Updates from reviewers

.gitignore

@@ -1,3 +1,3 @@
-# Coverage output
+.claude
 *.out
 todo.md

README.md

@@ -38,14 +38,18 @@ fakes -data-path ./data/sv_SE -data-path ./mydata word  # layer dirs; the last w
 fakes -seed 42 -data-path ./data/sv_SE address
 fakes -repeat 3 -data-path ./data/sv_SE person             # three values, one per line
 fakes -repeat 3 -separator ', ' -data-path ./data/sv_SE word  # nät, barn, sol
+fakes -data-path ./data/sv_SE -list                        # every path this data offers
 ```
 
-`-data-path` is repeatable (last wins a name clash) and the path comes last, after
+`-data-path` is repeatable (last wins a name clash) and the path comes last —
-the flags. `-repeat N` renders the path N times — each an independent draw — joined
+all flags must precede it. `-repeat N` renders the path N times — each an
-by `-separator` (default a newline, so values land one per line).
+independent draw — joined by `-separator` (default a newline, so values land one
+per line). Not sure what a data set offers? `-list` prints every path you can ask
+for; `-version` prints the build version.
 
 Without installing, run it from a checkout with `go run ./cmd/fakes …`. Exit
-codes: `0` success, `1` runtime error (missing dir, unknown path), `2` misuse.
+codes: `0` success (including `-list`, `-version`, `-h`), `1` runtime error
+(missing dir, unknown path), `2` misuse.
 
 ### Generating a file from a custom template
 
@@ -136,6 +140,9 @@ bv, _ := b.Fake("person")
 av == bv // true
 ```
 
+`f.List()` returns the sorted paths the loaded data offers — the categories, their
+dotted fields and folder segments (what the CLI's `-list` prints).
+
 A `*Fakes` is **not** safe for concurrent use — create one per goroutine.
 
 ## Data
@@ -248,7 +255,9 @@ There are four kinds. **Derivations** read the digits emitted so far, so put the
 after their payload; **generators** read only the rng, so they stand alone; one
 **session counter** (`seq`) advances state held on the faker; and one
 **computation** (`calc`) evaluates arithmetic over sibling fields. Arguments are
-validated at `New` (a bad count, range, country, or expression fails fast).
+validated at `New` (a bad count, range, country, or expression fails fast); a
+length, count or decimal place beyond a sane maximum is rejected there too, so a
+fat-fingered `hex(2000000000)` can't try to allocate gigabytes at render.
 
 | Function | Kind | Emits |
 |----------|------|-------|
@@ -292,7 +301,8 @@ hyphenated field name can't be an operand. The expression is checked at `New`
 renders `19.99 x 3 = 59.97`. Each name is rendered where it appears, so a field
 shown *and* used in a `calc` is drawn twice — keep a shared operand in a
 single-value field if the two must agree. A field that doesn't render to a number
-yields `NaN`, which prints rather than failing the render.
+yields `NaN`, and a division by zero yields `Inf`; both print rather than failing
+the render.
 
 **References.** A `{..path}` token renders a node from the **data root** instead
 of a sibling field — the dot path is the one `Fake` takes, resolved across every
@@ -305,8 +315,9 @@ data dirs:
 
 renders e.g. `Hej, Pat Smith!`. References are bound when you create the faker, so
 a path that is unknown, names a folder, or steps through a multi-variant choice
-fails at `New`. A reference must not lead back to its own value (directly or
+fails at `New`. A reference that leads back to its own value (directly, mutually,
-through a chain), or rendering won't terminate.
+or through a chain) is a cycle that would never finish rendering, so it too is
+rejected at `New`.
 
 **Format string.** Every character is literal except:
 
@@ -324,6 +335,11 @@ through a chain), or rendering won't terminate.
 `{a|b}` renders one of the sibling fields `a` or `b`, chosen at random; an arm
 may be a `{..path}` reference too (`{name|..en_US.person}`).
 
+Inside a `format`, `0 1 A a` are **always** character classes — so a fixed digit
+or letter must be escaped (`#1`, `#A`) or it becomes random. A format of
+`100 Main St` renders e.g. `506 mdin street`, not `100 Main St`. For a value with
+no tokens at all, use a bare string node (`"100 Main St"`), emitted verbatim.
+
 **Putting it together** (`person.json`):
 
 ```json
@@ -396,12 +412,15 @@ docker compose run --rm test                   # latest
 ## Layout
 
 ```
-fakes.go        Fakes, New, options, seeding
+fakes.go        Fakes, New, List, options, seeding
-template.go     Fake, the recursive renderer (choices, format strings, paths)
+node.go         the node model and JSON -> node compilation
+render.go       Fake and the recursive renderer (choices, format strings, paths)
+template.go     the {token} grammar: scanning, function tokens, validation
+reference.go    {..path} binding across the tree, and cycle detection
 builtins.go     the {name()} function registry and its implementations
 calc.go         the {calc()} arithmetic evaluator: parser, eval, validation
 data.go         data loading: folders/files -> namespace tree, multi-path merge
-cmd/fakes/      the `fakes` CLI (New + Fake over stdout)
+cmd/fakes/      the `fakes` CLI (New + Fake/List over stdout)
 data/           shipped data (JSON): locale folders + a misc folder
 ```
 

builtins.go

@@ -3,10 +3,19 @@ package fakes
 import (
 	"encoding/base64"
 	"fmt"
+	"math"
 	"strconv"
 	"strings"
 )
 
+// maxLen caps generator output lengths (hex, nanoid, base64) and 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.
+const (
+	maxLen      = 1 << 20 // 1,048,576 chars/bytes
+	maxDecimals = 1024
+)
+
 // builtins is the registry of {name(args)} functions. Two kinds: derivations read
 // the digits emitted so far in the current expansion (luhn, mod11, ean — place
 // them after their payload); generators read only the rng (uuid, ulid, ...). All
@@ -15,26 +24,31 @@ import (
 // the wall clock. Add a builtin only for what data can't express: a random v4
 // UUID already ships as data (data/misc/uuid.json), so the builtin is v7.
 var builtins = map[string]builtin{
-	"luhn":     {arity: 0, call: func(_ *session, e string, _ []string) string { return string(rune('0' + luhnCheck(e))) }},
+	"luhn": {arity: 0, call: func(_ *session, e string, _ map[string]node, _ []string) string {
-	"mod11":    {arity: 0, call: func(_ *session, e string, _ []string) string { return mod11Check(e) }},
+		return string(rune('0' + luhnCheck(e)))
-	"ean":      {arity: 0, call: func(_ *session, e string, _ []string) string { return eanCheck(e) }},
+	}},
-	"uuid":     {arity: 0, call: func(s *session, _ string, _ []string) string { return uuidV7(s) }},
+	"mod11":    {arity: 0, call: func(_ *session, e string, _ map[string]node, _ []string) string { return mod11Check(e) }},
-	"ulid":     {arity: 0, call: func(s *session, _ string, _ []string) string { return ulid(s) }},
+	"ean":      {arity: 0, call: func(_ *session, e string, _ map[string]node, _ []string) string { return eanCheck(e) }},
-	"objectid": {arity: 0, call: func(s *session, _ string, _ []string) string { return randHex(s, 24) }},
+	"uuid":     {arity: 0, call: func(s *session, _ string, _ map[string]node, _ []string) string { return uuidV7(s) }},
-	"nanoid":   {arity: 1, check: posIntArg, call: func(s *session, _ string, a []string) string { return nanoid(s, atoi(a[0])) }},
+	"ulid":     {arity: 0, call: func(s *session, _ string, _ map[string]node, _ []string) string { return ulid(s) }},
-	"hex":      {arity: 1, check: posIntArg, call: func(s *session, _ string, a []string) string { return randHex(s, atoi(a[0])) }},
+	"objectid": {arity: 0, call: func(s *session, _ string, _ map[string]node, _ []string) string { return randHex(s, 24) }},
-	"base64": {arity: 1, check: posIntArg, call: func(s *session, _ string, a []string) string {
+	"nanoid":   {arity: 1, check: posIntArg, call: func(s *session, _ string, _ map[string]node, a []string) string { return nanoid(s, atoi(a[0])) }},
+	"hex":      {arity: 1, check: posIntArg, call: func(s *session, _ string, _ map[string]node, a []string) string { return randHex(s, atoi(a[0])) }},
+	"base64": {arity: 1, check: posIntArg, call: func(s *session, _ string, _ map[string]node, a []string) string {
 		return base64.StdEncoding.EncodeToString(randBytes(s, atoi(a[0])))
 	}},
-	"int": {arity: 2, check: intRangeArgs, call: func(s *session, _ string, a []string) string {
+	"int": {arity: 2, check: intRangeArgs, call: func(s *session, _ string, _ map[string]node, a []string) string {
 		return strconv.Itoa(atoi(a[0]) + s.IntN(atoi(a[1])-atoi(a[0])+1))
 	}},
 	"float": {arity: 3, check: floatArgs, call: floatCall},
-	"iban":  {arity: 1, check: ibanArg, call: func(s *session, _ string, a []string) string { return iban(s, a[0]) }},
+	"iban":  {arity: 1, check: ibanArg, call: func(s *session, _ string, _ map[string]node, a []string) string { return iban(s, a[0]) }},
+	// calc is the one builtin that reads the sibling fields (to render its operands);
+	// every other ignores them. 1 or 2 args: the expression and an optional decimals.
+	"calc": {arity: -1, check: checkCalc, call: calcCall},
 	// seq is the one stateful builtin: a per-session counter from 1, advancing on
 	// each call. An optional name selects an independent counter; no name uses the
 	// default one. Deterministic by construction, so a seeded faker stays stable.
-	"seq": {arity: -1, check: seqArg, call: func(s *session, _ string, a []string) string {
+	"seq": {arity: -1, check: seqArg, call: func(s *session, _ string, _ map[string]node, a []string) string {
 		key := ""
 		if len(a) == 1 {
 			key = a[0]
@@ -64,14 +78,18 @@ func randHex(r rng, n int) string {
 	return string(b)
 }
 
-func posIntArg(a []string) error {
+func posIntArg(_ map[string]node, a []string) error {
-	if n, err := strconv.Atoi(a[0]); err != nil || n < 1 {
+	n, err := strconv.Atoi(a[0])
+	if err != nil || n < 1 {
 		return fmt.Errorf("count %q must be a positive integer", a[0])
 	}
+	if n > maxLen {
+		return fmt.Errorf("count %d exceeds the maximum %d", n, maxLen)
+	}
 	return nil
 }
 
-func intRangeArgs(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 {
@@ -80,10 +98,13 @@ func intRangeArgs(a []string) error {
 	if lo > hi {
 		return fmt.Errorf("int(min,max): min %d > max %d", lo, hi)
 	}
+	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)
+	}
 	return nil
 }
 
-func floatArgs(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])
@@ -93,19 +114,22 @@ func floatArgs(a []string) error {
 	if lo > hi {
 		return fmt.Errorf("float(min,max,dp): min %v > max %v", lo, hi)
 	}
-	if dp < 0 {
+	if math.IsInf(hi-lo, 0) { // an overflowing span would render as "+Inf"
-		return fmt.Errorf("float(min,max,dp): decimals %d < 0", dp)
+		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)
 	}
 	return nil
 }
 
-func floatCall(s *session, _ string, a []string) string {
+func floatCall(s *session, _ string, _ map[string]node, a []string) string {
 	lo, _ := strconv.ParseFloat(a[0], 64)
 	hi, _ := strconv.ParseFloat(a[1], 64)
 	return strconv.FormatFloat(lo+s.Float64()*(hi-lo), 'f', atoi(a[2]), 64)
 }
 
-func seqArg(a []string) error {
+func seqArg(_ map[string]node, a []string) error {
 	if len(a) > 1 {
 		return fmt.Errorf("seq takes at most one name, got %d args", len(a))
 	}
@@ -231,7 +255,7 @@ func eanCheck(s string) string {
 // a left-to-right derivation; it generates the whole value instead.
 var ibanLen = map[string]int{"BE": 16, "DE": 22, "DK": 18, "ES": 24, "FI": 18, "NO": 15, "SE": 24}
 
-func ibanArg(a []string) error {
+func ibanArg(_ map[string]node, a []string) error {
 	if _, ok := ibanLen[a[0]]; !ok {
 		return fmt.Errorf("iban(%q): unsupported country code", a[0])
 	}

builtins_test.go

@@ -122,6 +122,31 @@ func TestBuiltinSeqPerSession(t *testing.T) {
 	}
 }
 
+// TestBuiltinArgLimits pins the New-time guards that keep a fat-fingered or
+// overflowing argument from panicking or OOM-ing at render. Each must be rejected
+// at compile, not detonate when the template is drawn.
+func TestBuiltinArgLimits(t *testing.T) {
+	bad := []string{
+		`{"format":"{int(-9223372036854775808,9223372036854775807)}"}`, // span overflows int -> IntN panic
+		`{"format":"{hex(2000000000)}"}`,                               // ~2 GB string
+		`{"format":"{nanoid(2000000000)}"}`,
+		`{"format":"{base64(2000000000)}"}`,
+		`{"format":"{float(-1e308,1e308,2)}"}`,           // span is +Inf
+		`{"format":"{float(0,1,100000)}"}`,               // decimals -> huge string
+		`{"format":"{calc(1+1,100000)}"}`,                // calc decimals -> huge string
+		`{"format":"{x}","x":["1"],"repeat":2000000000}`, // repeat -> multi-GB join
+	}
+	for _, tmpl := range bad {
+		if _, err := compile(parse(t, tmpl)); err == nil {
+			t.Errorf("compile(%s) = nil error, want a limit rejection", tmpl)
+		}
+	}
+	// Ordinary values still compile.
+	if _, err := compile(parse(t, `{"format":"{hex(16)} {int(-5,5)} {float(0,1,3)} {calc(1+1,2)}"}`)); err != nil {
+		t.Errorf("compile of normal args failed: %v", err)
+	}
+}
+
 func TestBuiltinIBAN(t *testing.T) {
 	wantLen := map[string]int{"SE": 24, "DE": 22, "NO": 15}
 	f := engine(1)

calc.go

@@ -10,9 +10,9 @@ import (
 
 // calc is the {calc(expr[, dp])} token: an arithmetic expression over number
 // literals and sibling-field names (rendered, then parsed as numbers), with
-// + - * /, unary minus and parentheses. Unlike a builtin it reads the field
+// + - * /, unary minus and parentheses. It is a registry builtin like any other
-// environment, so expand and checkTokens route it here rather than the builtins
+// {name(args)} function — the one whose check and call read the sibling fields (to
-// registry. The value prints in minimal decimal form, or rounded to dp decimals
+// render its operands). The value prints in minimal decimal form, or rounded to dp
 // when given. A field that doesn't render to a number becomes NaN, which
 // propagates and prints as "NaN" — visible, never a render error. The expression
 // is re-parsed each render, mirroring how expand re-scans the format; checkCalc
@@ -58,8 +58,10 @@ func (n calcBin) eval(s *session, fields map[string]node) float64 {
 }
 
 // checkCalc validates a calc token at compile time: a parseable expression whose
-// operands all name existing fields, and an optional non-negative integer dp.
+// operands all name existing fields, and an optional non-negative integer dp. It
-func checkCalc(args []string, fields map[string]node) error {
+// is a builtin check (fields first), so calc dispatches through the registry like
+// every other {name(args)} function.
+func checkCalc(fields map[string]node, args []string) error {
 	if len(args) < 1 || len(args) > 2 {
 		return fmt.Errorf("calc takes an expression and an optional decimals count, got %d args", len(args))
 	}
@@ -73,17 +75,18 @@ func checkCalc(args []string, fields map[string]node) error {
 		}
 	}
 	if len(args) == 2 {
-		if dp, err := strconv.Atoi(args[1]); err != nil || dp < 0 {
+		if dp, err := strconv.Atoi(args[1]); err != nil || dp < 0 || dp > maxDecimals {
-			return fmt.Errorf("calc decimals %q must be a non-negative integer", args[1])
+			return fmt.Errorf("calc decimals %q must be an integer in 0..%d", args[1], maxDecimals)
 		}
 	}
 	return nil
 }
 
-// calcEval renders a calc token. checkCalc proved the expression parses and the
+// calcCall renders a calc token. checkCalc proved the expression parses and the
 // decimals arg is valid, so neither step here can fail. dp -1 prints the minimal
-// form; a given dp rounds to that many places.
+// form; a given dp rounds to that many places. The emitted-so-far arg is unused —
-func calcEval(s *session, args []string, fields map[string]node) string {
+// calc reads sibling fields, not the preceding output.
+func calcCall(s *session, _ string, fields map[string]node, args []string) string {
 	expr, _ := parseCalc(args[0])
 	dp := -1
 	if len(args) == 2 {
@@ -92,6 +95,25 @@ func calcEval(s *session, args []string, fields map[string]node) string {
 	return strconv.FormatFloat(expr.eval(s, fields), 'f', dp, 64)
 }
 
+// calcOperands lists the sibling-field names every {calc(...)} token in a format
+// reads, so cycle detection sees the field edges calc renders through (a function
+// token otherwise carries no field edge).
+func calcOperands(format string) []string {
+	var names []string
+	_ = eachToken(format, func(t ftoken) error {
+		if t.kind != 'b' {
+			return nil
+		}
+		if name, args, ok := funcCall(t.body); ok && name == "calc" && len(args) >= 1 {
+			if expr, err := parseCalc(args[0]); err == nil {
+				names = append(names, calcVars(expr)...)
+			}
+		}
+		return nil
+	})
+	return names
+}
+
 // calcVars lists the field names an expression references, for the existence
 // check in checkCalc.
 func calcVars(n calcNode) []string {

cmd/fakes/main.go

@@ -10,10 +10,12 @@
 package main
 
 import (
+	"errors"
 	"flag"
 	"fmt"
 	"io"
 	"os"
+	"runtime/debug"
 	"strings"
 
 	"github.com/Timewave-AB/fakes"
@@ -25,7 +27,11 @@ const usage = `Usage: fakes -data-path D [-data-path D]... [-seed N] [-repeat N]
   <path>        a category, or a dotted path into one (person, person.last)
   -seed N       seed for reproducible output
   -repeat N     render the path N times (default 1)
-  -separator S  string between repeated values (default newline)`
+  -separator S  string between repeated values (default newline)
+  -list         list the paths the data offers, then exit
+  -version      print the version, then exit
+
+Flags must come before <path>.`
 
 // stringList collects a repeatable string flag, preserving order.
 type stringList []string
@@ -45,19 +51,24 @@ func run(args []string, stdout, stderr io.Writer) int {
 	seed := fs.Uint64("seed", 0, "seed for reproducible output")
 	repeat := fs.Int("repeat", 1, "render the path this many times")
 	sep := fs.String("separator", "\n", "string between repeated values")
+	list := fs.Bool("list", false, "list the paths the data offers, then exit")
+	showVersion := fs.Bool("version", false, "print the version, then exit")
 	var dirs stringList
 	fs.Var(&dirs, "data-path", "a data directory to load (repeatable)")
 	if err := fs.Parse(args); err != nil {
+		if errors.Is(err, flag.ErrHelp) { // -h/-help already printed usage
+			return 0
+		}
 		return 2
 	}
-	if len(dirs) == 0 || fs.NArg() != 1 {
+	if *showVersion {
+		fmt.Fprintln(stdout, "fakes "+buildVersion())
+		return 0
+	}
+	if len(dirs) == 0 {
 		fs.Usage()
 		return 2
 	}
-	if *repeat < 1 {
-		fmt.Fprintln(stderr, "repeat must be a positive integer")
-		return 2
-	}
 
 	var opts []fakes.Option
 	fs.Visit(func(fl *flag.Flag) {
@@ -66,6 +77,26 @@ func run(args []string, stdout, stderr io.Writer) int {
 		}
 	})
 
+	if *list {
+		f, err := fakes.New(dirs, opts...)
+		if err != nil {
+			fmt.Fprintln(stderr, err)
+			return 1
+		}
+		for _, p := range f.List() {
+			fmt.Fprintln(stdout, p)
+		}
+		return 0
+	}
+	if fs.NArg() != 1 {
+		fs.Usage()
+		return 2
+	}
+	if *repeat < 1 {
+		fmt.Fprintln(stderr, "repeat must be a positive integer")
+		return 2
+	}
+
 	path := fs.Arg(0)
 	f, err := fakes.New(dirs, opts...)
 	if err != nil {
@@ -82,3 +113,13 @@ func run(args []string, stdout, stderr io.Writer) int {
 	fmt.Fprintln(stdout, strings.Join(vals, *sep))
 	return 0
 }
+
+// buildVersion reports the module version stamped into the binary by `go install`
+// (or "devel" for a local build), read from the build info — no version constant
+// to bump, no extra dependency.
+func buildVersion() string {
+	if info, ok := debug.ReadBuildInfo(); ok && info.Main.Version != "" {
+		return info.Main.Version
+	}
+	return "devel"
+}

cmd/fakes/main_test.go

@@ -145,6 +145,42 @@ func TestRunUnknownCategoryFails(t *testing.T) {
 	}
 }
 
+func TestRunList(t *testing.T) {
+	// -list prints the discoverable paths and exits 0, no positional path needed.
+	code, out, errb := runOut("-data-path", svSE, "-list")
+	if code != 0 {
+		t.Fatalf("run = %d, stderr=%q", code, errb)
+	}
+	for _, want := range []string{"person", "person.last", "address"} {
+		if !strings.Contains(out, want) {
+			t.Errorf("list output missing %q:\n%s", want, out)
+		}
+	}
+}
+
+func TestRunHelpExitsZero(t *testing.T) {
+	// -h/-help is success (0), not misuse (2), so scripts under `set -e` survive.
+	for _, h := range []string{"-h", "-help"} {
+		code, _, errb := runOut(h)
+		if code != 0 {
+			t.Errorf("%s = %d, want 0", h, code)
+		}
+		if !strings.Contains(errb, "Usage") {
+			t.Errorf("%s: stderr = %q, want usage", h, errb)
+		}
+	}
+}
+
+func TestRunVersion(t *testing.T) {
+	code, out, errb := runOut("-version")
+	if code != 0 {
+		t.Fatalf("-version = %d, stderr=%q", code, errb)
+	}
+	if strings.TrimSpace(out) == "" {
+		t.Error("-version: want a version on stdout")
+	}
+}
+
 func TestRunMissingDirFails(t *testing.T) {
 	code, _, errb := runOut("-data-path", "../../data/nope", "person")
 	if code != 1 {

data.go

@@ -30,6 +30,9 @@ func loadData(paths []string) (map[string]node, error) {
 	if err := linkRefs(root); err != nil {
 		return nil, err
 	}
+	if err := checkNoCycles(root); err != nil {
+		return nil, err
+	}
 	return root, nil
 }
 

fakes.go

@@ -21,6 +21,7 @@ import (
 	"encoding/binary"
 	"fmt"
 	"math/rand/v2"
+	"sort"
 )
 
 // Fakes generates fake data from a loaded namespace tree. Create one with [New].
@@ -75,6 +76,50 @@ func New(paths []string, opts ...Option) (*Fakes, error) {
 	return &Fakes{rand: newRand(c.seed, c.seeded), categories: cats}, nil
 }
 
+// List returns the sorted dotted paths Fake can render: every category, the
+// dotted fields within a template, and folder segments — descending transparently
+// through single-variant choices the way a reference does. A multi-variant choice
+// is one path (its pick is random); its items are not separately addressable. It's
+// the discoverable map of what a loaded data set offers, powering the CLI's -list.
+func (f *Fakes) List() []string {
+	var out []string
+	var walk func(prefix string, n node)
+	walk = func(prefix string, n node) {
+		switch n := n.(type) {
+		case *group:
+			for name, c := range n.children {
+				walk(join(prefix, name), c)
+			}
+		case *choice:
+			if len(n.items) == 1 { // a single-variant choice is a transparent wrapper
+				walk(prefix, n.items[0])
+				return
+			}
+			out = append(out, prefix)
+		case *template:
+			out = append(out, prefix)
+			for name, c := range n.fields {
+				if isRef(name) { // a bound {..path} reference, not an authored field
+					continue
+				}
+				walk(join(prefix, name), c)
+			}
+		case literal:
+			out = append(out, prefix)
+		}
+	}
+	walk("", &group{children: f.categories})
+	sort.Strings(out)
+	return out
+}
+
+func join(prefix, name string) string {
+	if prefix == "" {
+		return name
+	}
+	return prefix + "." + name
+}
+
 func newRand(seed uint64, seeded bool) *session {
 	r := rand.New(rand.NewPCG(seed, seed^0x9e3779b97f4a7c15))
 	if !seeded {

loading_test.go

@@ -3,9 +3,25 @@ package fakes
 import (
 	"os"
 	"path/filepath"
+	"reflect"
 	"testing"
 )
 
+// TestList pins the discoverable paths: every category, its dotted fields, and
+// folder segments — descending transparently through single-variant choices.
+func TestList(t *testing.T) {
+	dir := writeData(t, map[string]string{
+		"person":   `{"format":"{first} {last}","first":["A"],"last":["B"]}`,
+		"word":     `["x", "y"]`,
+		"geo/city": `["Z"]`,
+	})
+	got := newFakes(t, dir, WithSeed(1)).List()
+	want := []string{"geo.city", "person", "person.first", "person.last", "word"}
+	if !reflect.DeepEqual(got, want) {
+		t.Fatalf("List() = %v, want %v", got, want)
+	}
+}
+
 // writeData builds a temp data directory from a map of relative path (without
 // ".json") -> file content, creating parent folders as needed. The directory's
 // name carries no meaning anymore, so callers pick any layout they like —

node.go

@@ -0,0 +1,164 @@
+package fakes
+
+import (
+	"fmt"
+	"math"
+)
+
+// node is a compiled template element: literal, choice, or template. Compiling
+// JSON into these once (see compile) means rendering never re-inspects the raw
+// JSON or re-sums weights.
+type node interface{ isNode() }
+
+// literal is emitted verbatim, never formatted.
+type literal string
+
+func (literal) isNode() {}
+
+// group is a namespace of named children, built from a directory of JSON files
+// and subdirectories. It has no value of its own: descend into a named child by
+// dot path; rendering one is an error (see Fake).
+type group struct{ children map[string]node }
+
+func (*group) isNode() {}
+
+// choice picks one of its items. cum holds cumulative weights for a weighted
+// pick; when nil the choice is uniform and selection is O(1).
+type choice struct {
+	items []node
+	cum   []float64
+}
+
+func (*choice) isNode() {}
+
+// template renders a format string, substituting {tokens} from fields. repeat
+// (default 1) renders that format that many times and joins the results with
+// separator (default ""), each render an independent pick.
+type template struct {
+	format    string
+	fields    map[string]node
+	repeat    int
+	separator string
+}
+
+func (*template) isNode() {}
+
+// compile converts parsed JSON into a node tree, validating structure up front.
+func compile(v any) (node, error) {
+	switch v := v.(type) {
+	case string:
+		return literal(v), nil
+	case []any:
+		return compileChoice(v)
+	case map[string]any:
+		return compileTemplate(v)
+	default:
+		return nil, fmt.Errorf("unsupported node type %T", v)
+	}
+}
+
+func compileChoice(items []any) (node, error) {
+	if len(items) == 0 {
+		return nil, fmt.Errorf("empty choice")
+	}
+	c := &choice{items: make([]node, len(items))}
+	cum := make([]float64, len(items))
+	var total float64
+	weighted := false
+	for i, raw := range items {
+		w, err := weightOf(raw)
+		if err != nil {
+			return nil, err
+		}
+		if w != 1 {
+			weighted = true
+		}
+		total += w
+		cum[i] = total
+		n, err := compile(raw)
+		if err != nil {
+			return nil, err
+		}
+		c.items[i] = n
+	}
+	if weighted { // uniform choices skip the weight table and pick in O(1)
+		if total <= 0 || math.IsInf(total, 1) {
+			return nil, fmt.Errorf("choice weights must sum to a finite positive number, got %v", total)
+		}
+		c.cum = cum
+	}
+	return c, nil
+}
+
+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)
+	if err != nil {
+		return nil, err
+	}
+	sep := ""
+	if sv, ok := m["separator"]; ok {
+		if sep, ok = sv.(string); !ok {
+			return nil, fmt.Errorf("separator must be a string, got %T", sv)
+		}
+	}
+	t := &template{format: format, fields: make(map[string]node, len(m)), repeat: repeat, separator: sep}
+	for k, v := range m {
+		if k == "format" || k == "weight" || k == "repeat" || k == "separator" {
+			continue
+		}
+		n, err := compile(v)
+		if err != nil {
+			return nil, fmt.Errorf("field %q: %w", k, err)
+		}
+		t.fields[k] = n
+	}
+	if err := checkTokens(format, t.fields); err != nil {
+		return nil, err
+	}
+	return t, nil
+}
+
+// repeatOf reads a template's "repeat" (default 1): how many times its format
+// is rendered and concatenated. A present one must be a positive integer.
+func repeatOf(m map[string]any) (int, error) {
+	rv, ok := m["repeat"]
+	if !ok {
+		return 1, nil
+	}
+	r, ok := rv.(float64)
+	if !ok {
+		return 0, fmt.Errorf("repeat must be a number, got %T", rv)
+	}
+	if math.IsNaN(r) || math.IsInf(r, 0) || r < 1 || r != math.Trunc(r) {
+		return 0, fmt.Errorf("repeat must be a positive integer, got %v", rv)
+	}
+	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)
+	}
+	return int(r), nil
+}
+
+// weightOf reads a node's "weight" (default 1) from its raw JSON form. Only
+// template objects carry weight; a present one must be finite and non-negative.
+func weightOf(raw any) (float64, error) {
+	m, ok := raw.(map[string]any)
+	if !ok {
+		return 1, nil
+	}
+	wv, ok := m["weight"]
+	if !ok {
+		return 1, nil
+	}
+	w, ok := wv.(float64)
+	if !ok {
+		return 0, fmt.Errorf("weight must be a number, got %T", wv)
+	}
+	if w < 0 || math.IsNaN(w) || math.IsInf(w, 0) {
+		return 0, fmt.Errorf("weight must be finite and non-negative, got %v", w)
+	}
+	return w, nil
+}

reference.go

@@ -0,0 +1,180 @@
+package fakes
+
+import (
+	"fmt"
+	"strings"
+)
+
+// refPrefix marks a {..path} token: a reference to a node elsewhere in the data
+// root rather than a sibling field. The path after it is the same dot path Fake
+// takes, resolved across every loaded directory (see linkRefs).
+const refPrefix = ".."
+
+func isRef(name string) bool { return strings.HasPrefix(name, refPrefix) }
+
+// linkRefs resolves every {..path} reference in the assembled tree, binding the
+// target node into the referring template's fields under the token's key so the
+// ordinary resolver renders it like a sibling. It runs once, after all data is
+// merged, so a reference sees the final (override-resolved) tree. A path that is
+// unknown, names a folder, or steps through a multi-variant choice fails here,
+// keeping a bad reference a New-time error, never a random render-time one. The
+// seen set skips shared nodes (and keeps this walk finite if data is cyclic);
+// checkNoCycles, run next, is what rejects an actual reference cycle.
+func linkRefs(root map[string]node) error {
+	seen := map[node]bool{}
+	var visit func(node) error
+	visit = func(n node) error {
+		if n == nil || seen[n] {
+			return nil
+		}
+		seen[n] = true
+		switch n := n.(type) {
+		case *group:
+			for _, c := range n.children {
+				if err := visit(c); err != nil {
+					return err
+				}
+			}
+		case *choice:
+			for _, it := range n.items {
+				if err := visit(it); err != nil {
+					return err
+				}
+			}
+		case *template:
+			for _, name := range refTokens(n.format) {
+				target, err := lookup(root, strings.Split(name[len(refPrefix):], "."))
+				if err != nil {
+					return fmt.Errorf("reference {%s}: %w", name, err)
+				}
+				n.fields[name] = target
+			}
+			for _, c := range n.fields {
+				if err := visit(c); err != nil {
+					return err
+				}
+			}
+		}
+		return nil
+	}
+	for _, c := range root {
+		if err := visit(c); err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+// lookup finds the single node a reference path names, walking groups and
+// template fields by segment and descending a single-variant choice as a
+// transparent wrapper. A missing segment, a folder target, or a step through a
+// multi-variant choice (which has no one value to bind) is an error.
+func lookup(root map[string]node, segments []string) (node, error) {
+	var n node = &group{children: root}
+	for i := 0; i < len(segments); i++ {
+		switch c := n.(type) {
+		case *group:
+			child, ok := c.children[segments[i]]
+			if !ok {
+				return nil, fmt.Errorf("no entry %q", segments[i])
+			}
+			n = child
+		case *template:
+			child, ok := c.fields[segments[i]]
+			if !ok {
+				return nil, fmt.Errorf("no field %q", segments[i])
+			}
+			n = child
+		case *choice:
+			if len(c.items) != 1 {
+				return nil, fmt.Errorf("%q steps through a %d-way choice", segments[i], len(c.items))
+			}
+			n, i = c.items[0], i-1 // a choice consumes no segment; reprocess it unwrapped
+		default:
+			return nil, fmt.Errorf("cannot descend into %T at %q", n, segments[i])
+		}
+	}
+	if _, ok := n.(*group); ok {
+		return nil, fmt.Errorf("names a folder, not a value")
+	}
+	return n, nil
+}
+
+// refTokens returns just the {..path} reference names among a format's field
+// tokens (linkRefs binds each into the template's fields).
+func refTokens(format string) []string {
+	var refs []string
+	for _, name := range fieldTokens(format) {
+		if isRef(name) {
+			refs = append(refs, name)
+		}
+	}
+	return refs
+}
+
+// renderEdge is a child a node renders into, labelled by the token that reaches it
+// (a field name, reference, or choice index) for a readable cycle report.
+type renderEdge struct {
+	to    node
+	label string
+}
+
+// renderEdges lists the children rendering n recurses into, mirroring expand: a
+// choice's items, and a template's field/reference tokens plus its calc operands.
+// A literal or 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{it, fmt.Sprintf("[%d]", i)}
+		}
+		return es
+	case *template:
+		var es []renderEdge
+		for _, name := range append(fieldTokens(n.format), calcOperands(n.format)...) {
+			if c, ok := n.fields[name]; ok {
+				es = append(es, renderEdge{c, name})
+			}
+		}
+		return es
+	default:
+		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.
+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
+	}
+	for name, c := range root {
+		if err := visit(c, name); err != nil {
+			return err
+		}
+	}
+	return nil
+}

reference_test.go

@@ -81,6 +81,13 @@ func TestReferenceErrors(t *testing.T) {
 			"card": `{"format":"{..who.f}"}`,
 		},
 		"empty reference path": {"card": `{"format":"{..}"}`},
+		// A reference that leads back to its own value never terminates at render,
+		// so New must reject the cycle up front (direct, mutual, or chained).
+		"direct cycle": {"a": `{"format":"x{..a}"}`},
+		"mutual cycle": {"a": `{"format":"{..b}"}`, "b": `{"format":"{..a}"}`},
+		"chain cycle":  {"a": `{"format":"{..b}"}`, "b": `{"format":"{..c}"}`, "c": `{"format":"{..a}"}`},
+		// calc renders its operands, so a cycle through one must be caught too.
+		"calc operand cycle": {"x": `{"format":"{calc(y)}","y":[{"format":"{..x}"}]}`},
 	}
 	for name, files := range cases {
 		if _, err := New([]string{writeData(t, files)}); err == nil {

render.go

@@ -0,0 +1,140 @@
+package fakes
+
+import (
+	"fmt"
+	"sort"
+	"strings"
+)
+
+// rng is the randomness the renderer draws from. Passing it in keeps the render
+// functions a pure core over an explicit effect; *rand.Rand satisfies it.
+type rng interface {
+	IntN(n int) int
+	Float64() float64
+}
+
+// Fake generates a value for a dot path. Each segment descends one level: folder
+// names and the category (JSON file) come first, then named fields within it,
+// e.g. "sv_SE.address" or "sv_SE.address.street". Choices along the way are
+// resolved at random. A path naming a folder (no value of its own) is an error.
+func (f *Fakes) Fake(path string) (string, error) {
+	n, err := descend(f.rand, &group{children: f.categories}, strings.Split(path, "."))
+	if err != nil {
+		return "", fmt.Errorf("fakes: %s: %w", path, err)
+	}
+	if _, ok := n.(*group); ok {
+		return "", fmt.Errorf("fakes: %s names a folder, not a value", path)
+	}
+	return render(f.rand, n), nil
+}
+
+// descend walks named fields, resolving choices it meets along the way. 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.
+func descend(s *session, n node, segments []string) (node, error) {
+	if len(segments) == 0 {
+		return n, nil
+	}
+	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:
+		return descend(s, pick(s, n), segments) // a choice consumes no path segment
+	default:
+		return nil, fmt.Errorf("cannot descend into %T at %q", n, segments[0])
+	}
+}
+
+// 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 {
+	switch n := n.(type) {
+	case literal:
+		return string(n)
+	case *choice:
+		return render(s, pick(s, n))
+	case *template:
+		if n.repeat == 1 {
+			return expand(s, n.format, n.fields)
+		}
+		var b strings.Builder
+		for i := 0; i < n.repeat; i++ {
+			if i > 0 {
+				b.WriteString(n.separator)
+			}
+			b.WriteString(expand(s, n.format, n.fields))
+		}
+		return b.String()
+	default:
+		panic(fmt.Sprintf("fakes: uncompiled node %T", n))
+	}
+}
+
+// pick selects one item. Uniform choices are O(1); weighted choices are an
+// O(log n) search over precomputed cumulative weights. compile guarantees a
+// non-empty choice and a finite positive total, so the index is always in range.
+func pick(r rng, c *choice) node {
+	if c.cum == nil {
+		return c.items[r.IntN(len(c.items))]
+	}
+	x := r.Float64() * c.cum[len(c.cum)-1]
+	i := sort.Search(len(c.cum), func(i int) bool { return c.cum[i] > x })
+	return c.items[i]
+}
+
+// classChar randomises one character-class rune: '0' digit 0-9, '1' digit 1-9,
+// 'A' letter A-Z, 'a' letter a-z.
+func classChar(s *session, c rune) byte {
+	switch c {
+	case '0':
+		return byte('0' + s.IntN(10))
+	case '1':
+		return byte('1' + s.IntN(9))
+	case 'A':
+		return byte('A' + s.IntN(26))
+	default: // 'a'
+		return byte('a' + s.IntN(26))
+	}
+}
+
+// expand renders a format string: literals verbatim, class chars randomised, a
+// {name(args)} token via its builtin, any other {token} via resolve. checkTokens
+// validated every token at compile time, so the scan cannot fail here (the
+// eachToken error is unreachable and ignored).
+func expand(s *session, format string, fields map[string]node) string {
+	var b strings.Builder
+	_ = eachToken(format, func(t ftoken) error {
+		switch t.kind {
+		case 'l':
+			b.WriteRune(t.r)
+		case 'c':
+			b.WriteByte(classChar(s, t.r))
+		case 'b':
+			if name, args, ok := funcCall(t.body); ok {
+				b.WriteString(builtins[name].call(s, b.String(), fields, args)) // b.String() is the output so far
+			} else {
+				b.WriteString(resolve(s, t.body, fields))
+			}
+		}
+		return nil
+	})
+	return b.String()
+}
+
+// resolve renders a "{token}" body: one or more names separated by '|', one
+// picked at random. A name is a sibling field or a {..path} reference, which
+// linkRefs bound into fields too; checkTokens and linkRefs guarantee both exist.
+func resolve(s *session, token string, fields map[string]node) string {
+	names := strings.Split(token, "|")
+	return render(s, fields[names[s.IntN(len(names))]])
+}

template.go

@@ -2,205 +2,75 @@ package fakes
 
 import (
 	"fmt"
-	"math"
-	"sort"
 	"strings"
 )
 
-// node is a compiled template element: literal, choice, or template. Compiling
+// This file is the {token} grammar of a format string: how it is scanned
-// JSON into these once (see compile) means rendering never re-inspects the raw
+// (eachToken), how a function token is parsed (funcCall) and validated
-// JSON or re-sums weights.
+// (checkFunc, checkTokens), and the builtin contract its functions implement.
-type node interface{ isNode() }
+// render.go evaluates these tokens; node.go compiles the surrounding JSON;
+// reference.go binds {..path} tokens across the tree.
 
-// rng is the randomness the renderer draws from. Passing it in keeps the render
+// ftoken is one unit of a scanned format string: a literal rune to emit, a class
-// functions a pure core over an explicit effect; *rand.Rand satisfies it.
+// char to randomise ('0' '1' 'A' 'a'), or the body of a {…} token.
-type rng interface {
+type ftoken struct {
-	IntN(n int) int
+	kind byte // 'l' literal rune, 'c' class char, 'b' brace body
-	Float64() float64
+	r    rune // for kinds 'l' and 'c'
+	body string
 }
 
-// literal is emitted verbatim, never formatted.
+// eachToken scans a format string once and calls fn for each unit, the single
-type literal string
+// source of truth for how '#' escapes and {…} braces are read — expand,
-
+// checkTokens, fieldTokens and refTokens all drive off it so the grammar can't
-func (literal) isNode() {}
+// drift between the validator and the renderer. '#' escapes the next char to a
-
+// literal ("#0" -> '0', "##" -> '#'); a '{' must reach a '}' (else an error); an
-// group is a namespace of named children, built from a directory of JSON files
+// unmatched '}' is an ordinary literal. The error stops the scan early.
-// and subdirectories. It has no value of its own: descend into a named child by
+func eachToken(format string, fn func(ftoken) error) error {
-// dot path; rendering one is an error (see Fake).
-type group struct{ children map[string]node }
-
-func (*group) isNode() {}
-
-// choice picks one of its items. cum holds cumulative weights for a weighted
-// pick; when nil the choice is uniform and selection is O(1).
-type choice struct {
-	items []node
-	cum   []float64
-}
-
-func (*choice) isNode() {}
-
-// template renders a format string, substituting {tokens} from fields. repeat
-// (default 1) renders that format that many times and joins the results with
-// separator (default ""), each render an independent pick.
-type template struct {
-	format    string
-	fields    map[string]node
-	repeat    int
-	separator string
-}
-
-func (*template) isNode() {}
-
-// Fake generates a value for a dot path. Each segment descends one level: folder
-// names and the category (JSON file) come first, then named fields within it,
-// e.g. "sv_SE.address" or "sv_SE.address.street". Choices along the way are
-// resolved at random. A path naming a folder (no value of its own) is an error.
-func (f *Fakes) Fake(path string) (string, error) {
-	n, err := descend(f.rand, &group{children: f.categories}, strings.Split(path, "."))
-	if err != nil {
-		return "", fmt.Errorf("fakes: %s: %w", path, err)
-	}
-	if _, ok := n.(*group); ok {
-		return "", fmt.Errorf("fakes: %s names a folder, not a value", path)
-	}
-	return render(f.rand, n), nil
-}
-
-// descend walks named fields, resolving choices it meets along the way. 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.
-func descend(s *session, n node, segments []string) (node, error) {
-	if len(segments) == 0 {
-		return n, nil
-	}
-	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:
-		return descend(s, pick(s, n), segments) // a choice consumes no path segment
-	default:
-		return nil, fmt.Errorf("cannot descend into %T at %q", n, segments[0])
-	}
-}
-
-// 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 {
-	switch n := n.(type) {
-	case literal:
-		return string(n)
-	case *choice:
-		return render(s, pick(s, n))
-	case *template:
-		if n.repeat == 1 {
-			return expand(s, n.format, n.fields)
-		}
-		var b strings.Builder
-		for i := 0; i < n.repeat; i++ {
-			if i > 0 {
-				b.WriteString(n.separator)
-			}
-			b.WriteString(expand(s, n.format, n.fields))
-		}
-		return b.String()
-	default:
-		panic(fmt.Sprintf("fakes: uncompiled node %T", n))
-	}
-}
-
-// pick selects one item. Uniform choices are O(1); weighted choices are an
-// O(log n) search over precomputed cumulative weights. compile guarantees a
-// non-empty choice and a finite positive total, so the index is always in range.
-func pick(r rng, c *choice) node {
-	if c.cum == nil {
-		return c.items[r.IntN(len(c.items))]
-	}
-	x := r.Float64() * c.cum[len(c.cum)-1]
-	i := sort.Search(len(c.cum), func(i int) bool { return c.cum[i] > x })
-	return c.items[i]
-}
-
-// expand renders a format string. Character classes: '0' digit 0-9, '1' digit
-// 1-9, 'A' letter A-Z, 'a' letter a-z. '#' escapes the next character to a
-// literal ("#0" -> "0", "##" -> "#"). A "{name(args)}" token calls a builtin;
-// any other "{token}" is substituted via resolve; every other rune is literal.
-// checkTokens validated the braces and tokens at compile time, so this scan
-// cannot fail.
-func expand(s *session, format string, fields map[string]node) string {
-	var b strings.Builder
 	rs := []rune(format)
 	for i := 0; i < len(rs); i++ {
+		var t ftoken
 		switch c := rs[i]; c {
 		case '#':
+			t.kind, t.r = 'l', '#'
 			if i++; i < len(rs) {
-				b.WriteRune(rs[i])
+				t.r = rs[i]
-			} else {
-				b.WriteRune('#')
 			}
-		case '0':
+		case '0', '1', 'A', 'a':
-			b.WriteByte(byte('0' + s.IntN(10)))
+			t.kind, t.r = 'c', c
-		case '1':
-			b.WriteByte(byte('1' + s.IntN(9)))
-		case 'A':
-			b.WriteByte(byte('A' + s.IntN(26)))
-		case 'a':
-			b.WriteByte(byte('a' + s.IntN(26)))
 		case '{':
 			end := i + 1
-			for rs[end] != '}' { // checkTokens guarantees a closing '}'
+			for end < len(rs) && rs[end] != '}' {
 				end++
 			}
-			body := string(rs[i+1 : end])
+			if end >= len(rs) {
-			if name, args, ok := funcCall(body); ok {
+				return fmt.Errorf("unterminated '{' in %q", format)
-				if name == "calc" { // calc reads sibling fields, not just (rng, emitted)
-					b.WriteString(calcEval(s, args, fields))
-				} else {
-					b.WriteString(builtins[name].call(s, b.String(), args)) // b.String() is the output so far
-				}
-			} else {
-				b.WriteString(resolve(s, body, fields))
 			}
+			t.kind, t.body = 'b', string(rs[i+1:end])
 			i = end
 		default:
-			b.WriteRune(c)
+			t.kind, t.r = 'l', c
+		}
+		if err := fn(t); err != nil {
+			return err
 		}
 	}
-	return b.String()
+	return nil
-}
-
-// resolve renders a "{token}" body: one or more names separated by '|', one
-// picked at random. A name is a sibling field or a {..path} reference, which
-// linkRefs bound into fields too; checkTokens and linkRefs guarantee both exist.
-func resolve(s *session, token string, fields map[string]node) string {
-	names := strings.Split(token, "|")
-	return render(s, fields[names[s.IntN(len(names))]])
 }
 
 // builtin is a format-string function invoked as {name(args)}. It receives the
 // session (its rng, and the {seq()} counters), the output emitted so far in the
 // current expansion (for derivations such as a checksum over preceding digits),
-// and its args. Almost all are pure over (rng, emitted, args) — no wall-clock, no
+// the sibling fields (only calc reads them, to render its operands), and its args.
-// crypto/rand — so seeding stays reproducible; a time-based id derives its time
+// Almost all are pure over (rng, emitted, args) — no wall-clock, no crypto/rand —
-// from the rng. seq is the one exception: it advances per-session counter state,
+// so seeding stays reproducible; a time-based id derives its time from the rng.
-// which is itself deterministic (1, 2, 3 …). arity is the exact arg count, or -1
+// seq is the one exception: it advances per-session counter state, which is itself
-// for variadic (then check does all the validation). The optional check validates
+// deterministic (1, 2, 3 …). arity is the exact arg count, or -1 for variadic
-// args at compile time (their values, beyond the count). The registry lives in
+// (then check does all the validation). The optional check validates args at
-// builtins.go.
+// compile time (their values, beyond the count). The registry lives in builtins.go.
 type builtin struct {
 	arity int
-	check func(args []string) error
+	check func(fields map[string]node, args []string) error
-	call  func(s *session, emitted string, args []string) string
+	call  func(s *session, emitted string, fields map[string]node, args []string) string
 }
 
 // funcCall splits a "{token}" body shaped name(args) into its parts; ok is false
@@ -227,8 +97,9 @@ func splitArgs(s string) []string {
 }
 
 // checkFunc validates a function token at compile time: well-formed, naming a
-// known builtin, with the arg count that builtin takes.
+// known builtin, with the arg count that builtin takes and args its check accepts.
-func checkFunc(body string) error {
+// fields is passed through for the one builtin (calc) that validates against them.
+func checkFunc(body string, fields map[string]node) error {
 	name, args, ok := funcCall(body)
 	if !ok {
 		return fmt.Errorf("malformed function token {%s}", body)
@@ -241,296 +112,50 @@ func checkFunc(body string) error {
 		return fmt.Errorf("token {%s}: %s takes %d args, got %d", body, name, b.arity, len(args))
 	}
 	if b.check != nil {
-		if err := b.check(args); err != nil {
+		if err := b.check(fields, args); err != nil {
 			return fmt.Errorf("token {%s}: %w", body, err)
 		}
 	}
 	return nil
 }
 
-// compile converts parsed JSON into a node tree, validating structure up front.
-func compile(v any) (node, error) {
-	switch v := v.(type) {
-	case string:
-		return literal(v), nil
-	case []any:
-		return compileChoice(v)
-	case map[string]any:
-		return compileTemplate(v)
-	default:
-		return nil, fmt.Errorf("unsupported node type %T", v)
-	}
-}
-
-func compileChoice(items []any) (node, error) {
-	if len(items) == 0 {
-		return nil, fmt.Errorf("empty choice")
-	}
-	c := &choice{items: make([]node, len(items))}
-	cum := make([]float64, len(items))
-	var total float64
-	weighted := false
-	for i, raw := range items {
-		w, err := weightOf(raw)
-		if err != nil {
-			return nil, err
-		}
-		if w != 1 {
-			weighted = true
-		}
-		total += w
-		cum[i] = total
-		n, err := compile(raw)
-		if err != nil {
-			return nil, err
-		}
-		c.items[i] = n
-	}
-	if weighted { // uniform choices skip the weight table and pick in O(1)
-		if total <= 0 || math.IsInf(total, 1) {
-			return nil, fmt.Errorf("choice weights must sum to a finite positive number, got %v", total)
-		}
-		c.cum = cum
-	}
-	return c, nil
-}
-
-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)
-	if err != nil {
-		return nil, err
-	}
-	sep := ""
-	if sv, ok := m["separator"]; ok {
-		if sep, ok = sv.(string); !ok {
-			return nil, fmt.Errorf("separator must be a string, got %T", sv)
-		}
-	}
-	t := &template{format: format, fields: make(map[string]node, len(m)), repeat: repeat, separator: sep}
-	for k, v := range m {
-		if k == "format" || k == "weight" || k == "repeat" || k == "separator" {
-			continue
-		}
-		n, err := compile(v)
-		if err != nil {
-			return nil, fmt.Errorf("field %q: %w", k, err)
-		}
-		t.fields[k] = n
-	}
-	if err := checkTokens(format, t.fields); err != nil {
-		return nil, err
-	}
-	return t, nil
-}
-
 // checkTokens validates a format string the way expand scans it, so every
-// "{token}" is balanced and names an existing field. This makes a typo'd or
+// "{token}" is balanced and names an existing field (or a known function). This
-// dangling reference a New-time error, never a random render-time one.
+// makes a typo'd or dangling reference a New-time error, never a random
+// render-time one.
 func checkTokens(format string, fields map[string]node) error {
-	rs := []rune(format)
+	return eachToken(format, func(t ftoken) error {
-	for i := 0; i < len(rs); i++ {
+		if t.kind != 'b' {
-		switch rs[i] {
-		case '#':
-			i++ // an escaped char is literal, never a token delimiter
-		case '{':
-			end := i + 1
-			for end < len(rs) && rs[end] != '}' {
-				end++
-			}
-			if end >= len(rs) {
-				return fmt.Errorf("unterminated '{' in %q", format)
-			}
-			body := string(rs[i+1 : end])
-			if strings.IndexByte(body, '(') >= 0 { // a function token, not a field
-				if name, args, ok := funcCall(body); ok && name == "calc" {
-					if err := checkCalc(args, fields); err != nil {
-						return fmt.Errorf("token {%s}: %w", body, err)
-					}
-				} else if err := checkFunc(body); err != nil {
-					return err
-				}
-			} else {
-				for _, name := range strings.Split(body, "|") {
-					if isRef(name) {
-						if name == refPrefix {
-							return fmt.Errorf("token {%s}: reference has no path", body)
-						}
-						continue // a root reference; its target is checked at New (see linkRefs)
-					}
-					if _, ok := fields[name]; !ok {
-						return fmt.Errorf("token {%s}: no field %q", body, name)
-					}
-				}
-			}
-			i = end
-		}
-	}
-	return nil
-}
-
-// refPrefix marks a {..path} token: a reference to a node elsewhere in the data
-// root rather than a sibling field. The path after it is the same dot path Fake
-// takes, resolved across every loaded directory (see linkRefs).
-const refPrefix = ".."
-
-func isRef(name string) bool { return strings.HasPrefix(name, refPrefix) }
-
-// linkRefs resolves every {..path} reference in the assembled tree, binding the
-// target node into the referring template's fields under the token's key so the
-// ordinary resolver renders it like a sibling. It runs once, after all data is
-// merged, so a reference sees the final (override-resolved) tree. A path that is
-// unknown, names a folder, or steps through a multi-variant choice fails here,
-// keeping a bad reference a New-time error, never a random render-time one. The
-// seen set both skips shared nodes and stops a cyclic reference looping the walk.
-func linkRefs(root map[string]node) error {
-	seen := map[node]bool{}
-	var visit func(node) error
-	visit = func(n node) error {
-		if n == nil || seen[n] {
 			return nil
 		}
-		seen[n] = true
+		if strings.IndexByte(t.body, '(') >= 0 { // a function token, not a field
-		switch n := n.(type) {
+			return checkFunc(t.body, fields)
-		case *group:
+		}
-			for _, c := range n.children {
+		for _, name := range strings.Split(t.body, "|") {
-				if err := visit(c); err != nil {
+			if isRef(name) {
-					return err
+				if name == refPrefix {
+					return fmt.Errorf("token {%s}: reference has no path", t.body)
 				}
+				continue // a root reference; its target is checked at New (see linkRefs)
 			}
-		case *choice:
+			if _, ok := fields[name]; !ok {
-			for _, it := range n.items {
+				return fmt.Errorf("token {%s}: no field %q", t.body, name)
-				if err := visit(it); err != nil {
-					return err
-				}
-			}
-		case *template:
-			for _, name := range refTokens(n.format) {
-				target, err := lookup(root, strings.Split(name[len(refPrefix):], "."))
-				if err != nil {
-					return fmt.Errorf("reference {%s}: %w", name, err)
-				}
-				n.fields[name] = target
-			}
-			for _, c := range n.fields {
-				if err := visit(c); err != nil {
-					return err
-				}
 			}
 		}
 		return nil
-	}
+	})
-	for _, c := range root {
+}
-		if err := visit(c); err != nil {
+
-			return err
+// fieldTokens returns the field and reference names a format renders via {name}
+// or {a|..b} tokens (function tokens, which carry no field edges, are excluded).
+// These are exactly the child nodes expand's resolve recurses into.
+func fieldTokens(format string) []string {
+	var names []string
+	_ = eachToken(format, func(t ftoken) error {
+		if t.kind == 'b' && strings.IndexByte(t.body, '(') < 0 {
+			names = append(names, strings.Split(t.body, "|")...)
 		}
-	}
+		return nil
-	return nil
+	})
-}
+	return names
-
-// lookup finds the single node a reference path names, walking groups and
-// template fields by segment and descending a single-variant choice as a
-// transparent wrapper. A missing segment, a folder target, or a step through a
-// multi-variant choice (which has no one value to bind) is an error.
-func lookup(root map[string]node, segments []string) (node, error) {
-	var n node = &group{children: root}
-	for i := 0; i < len(segments); i++ {
-		switch c := n.(type) {
-		case *group:
-			child, ok := c.children[segments[i]]
-			if !ok {
-				return nil, fmt.Errorf("no entry %q", segments[i])
-			}
-			n = child
-		case *template:
-			child, ok := c.fields[segments[i]]
-			if !ok {
-				return nil, fmt.Errorf("no field %q", segments[i])
-			}
-			n = child
-		case *choice:
-			if len(c.items) != 1 {
-				return nil, fmt.Errorf("%q steps through a %d-way choice", segments[i], len(c.items))
-			}
-			n, i = c.items[0], i-1 // a choice consumes no segment; reprocess it unwrapped
-		default:
-			return nil, fmt.Errorf("cannot descend into %T at %q", n, segments[i])
-		}
-	}
-	if _, ok := n.(*group); ok {
-		return nil, fmt.Errorf("names a folder, not a value")
-	}
-	return n, nil
-}
-
-// refTokens returns the reference names ({..path}, including ones inside a
-// {a|..path} alternation) used in a format string, scanning braces the way
-// expand does. Function tokens carry no references.
-func refTokens(format string) []string {
-	var refs []string
-	rs := []rune(format)
-	for i := 0; i < len(rs); i++ {
-		switch rs[i] {
-		case '#':
-			i++
-		case '{':
-			end := i + 1
-			for end < len(rs) && rs[end] != '}' {
-				end++
-			}
-			if end >= len(rs) {
-				return refs // checkTokens already rejected the unterminated brace
-			}
-			if body := string(rs[i+1 : end]); strings.IndexByte(body, '(') < 0 {
-				for _, name := range strings.Split(body, "|") {
-					if isRef(name) {
-						refs = append(refs, name)
-					}
-				}
-			}
-			i = end
-		}
-	}
-	return refs
-}
-
-// repeatOf reads a template's "repeat" (default 1): how many times its format
-// is rendered and concatenated. A present one must be a positive integer.
-func repeatOf(m map[string]any) (int, error) {
-	rv, ok := m["repeat"]
-	if !ok {
-		return 1, nil
-	}
-	r, ok := rv.(float64)
-	if !ok {
-		return 0, fmt.Errorf("repeat must be a number, got %T", rv)
-	}
-	if math.IsNaN(r) || math.IsInf(r, 0) || r < 1 || r != math.Trunc(r) {
-		return 0, fmt.Errorf("repeat must be a positive integer, got %v", rv)
-	}
-	return int(r), nil
-}
-
-// weightOf reads a node's "weight" (default 1) from its raw JSON form. Only
-// template objects carry weight; a present one must be finite and non-negative.
-func weightOf(raw any) (float64, error) {
-	m, ok := raw.(map[string]any)
-	if !ok {
-		return 1, nil
-	}
-	wv, ok := m["weight"]
-	if !ok {
-		return 1, nil
-	}
-	w, ok := wv.(float64)
-	if !ok {
-		return 0, fmt.Errorf("weight must be a number, got %T", wv)
-	}
-	if w < 0 || math.IsNaN(w) || math.IsInf(w, 0) {
-		return 0, fmt.Errorf("weight must be finite and non-negative, got %v", w)
-	}
-	return w, nil
 }