476 lines
23 KiB
Markdown
476 lines
23 KiB
Markdown
# fejkdata
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A Go library and CLI for generating locale-aware fake data from JSON templates.
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Forked from [github.com/Timewave-AB/fakes](https://github.com/Timewave-AB/fakes).
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## Goals
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1. **Valid by construction** — every value passes the check its real consumer
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applies; facts that belong together come from one draw, within a value and
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across categories.
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2. **Text means what it says** — a format renders as written; only `{…}` varies,
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random characters included (`{digits(3)}`). One spelling per result; the wrong
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one is a load error naming the right one.
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3. **Every mistake is a load error** — `New` rejects; `Fake` on a loaded generator
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fails only for an unknown path.
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4. **Zero to a value in one command** — `go install`, then `fejkdata sv_SE.person`:
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no checkout, no flag. Flags are GNU-form (`--seed 42`, `-n 3`) in any position;
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the first custom template needs no escape and no option.
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5. **Data lives in JSON** — a builtin only for what data can't express.
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6. **Reproducible** — seed in, same stream out; no builtin reads a clock.
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7. **Zero dependencies** — standard library only.
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8. **Docs index the grammar** — every syntax feature is a heading; every example
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runs under test and shows its output; a rule is stated once.
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## CLI
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```sh
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go install gitea.larvit.se/larvit/fejkdata/cmd/fejkdata@latest
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fejkdata sv_SE.person # Sara Eriksson
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fejkdata sv_SE.person.last # Eriksson
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fejkdata --seed 42 sv_SE.address # the same address every run
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fejkdata -n 3 --separator ', ' sv_SE.word # nät, barn, sol
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fejkdata --list # every path the data offers
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fejkdata --data-path ./mydata sv_SE.word # layer a directory over the shipped data
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fejkdata --no-shipped-data -d ./mydata --list # only your data
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fejkdata 'name: {/sv_SE.person.last}' # name: Eriksson (an inline template)
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fejkdata '{"format":"name: {x}","x":["bosse","lina"]}' # name: bosse
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```
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A path names a category, or a field inside one: each dot segment descends one
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level — folders, then the category (a JSON file), then fields. An argument that is
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a JSON object or array, or that carries a `{` token, is instead an **inline
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template**: a format string or a JSON value compiled and rendered on the spot. Its
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tokens reach the data by reference from the root — `{/sv_SE.person.last}` (or
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`{.name}`, which means the same here), so shipped and `--data-path` categories are
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available — and `{..name}` is rejected, an inline template having no folder to step
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up from. A path never contains a brace, so the two cannot collide (see
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[Decisions](#decisions)).
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| Flag | |
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|------|--|
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| `-d`, `--data-path D` | a directory to layer over the shipped data; repeatable, the last wins a name clash |
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| `--no-shipped-data` | load only the `--data-path` directories |
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| `-s`, `--seed N` | reproducible output |
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| `-n`, `--repeat N` | render the value N times (up to 1048576), each an independent draw, streamed |
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| `--separator S` | between repeated values (default a newline) |
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| `--list` | print every path, then exit |
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| `--version`, `-h`, `--help` | print, then exit |
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`--name value` and `--name=value` both work, a short flag's value attaches or
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follows (`-n3`, `-n 3`) and short flags bundle (`-hn 3`) — see
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[Decisions](#decisions); flags go anywhere, `--` ends them. Exit codes: `0` success, `1` runtime error (missing
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dir, unknown path), `2` misuse. From a checkout: `go run ./cmd/fejkdata …`.
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### Your own data
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A category is a JSON file in a directory. Save this as `mydata/sql.json`:
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```json
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{
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"format": "INSERT INTO users VALUES({sql-username});",
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"sql-username": {
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"format": "'{username}'",
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"repeat": 3,
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"separator": "),(",
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"username": ["pixelfox", "snork", "turbohund", "blip", "zoom", "wahoo"]
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}
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}
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```
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```sh
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fejkdata --seed 1 --data-path ./mydata sql
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# INSERT INTO users VALUES('zoom'),('wahoo'),('blip');
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fejkdata --repeat 100 --data-path ./mydata sql > seed.sql
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```
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## Library
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```sh
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go get gitea.larvit.se/larvit/fejkdata # Go 1.22+
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```
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```go
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f, err := fejkdata.New(fejkdata.WithSeed(42))
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if err != nil {
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log.Fatal(err)
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}
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v, err := f.Fake("sv_SE.address") // "Kungsvägen 68\n379 17 Stockholm"
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paths := f.List() // every path Fake accepts, sorted
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v, err = f.FakeTemplate("name: {/sv_SE.person.last}") // an inline template
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v, err = f.FakeTemplate(`{"format":"name: {x}","x":["bosse","lina"]}`)
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```
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| Option | |
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|--------|--|
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| `WithSeed(n)` | same seed, same data: identical sequence |
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| `WithDataPath(dir)` | layer a directory; repeat to layer several, the last wins a clash |
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| `WithDataFS(fsys)` | layer an `fs.FS`, such as your own `embed.FS` |
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| `WithoutShippedData()` | load only what you give |
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A `*Generator` is safe for concurrent use; a seeded sequence is reproducible only
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when drawn from one goroutine. Changing how a value is composed shifts the seeded
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stream for that value and everything drawn after it.
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## Data
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The shipped set under [`data/`](data) — one folder per locale (`en_US`, `sv_SE`)
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plus a locale-neutral `misc` folder — is embedded, so the CLI and the library
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work with no data on disk. A directory is a namespace: each JSON file is a
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category named after the file, each subdirectory a dot-path segment, so
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`mydata/sv_SE/person.json` is `sv_SE.person` and replaces the shipped one.
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Sources merge in order; matching folders combine, any other clash is won by the
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last loaded. Names may not use `.`, `|`, `(`, `{`, `}` or `/`; dot-prefixed entries
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are skipped, so a data directory can also be a checkout.
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Each locale carries `address`, `color`, `company`, `date`, `email`, `ip`,
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`person`, `phone`, `price`, `sentence`, `ssn`, `time`, `url`, `username`,
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`version` and `word`, formatted per locale. `misc` carries `car`, `coordinate`,
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`country` (ISO 3166), `creditcard` (Luhn-valid), `currency` (ISO 4217), `emoji`,
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`httpstatus`, `language` (ISO 639), `mac`, `mimetype`, `objectid`, `timezone`
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(IANA), `useragent` and `uuid` (v4). Many carry sub-fields — `misc.currency.symbol`,
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`misc.country.alpha2`, `misc.httpstatus.code` — which `--list` shows.
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## Data format
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Every value is a **node**, nestable without limit:
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| Node | JSON | Renders |
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|------|------|---------|
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| string | `"Malmö"` | its text, with any `{…}` tokens expanded |
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| choice | `["a", "b", …]` | one item, picked at random |
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| template | `{"format": "…", …}` | its format, with `{name}` tokens rendering the named fields |
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### Format string
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Every character is literal except a `{…}` token:
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| Token | Renders |
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|-------|---------|
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| `{name}` | the sibling field `name` |
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| `{name.field}` | `field` of one draw of `name` ([Correlated fields](#correlated-fields)) |
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| `{a\|b}` | one of the named fields, even odds |
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| `{fn(args)}` | a builtin ([Functions](#functions)) |
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| `{/path}`, `{.path}`, `{..path}` | a node reached from the data root, this file's folder, or the folder above ([References](#references)) |
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| `{{`, `}}` | a literal `{` or `}` |
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```json
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"100 Main St, Apt {int(1,9)}{upper(1)} — tel {digits(3)}-{digits(4)}"
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```
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Renders e.g. `100 Main St, Apt 4B — tel 555-0199`. Random characters come from
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the sample functions, so a phone pattern is `070-{digits(3)} {digits(2)} {digits(2)}`.
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A lone `}` is a load error naming `}}`; the arms of `{a|b}` must differ, a
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repeated arm being a second spelling of [weight](#weight).
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### Weight
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An item of a choice may carry a `weight` (default `1`) to skew its odds:
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```json
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[
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{ "format": "070-{digits(3)} {digits(2)} {digits(2)}", "weight": 10 },
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"08-{digits(3)} {digits(2)} {digits(2)}",
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{ "format": "010-{digits(3)} {digits(2)} {digits(2)}", "weight": 2 }
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]
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```
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Renders `070-412 38 91` ten times as often as `08-…`. A string item is weighted by
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writing it as `{ "format": "AB", "weight": 3 }`. Rejected at load: a weight that
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is negative, non-numeric, `0` (never drawn — remove the item), `1` (the default)
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or outside a choice, and a repeated item — `["a", "a", "b"]` is `[{ "format": "a", "weight": 2 }, "b"]`,
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and the error says so.
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### Repeat
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A template may carry `repeat` (an integer above `1`) to render its format that
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many times — each an independent draw — joined by `separator` (default `""`):
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```json
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{ "format": "{word}", "repeat": 3, "separator": " ", "word": ["foo", "bar", "baz"] }
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```
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Renders e.g. `bar foo baz`. Rejected at load: a `separator` without a `repeat`,
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a `separator` of `""` (the default), and a `repeat` that multiplies to more than
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1 048 576 renders along any path of nested repeats.
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### Options and fields
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`format`, `weight`, `repeat` and `separator` are the only options; **any other
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key is a field** (see [Decisions](#decisions)). An object that does nothing a
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string can't — only a `format` — is rejected naming the string, as is a one-item
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choice naming its item.
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### Functions
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A `{name(args)}` token calls a builtin. Arguments are checked at `New`: a bad
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count, range, country or expression fails fast; an integer is written plain
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(`5`, not `+5` or `05`); bounds are finite; a sample that could only ever emit one
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value (`int(5,5)`, `float(1,1,2)`) is rejected naming the text to write instead;
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and a length, count or decimal place beyond a sane maximum is rejected, so a
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fat-fingered `hex(2000000000)` never tries to allocate gigabytes. Every builtin draws only from the seed — a
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time-based id takes its timestamp from the rng, not the clock — so seeded output
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stays reproducible.
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| Function | Kind | Renders |
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|----------|------|---------|
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| `{luhn()}` | derivation | Luhn check digit over the digits emitted so far in this expansion |
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| `{mod11()}` | derivation | weighted mod-11 check char (weights 2–7 from the right); `X` for 10 |
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| `{ean()}` | derivation | EAN-13 / UPC-A / ISBN-13 / GTIN check digit |
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| `{digits(n)}` | sample | `n` digits 0–9 |
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| `{upper(n)}`, `{lower(n)}` | sample | `n` letters A–Z, a–z |
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| `{int(min,max)}` | sample | uniform integer in `[min, max]` |
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| `{float(min,max,dp)}` | sample | number in `[min, max]` with `dp` decimals |
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| `{hex(n)}` | sample | `n` lowercase hex digits |
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| `{base64(n)}` | sample | `n` random bytes, base64 |
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| `{uuid()}` | sample | UUID v7 (`misc.uuid` ships v4 as data) |
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| `{ulid()}` | sample | ULID, 26 Crockford base32 chars |
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| `{nanoid(n)}` | sample | URL-safe Nano ID, `n` chars |
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| `{iban(CC)}` | sample | length- and mod-97-valid IBAN for BE, DE, DK, ES, FI, NO or SE |
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| `{seq()}`, `{seq(name)}` | counter | next integer from 1 in this generator; `name` selects an independent counter |
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| `{calc(expr)}`, `{calc(expr,dp)}` | computation | an arithmetic expression over sibling fields ([Computation](#computation)) |
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| `{lowercase(x)}`, `{uppercase(x)}`, `{ascii(x)}` | transform | a field's value rewritten ([Transforms](#transforms)) |
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A derivation reads what is to its left, so place it after its payload; the
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buffer is per expansion, so a nested template keeps fixed parts out of the sum. A
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Swedish personnummer is a Luhn checksum over the nine digits before it:
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```json
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{ "format": "{century}{core}", "century": ["19", "20"],
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"core": { "format": "{digits(2)}{mmdd}-{digits(3)}{luhn()}", "mmdd": ["0115", "0704", "1218"] } }
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```
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Renders e.g. `19811218-9876`. `{seq()}` spans `Fake` calls and `repeat`, resets
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with a new generator, and is the natural primary key for the SQL example above.
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### Computation
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`{calc(expr)}` evaluates `+ - * /`, parentheses and unary minus over number
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literals and sibling field names, each rendered then read as a number; a second
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argument rounds to that many decimals. A hyphen is always subtraction, so a
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hyphenated field can't be an operand.
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```json
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{ "format": "{net} x {qty} = {calc(net * qty, 2)}", "net": ["19.99", "5.00"], "qty": ["3", "7"] }
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```
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Renders e.g. `19.99 x 3 = 59.97`. An operand that can never be a number (`"abc"`,
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or a choice of such) is rejected at load, as is a division by a constant zero
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(`1/0`, or a fixed `"0"` field); an operand that sometimes is not a number yields
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`NaN`, and a division by one that is not constant `Inf` — both print rather than
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fail.
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### Transforms
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`{lowercase(x)}`, `{uppercase(x)}` and `{ascii(x)}` rewrite the value of `x` — a
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field, a path or a `..path` — and nest. `ascii` folds Latin letters (`Åsa Öberg`
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→ `Asa Oberg`) and drops any other non-ASCII rune. `x` is held
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([One draw, one spelling](#one-draw-one-spelling)), so an email built from a name
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matches the name beside it:
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```json
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{ "format": "{p.first} {p.last} <{lowercase(ascii(p.first))}.{lowercase(ascii(p.last))}@example.com>",
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"p": [
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{ "format": "{first} {last}", "first": "Åsa", "last": "Öberg" },
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{ "format": "{first} {last}", "first": "Bo", "last": "Ek" }
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] }
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```
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Renders `Åsa Öberg <asa.oberg@example.com>` or `Bo Ek <bo.ek@example.com>`, never
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a mix.
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### References
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A reference renders a node from elsewhere in the data — the path `Fake` takes,
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across every loaded source — so one category borrows another. The sigil says
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where the path starts, as in a filesystem: `{/en_US.person}` from the data root,
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`{.username}` from the folder this file sits in, `{..username}` from the folder
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above. `data/sv_SE/email.json` can therefore read its own locale's `username`
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without naming `sv_SE`:
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```json
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"Hej, {/en_US.person}!"
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```
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Renders e.g. `Hej, Pat Smith!`. A reference into a category is held like a
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[correlated](#correlated-fields) path — `{.person.first} {.person.last}` name one
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person, `{lowercase(.person.first)}` reads that same draw, and `{.person.first}`
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beside `{/sv_SE.person.last}` in `sv_SE` is one person too — while a bare
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`{/misc.uuid} {/misc.uuid}` is two draws. Rejected at `New`: a path that is
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unknown, names a folder, has no folder above, or reads a field not every variant
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of a choice carries, and a reference that leads back to its own value, directly,
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mutually or through a chain.
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### Correlated fields
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`{name.field}` reads a path into a sibling, which holds the sibling to one draw
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([One draw, one spelling](#one-draw-one-spelling)), so several tokens read one
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row — a locality and the postal code that really covers it:
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```json
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{ "format": "{street} {int(1,99)}\n{place.postal-code} {place.locality}",
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"street": ["Kungsgatan", "Storgatan"],
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"place": [
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{ "format": "{locality}", "locality": "Stockholm", "postal-code": "1{digits(2)} {digits(2)}", "weight": 975 },
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{ "format": "{locality}", "locality": "Tranås", "postal-code": "573 {digits(2)}", "weight": 18 }
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] }
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```
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Renders e.g. `Kungsgatan 35` / `176 99 Stockholm`, never a Stockholm code beside
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Tranås; each row's `weight` says how often it appears. A path is held at every
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level it passes through: `{p.geo.town.name} {p.geo.town.zip}` share the town.
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Every variant of a choice on the path must carry the rest of it, so a row missing
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a field is named at load:
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```text
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token {place.postal-code}: field "place": not every variant of this 2-way choice carries "postal-code"; all carry [locality]
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```
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The sub-fields stay addressable — `Fake("address.place.locality")` renders, and
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`List` advertises it. A path may not read into a level carrying a `repeat`.
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### One draw, one spelling
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A name any token reads as a path (`{p.first}`) or as an operand (`{calc(net * 2)}`,
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`{uppercase(w)}`) is drawn **once per expansion**, and every other route to it —
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a bare `{p}`, a second bare `{w}`, `{/cat.net}`, a nested template rendering
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`{/cat.p.last}`, at any depth — is a load error naming the spelling to use. A
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name nothing reads that way is drawn each time: `{word} {word}` differs. An
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expansion is one render of one format, so each `repeat` iteration and each nested
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template draws again.
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```text
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token {p} renders a level that {p.first} reads a path into; name the fields you want instead
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token {w} is repeated, and uppercase operand "w" holds "w" to one draw per expansion; write {w} once
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```
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### Performance
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Each file is parsed, validated and weight-indexed once, in `New`. A `Fake` call
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then costs about what its output costs: an unweighted pick is O(1) whatever the
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list's length, a weighted one O(log n), and long formats, deep nesting and many
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tokens add cost in proportion to the output.
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## Decisions
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- **Options and fields share one namespace.** `format`, `weight`, `repeat` and
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`separator` are reserved; every other key is a field. Nesting fields under a
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key, or prefixing options, would tax every template to guard against a
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misspelt option.
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- **`{a|b}` stays beside nested choices.** `[[…], […]]` picks the same way, but
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its arms are anonymous; `{femalefirst|malefirst}` keeps `person.femalefirst`
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addressable.
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- **Flags follow getopt_long.** `--name value` and `--name=value` both work; a
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short flag's value attaches or follows (`-s42`, `-s 42`) and short flags bundle
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(`-hn 3`), as every shell user expects. A single-dash long flag is rejected
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naming the double-dash spelling, and `-s=42` is rejected naming both short
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spellings: `=` belongs to the long form, and reading `=42` as the value would
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make `-d=./x` a directory named `=./x`.
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- **An argument is a template by its shape, not by a flag.** A JSON object or
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array, or a string carrying a `{` token, is an inline template; anything else is
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a path. A path can never contain a brace — a name may not use one — so the two
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never collide, and no `--template` flag is needed to disambiguate them.
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- **The shipped data is embedded, not discovered.** A directory a machine happens
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to have would make `--seed 42` machine-dependent. Data still lives in `data/`
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as JSON; `--data-path` layers over it.
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- **A bare reference draws each time; a reference path is held.** `{/p} {/p}`
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is two draws, as `{word} {word}` is, while `{/p.first}` beside a nested template
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rendering `{/p.first}` is a load error: a bare token is by contract an
|
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independent draw, a path pins its level, and any route into a pinned level from
|
||
another expansion could show another row.
|
||
- **Reference sigils follow the filesystem.** `/` is the root, `.` this file's
|
||
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.
|
||
|
||
## Development
|
||
|
||
Everything runs in Docker — **no local tooling beyond Docker is needed**.
|
||
Source is bind-mounted; build caches persist in the `gocache` volume.
|
||
|
||
```sh
|
||
docker compose run --rm test # go test -race
|
||
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
|
||
```
|
||
|
||
Commands that rewrite source keep your file ownership when run with `--user`:
|
||
|
||
```sh
|
||
docker compose run --rm --user "$(id -u):$(id -g)" fmt # gofmt -w .
|
||
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, complexity, format check and tests — so run it locally before pushing:
|
||
|
||
```sh
|
||
docker build . # latest
|
||
docker build --build-arg GO_VERSION=1.22.12 . # lowest supported
|
||
GO_VERSION=1.22.12 docker compose run --rm test # the same tests, without the image build
|
||
```
|
||
|
||
## Layout
|
||
|
||
```
|
||
fejkdata.go Generator, New, options, the embedded data set, List
|
||
node.go the node model and JSON -> node compilation
|
||
path.go the dotted-path walk, and proving a path resolves
|
||
render.go Fake and the recursive renderer (choices, format strings, expansions)
|
||
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
|
||
cmd/fejkdata/ the fejkdata CLI
|
||
data/ shipped data (JSON), embedded at build: locale folders + a misc folder
|
||
```
|
||
|
||
## License
|
||
|
||
MIT — see [LICENSE](LICENSE).
|