
@@ -151,7 +151,9 @@ func checkTokens(format string, fields map[string]node) error {
 }
 
 // checkArm validates one sibling name or path against a template's fields.
-func checkArm(name string, fields map[string]node) error {
+// wholeToken says the name is the token's entire body, so {/name} would render
+// the same value and can be offered as the reference spelling.
+func checkArm(name string, fields map[string]node, wholeToken bool) error {
 	a := splitArm(name, nil)
 	if err := checkSegments(a); err != nil {
 		return err
@@ -164,6 +166,13 @@ func checkArm(name string, fields map[string]node) error {
 		if isOption(a.key) {
 			return fmt.Errorf("%q is an option and can never be a field", a.key)
 		}
+		if len(fields) == 0 {
+			hint := ""
+			if wholeToken && hintableRef(name) {
+				hint = fmt.Sprintf(" — write {/%s} to reference the data", name)
+			}
+			return fmt.Errorf("no field %q; a token names a sibling field, and this template has none%s", a.key, hint)
+		}
 		return fmt.Errorf("no field %q", a.key)
 	}
 	if err := checkPath(head, a.tail, a.key); err != nil {
@@ -172,6 +181,17 @@ func checkArm(name string, fields map[string]node) error {
 	return nil
 }
 
+// hintableRef reports whether {/name} is a reference the grammar accepts, so the
+// hint never names a spelling that fails too.
+func hintableRef(name string) bool {
+	for _, seg := range strings.Split(name, ".") {
+		if checkName(seg) != nil {
+			return false
+		}
+	}
+	return true
+}
+
 // tokenOperands lists the fields one {token} body reads as operands, empty for a
 // field token or a builtin that reads none.
 func tokenOperands(body string) []string {
@@ -235,10 +255,11 @@ func fieldTokens(format string) []string {
 // tail is what makes the arm a bound draw: its head is drawn once per expansion
 // (see compileOps).
 type arm struct {
-	name  string // as written, and the key a bound draw's value is held under
+	name  string // as written, for messages
 	key   string
 	tail  []string
 	steps []string // key per level passed through; the head and leaf hold their own
+	path  string   // key and tail, the one spelling every way of writing this read shares
 }
 
 // splitArm splits one name into key and tail. refs maps a reference to what
@@ -251,13 +272,13 @@ func splitArm(name string, refs map[string]refBinding) arm {
 			if bound {
 				key = b.key
 			}
-			return arm{name: name, key: key}
+			return arm{name: name, key: key, path: key}
 		}
 		return pathArm(name, b.key, b.tail)
 	}
 	head, tail, dotted := strings.Cut(name, ".")
 	if !dotted {
-		return arm{name: name, key: name}
+		return arm{name: name, key: name, path: name}
 	}
 	return pathArm(name, head, strings.Split(tail, "."))
 }
@@ -267,62 +288,21 @@ func pathArm(name, key string, segs []string) arm {
 	for i := 0; i < len(segs)-1; i++ { // every level except the leaf's own
 		steps = append(steps, key+"."+strings.Join(segs[:i+1], "."))
 	}
-	return arm{name: name, key: key, tail: segs, steps: steps}
+	return arm{name: name, key: key, tail: segs, steps: steps, path: key + "." + strings.Join(segs, ".")}
 }
 
-// checkNoOverlap rejects a format that both renders a level and reads a path into
-// it — {p} beside {p.first}, or {p.addr} beside {p.addr.city}. The path reads the
-// level's held draw while rendering the level expands it afresh, so their values
-// would disagree. Names are compared in sorted order, so which pair is reported
-// does not depend on where the tokens sit.
-func checkNoOverlap(format string, bound map[string]string, refs map[string]refBinding) error {
-	names := boundReaders(format, bound, refs)
-	// Stable over one format-order scan, so two readers of one name (a token and a
-	// calc operand both naming "p") are reported as the format writes them.
-	sort.SliceStable(names, func(i, j int) bool { return names[i].name < names[j].name })
-	for i, level := range names {
-		for _, path := range names[i+1:] {
-			if strings.HasPrefix(path.name, level.name+".") {
-				return fmt.Errorf("%s renders a level that {%s} reads a path into; name the fields you want instead", level.label, path.name)
+// splitArms splits a token body's '|' alternatives.
+func splitArms(body string, refs map[string]refBinding) []arm {
+	parts := strings.Split(body, "|")
+	arms := make([]arm, len(parts))
+	for i, p := range parts {
+		arms[i] = splitArm(p, refs)
 	}
-		}
-	}
-	return nil
+	return arms
 }
 
-// reader is one way a format reaches a bound field, and how to name that spelling.
-type reader struct{ name, label string }
-
-// boundReaders lists every way a format reaches a bound field, in the order the
-// format writes them. An operand renders its field, so it names a level exactly
-// as a token does; one scan finds both, which is what puts them in one order.
-func boundReaders(format string, bound map[string]string, refs map[string]refBinding) []reader {
-	var names []reader
-	_ = eachToken(format, func(t ftoken) error {
-		if t.kind != 'b' {
-			return nil
-		}
-		if fn, _, isFunc := funcCall(t.body); isFunc {
-			for _, operand := range tokenOperands(t.body) {
-				a := splitArm(operand, refs)
-				if _, isBound := bound[a.key]; isBound {
-					names = append(names, reader{a.name, fmt.Sprintf("%s operand %q", fn, operand)})
-				}
-			}
-			return nil
-		}
-		for _, a := range splitArms(t.body, refs) {
-			if _, isBound := bound[a.key]; isBound {
-				names = append(names, reader{a.name, "token {" + a.name + "}"})
-			}
-		}
-		return nil
-	})
-	return names
-}
-
-// checkSegments rejects an unfinished path: "{a.}", "{.b}" and "{a..b}" each have
-// a segment naming nothing. A field really named "" would otherwise make them
+// checkSegments rejects an unfinished path: "{a.}" and "{a..b}" each have a
+// segment naming nothing. A field really named "" would otherwise make them
 // resolve, so a typo would read as a path that worked.
 func checkSegments(a arm) error {
 	if len(a.tail) == 0 {
@@ -339,16 +319,6 @@ func checkSegments(a arm) error {
 	return nil
 }
 
-// splitArms splits a token body's '|' alternatives.
-func splitArms(body string, refs map[string]refBinding) []arm {
-	parts := strings.Split(body, "|")
-	arms := make([]arm, len(parts))
-	for i, p := range parts {
-		arms[i] = splitArm(p, refs)
-	}
-	return arms
-}
-
 // callFn is a builtin bound to one call site: its args already parsed. It reads the
 // output emitted so far in the current expansion (a derivation's payload) and the
 // values of the operands it named, which expand read for it.
@@ -450,27 +420,3 @@ func compileOps(format string, refs map[string]refBinding) formatOps {
 	flush()
 	return c
 }
-
-// checkNoRepeatedRead rejects a bare token repeated on a held name: {w} {w} beside
-// {uppercase(w)} would read one draw twice, where {w} {w} alone draws twice. The
-// error names the single-token spelling.
-func checkNoRepeatedRead(format string, c formatOps, refs map[string]refBinding) error {
-	count := map[string]int{}
-	return eachToken(format, func(t ftoken) error {
-		if t.kind != 'b' {
-			return nil
-		}
-		if _, _, isFunc := funcCall(t.body); isFunc {
-			return nil
-		}
-		for _, a := range splitArms(t.body, refs) {
-			if len(a.tail) > 0 || !c.held[a.key] {
-				continue
-			}
-			if count[a.key]++; count[a.key] > 1 {
-				return fmt.Errorf("token {%s} is repeated, and %s holds %q to one draw per expansion; write {%s} once", a.name, c.holder[a.key], a.key, a.name)
-			}
-		}
-		return nil
-	})
-}

template_test.go

@@ -8,7 +8,13 @@ import (
 )
 
 // engine builds a seeded generator with no loaded categories, for rendering tests.
-func engine(seed uint64) *Generator { return &Generator{rand: newRand(seed, true)} }
+func engine(seed uint64) *Generator {
+	s, err := newRand(seed, true)
+	if err != nil {
+		panic(err)
+	}
+	return &Generator{rand: s}
+}
 
 // parse unmarshals a JSON template fragment into its dynamic form.
 func parse(t *testing.T, s string) any {
@@ -32,7 +38,7 @@ func compiled(t *testing.T, s string) node {
 
 func mustRender(t *testing.T, f *Generator, s string) string {
 	t.Helper()
-	return render(f.rand, compiled(t, s))
+	return render(f.rand, compiled(t, s), nil)
 }
 
 func TestStringIsAFormat(t *testing.T) {
@@ -238,6 +244,18 @@ func TestCompileErrors(t *testing.T) {
 		`"{base64(0)}"`,        // count must be positive
 		`"{float(1,2)}"`,       // wrong arity
 		`"{float(1,2,-1)}"`,    // negative decimals
+		`"{float(NaN,NaN,2)}"`, // bounds must be finite
+		`"{float(Inf,Inf,2)}"`, // same-sign infinities
+		`"{float(1,NaN,2)}"`,   // one NaN bound
+		`"{float(-Inf,1,2)}"`,  // one infinite bound
+		`"{digits(+5)}"`,       // a count is a plain integer
+		`"{digits(05)}"`,       // no leading zero
+		`"{int(+1,5)}"`,        // a bound is a plain integer
+		`"{int(5,5)}"`,         // a constant is written as text
+		`"{float(1,1,2)}"`,     // a constant is written as text
+		`{"format":"{x}","x":"v","repeat":2,"separator":""}`, // separator "" is the default
+		`"{calc(1/0)}"`, // a constant zero divisor
+		`{"format":"{calc(x/y)}","x":"1","y":"0"}`, // a fixed zero divisor
 		`"{iban(US)}"`,      // unsupported country
 		`"{seq(a,b)}"`,      // seq takes at most one name
 		`"{calc()}"`,        // calc needs an expression
@@ -297,7 +315,7 @@ func TestGrowIsALowerBound(t *testing.T) {
 			t.Fatalf("format %q did not compile to a template", format)
 		}
 		for i := 0; i < 50; i++ {
-			if got := len(expand(f.rand, tmpl)); got < tmpl.grow {
+			if got := len(expand(f.rand, tmpl, nil)); got < tmpl.grow {
 				t.Errorf("format %q: expand emitted %d bytes, below grow %d", format, got, tmpl.grow)
 			}
 		}
@@ -311,3 +329,53 @@ func quote(s string) string {
 	}
 	return string(b)
 }
+
+func TestHashIsLiteral(t *testing.T) {
+	f := engine(1)
+	cases := map[string]string{
+		`{"format":"","x":"v"}`:         "",
+		`{"format":"#","x":"v"}`:        "#",
+		`{"format":"##","x":"v"}`:       "##",
+		`{"format":"#0#1#A#a","x":"v"}`: "#0#1#A#a",
+		`{"format":"#{x}","x":"v"}`:     "#v",
+	}
+	for tmpl, want := range cases {
+		if got := mustRender(t, f, tmpl); got != want {
+			t.Errorf("render(%s) = %q, want %q", tmpl, got, want)
+		}
+	}
+}
+
+func TestMultibyteFormat(t *testing.T) {
+	// Scanning is rune-aware: multibyte literals coexist with class chars and
+	// tokens without corrupting indices.
+	got := mustRender(t, engine(2), `{"format":"Öster{x}-{digits(1)}å","x":"väg"}`)
+	if !regexp.MustCompile(`^Österväg-[0-9]å$`).MatchString(got) {
+		t.Fatalf("multibyte format = %q", got)
+	}
+}
+
+func TestAlternationThreeWay(t *testing.T) {
+	f := engine(4)
+	seen := map[string]bool{}
+	for i := 0; i < 200; i++ {
+		seen[mustRender(t, f, `{"format":"{a|b|c}","a":"A","b":"B","c":"C"}`)] = true
+	}
+	if !seen["A"] || !seen["B"] || !seen["C"] || len(seen) != 3 {
+		t.Fatalf("3-way alternation produced %v, want A, B and C", seen)
+	}
+}
+
+func TestArgErrorsNameTheSpelling(t *testing.T) {
+	for src, want := range map[string]string{
+		`"{float(1,2,02)}"`:                 "write 2",
+		`{"format":"{calc(a,02)}","a":"1"}`: "write 2",
+		`"{digits(+5)}"`:                    "write 5",
+		`"{hex(99999999999999999999)}"`:     "exceeds the maximum",
+		`"{int(007,9)}"`:                    "write 7",
+	} {
+		if _, err := compile(parse(t, src)); err == nil || !strings.Contains(err.Error(), want) {
+			t.Errorf("compile(%s) = %v, want an error saying %q", src, err, want)
+		}
+	}
+}

transform_test.go

@@ -51,3 +51,9 @@ func TestTransformArgs(t *testing.T) {
 		}
 	}
 }
+
+func TestAsciiKeepsDEL(t *testing.T) {
+	if got := mustRender(t, engine(1), `{"format":"{ascii(x)}","x":"a\u007fb"}`); got != "a\u007fb" {
+		t.Errorf("ascii over DEL = %q, want it kept: DEL is ASCII", got)
+	}
+}