Add repeat/separator templates; validate at compile time; support Go 1.22+ via GO_VERSION

Dockerfile

@@ -1,7 +1,9 @@
 # Reproducible test/build image. `docker build .` fails if vet or tests fail,
-# so it doubles as CI. Day-to-day, prefer the Makefile (bind-mounts source,
+# so it doubles as CI. Day-to-day, prefer `docker compose run` (bind-mounts
-# no rebuilds). Pin matches the latest stable Go at time of writing.
+# source, no rebuilds). GO_VERSION defaults to the latest stable Go; override it
-FROM golang:1.26.4
+# to test the lowest supported version: docker build --build-arg GO_VERSION=1.22 .
+ARG GO_VERSION=1.26.4
+FROM golang:${GO_VERSION}
 
 WORKDIR /app
 

README.md

@@ -23,7 +23,7 @@ format control we needed.
 go get github.com/Timewave-AB/fakes
 ```
 
-Requires Go 1.26+.
+Requires Go 1.22+ (for `math/rand/v2`).
 
 ## Usage
 
@@ -119,6 +119,22 @@ within a choice:
 ]
 ```
 
+Only template (object) nodes carry `weight` — a bare string or nested array in a
+choice always counts as `1`. Weights are checked when you create the faker: a
+negative, non-numeric, or all-zero set is rejected at `New`, so a typo fails
+fast instead of silently skewing output.
+
+**Repeat.** A template node may carry a `repeat` (default `1`) to render its
+`format` that many times — each render an independent pick — joined by
+`separator` (default `""`):
+
+```json
+{ "format": "{word}", "repeat": 3, "separator": " ", "word": ["foo", "bar", "baz"] }
+```
+
+This yields e.g. `bar foo baz`. `repeat` must be a positive integer and
+`separator` a string, both checked at `New`.
+
 **Format string.** Every character is literal except:
 
 | Token | Expands to |
@@ -193,6 +209,14 @@ docker compose run --rm --user "$(id -u):$(id -g)" tidy  # go mod tidy
 
 `docker build .` runs `go vet` and the tests, so it works as a CI gate too.
 
+Tests run against the latest Go by default. Set `GO_VERSION` to check the lowest
+supported version too:
+
+```sh
+GO_VERSION=1.22 docker compose run --rm test   # lowest supported
+docker compose run --rm test                   # latest
+```
+
 ## Layout
 
 ```

compose.yaml

@@ -8,9 +8,12 @@
 # Build caches persist in the `gocache` volume, so re-runs are fast and the
 # host tree stays clean. Commands that rewrite source (fmt, tidy) keep your
 # ownership when run with `--user "$(id -u):$(id -g)"`.
+#
+# GO_VERSION picks the toolchain image (default: latest stable). Test the lowest
+# supported Go with: GO_VERSION=1.22 docker compose run --rm test
 
 x-go: &go
-  image: golang:1.26.4
+  image: golang:${GO_VERSION:-1.26.4}
   working_dir: /app
   volumes:
     - .:/app

edge_test.go

@@ -22,7 +22,7 @@ func TestEscapeEdgeCases(t *testing.T) {
 		`{"format":"x}y"}`:      "x}y",  // an unmatched } is literal (x, y aren't classes)
 	}
 	for tmpl, want := range cases {
-		if got := render(t, f, tmpl); got != want {
+		if got := mustRender(t, f, tmpl); got != want {
 			t.Errorf("render(%s) = %q, want %q", tmpl, got, want)
 		}
 	}
@@ -31,7 +31,7 @@ func TestEscapeEdgeCases(t *testing.T) {
 func TestMultibyteFormat(t *testing.T) {
 	// Scanning is rune-aware: multibyte literals coexist with class chars and
 	// tokens without corrupting indices.
-	got := render(t, engine(2), `{"format":"Öster{x}-0å","x":["väg"]}`)
+	got := mustRender(t, engine(2), `{"format":"Öster{x}-0å","x":["väg"]}`)
 	if !regexp.MustCompile(`^Österväg-[0-9]å$`).MatchString(got) {
 		t.Fatalf("multibyte format = %q", got)
 	}
@@ -41,19 +41,13 @@ func TestAlternationThreeWay(t *testing.T) {
 	f := engine(4)
 	seen := map[string]bool{}
 	for i := 0; i < 200; i++ {
-		seen[render(t, f, `{"format":"{a|b|c}","a":["A"],"b":["B"],"c":["C"]}`)] = true
+		seen[mustRender(t, f, `{"format":"{a|b|c}","a":["A"],"b":["B"],"c":["C"]}`)] = true
 	}
 	if !seen["A"] || !seen["B"] || !seen["C"] || len(seen) != 3 {
 		t.Fatalf("3-way alternation produced %v, want A, B and C", seen)
 	}
 }
 
-func TestEmptyChoiceErrors(t *testing.T) {
-	if _, err := engine(1).render(compiled(t, `[]`)); err == nil {
-		t.Fatal("render([]) = nil error, want empty-choice error")
-	}
-}
-
 func TestNewErrors(t *testing.T) {
 	// Pointing New at a file (not a directory) fails.
 	file := filepath.Join(t.TempDir(), "xx_XX")
@@ -69,20 +63,6 @@ func TestNewErrors(t *testing.T) {
 	}
 }
 
-func TestFakeSurfacesErrors(t *testing.T) {
-	f := engine(1)
-	f.categories = map[string]node{
-		"bad":   compiled(t, `[{"format":"{y}","x":["Q"]}]`), // token names a missing field -> render error
-		"empty": compiled(t, `[]`),                           // empty choice met on a path -> descend error
-	}
-	if _, err := f.Fake("bad"); err == nil {
-		t.Error("Fake(bad) = nil error, want render error")
-	}
-	if _, err := f.Fake("empty.foo"); err == nil {
-		t.Error("Fake(empty.foo) = nil error, want empty-choice error")
-	}
-}
-
 // --- deep path navigation ---
 
 func TestDeepDottedPath(t *testing.T) {

fakes.go

@@ -74,7 +74,3 @@ func newRand(seed uint64, seeded bool) *rand.Rand {
 	}
 	return rand.New(rand.NewPCG(seed, seed^0x9e3779b97f4a7c15))
 }
-
-// intn returns a random int in [0, n). It panics for n <= 0, matching the
-// stdlib contract.
-func (f *Fakes) intn(n int) int { return f.rand.IntN(n) }

go.mod

@@ -1,3 +1,3 @@
 module github.com/Timewave-AB/fakes
 
-go 1.26
+go 1.22

template.go

@@ -2,6 +2,7 @@ package fakes
 
 import (
 	"fmt"
+	"math"
 	"sort"
 	"strings"
 )
@@ -11,6 +12,13 @@ import (
 // JSON or re-sums weights.
 type node interface{ isNode() }
 
+// 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
+}
+
 // literal is emitted verbatim, never formatted.
 type literal string
 
@@ -25,10 +33,14 @@ type choice struct {
 
 func (*choice) isNode() {}
 
-// template renders a format string, substituting {tokens} from fields.
+// 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
+	format    string
-	fields map[string]node
+	fields    map[string]node
+	repeat    int
+	separator string
 }
 
 func (*template) isNode() {}
@@ -43,19 +55,17 @@ func (f *Fakes) Fake(path string) (string, error) {
 	if !ok {
 		return "", fmt.Errorf("fakes: unknown category %q", segments[0])
 	}
-	n, err := f.descend(n, segments[1:])
+	n, err := descend(f.rand, n, segments[1:])
 	if err != nil {
 		return "", fmt.Errorf("fakes: %s: %w", path, err)
 	}
-	s, err := f.render(n)
+	return render(f.rand, n), nil
-	if err != nil {
-		return "", fmt.Errorf("fakes: %s: %w", path, err)
-	}
-	return s, nil
 }
 
-// descend walks named fields, resolving choices it meets along the way.
+// descend walks named fields, resolving choices it meets along the way. It is
-func (f *Fakes) descend(n node, segments []string) (node, error) {
+// 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(r rng, n node, segments []string) (node, error) {
 	if len(segments) == 0 {
 		return n, nil
 	}
@@ -65,58 +75,57 @@ func (f *Fakes) descend(n node, segments []string) (node, error) {
 		if !ok {
 			return nil, fmt.Errorf("no field %q", segments[0])
 		}
-		return f.descend(child, segments[1:])
+		return descend(r, child, segments[1:])
 	case *choice:
-		chosen, err := f.pick(n)
+		return descend(r, pick(r, n), segments) // a choice consumes no path segment
-		if err != nil {
-			return nil, err
-		}
-		return f.descend(chosen, segments) // a choice consumes no path segment
 	default:
 		return nil, fmt.Errorf("cannot descend into %T at %q", n, segments[0])
 	}
 }
 
-// render evaluates a node to a string.
+// render evaluates a compiled node to a string. compile validates every node up
-func (f *Fakes) render(n node) (string, error) {
+// front, so rendering a compiled tree cannot fail.
+func render(r rng, n node) string {
 	switch n := n.(type) {
 	case literal:
-		return string(n), nil
+		return string(n)
 	case *choice:
-		chosen, err := f.pick(n)
+		return render(r, pick(r, n))
-		if err != nil {
-			return "", err
-		}
-		return f.render(chosen)
 	case *template:
-		return f.expand(n.format, n.fields)
+		if n.repeat == 1 {
+			return expand(r, 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(r, n.format, n.fields))
+		}
+		return b.String()
 	default:
-		return "", fmt.Errorf("unsupported node %T", n)
+		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.
+// O(log n) search over precomputed cumulative weights. compile guarantees a
-func (f *Fakes) pick(c *choice) (node, error) {
+// non-empty choice and a finite positive total, so the index is always in range.
-	if len(c.items) == 0 {
+func pick(r rng, c *choice) node {
-		return nil, fmt.Errorf("empty choice")
-	}
 	if c.cum == nil {
-		return c.items[f.intn(len(c.items))], nil
+		return c.items[r.IntN(len(c.items))]
 	}
-	r := f.rand.Float64() * c.cum[len(c.cum)-1]
+	x := r.Float64() * c.cum[len(c.cum)-1]
-	i := sort.Search(len(c.cum), func(i int) bool { return c.cum[i] > r })
+	i := sort.Search(len(c.cum), func(i int) bool { return c.cum[i] > x })
-	if i >= len(c.items) {
+	return c.items[i]
-		i = len(c.items) - 1
-	}
-	return c.items[i], nil
 }
 
 // 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", "##" -> "#"). "{token}" is substituted via resolve;
-// every other rune is literal.
+// every other rune is literal. checkTokens validated the braces and tokens at
-func (f *Fakes) expand(format string, fields map[string]node) (string, error) {
+// compile time, so this scan cannot fail.
+func expand(r rng, format string, fields map[string]node) string {
 	var b strings.Builder
 	rs := []rune(format)
 	for i := 0; i < len(rs); i++ {
@@ -128,44 +137,32 @@ func (f *Fakes) expand(format string, fields map[string]node) (string, error) {
 				b.WriteRune('#')
 			}
 		case '0':
-			b.WriteByte(byte('0' + f.intn(10)))
+			b.WriteByte(byte('0' + r.IntN(10)))
 		case '1':
-			b.WriteByte(byte('1' + f.intn(9)))
+			b.WriteByte(byte('1' + r.IntN(9)))
 		case 'A':
-			b.WriteByte(byte('A' + f.intn(26)))
+			b.WriteByte(byte('A' + r.IntN(26)))
 		case 'a':
-			b.WriteByte(byte('a' + f.intn(26)))
+			b.WriteByte(byte('a' + r.IntN(26)))
 		case '{':
 			end := i + 1
-			for end < len(rs) && rs[end] != '}' {
+			for rs[end] != '}' { // checkTokens guarantees a closing '}'
 				end++
 			}
-			if end >= len(rs) {
+			b.WriteString(resolve(r, string(rs[i+1:end]), fields))
-				return "", fmt.Errorf("unterminated '{' in %q", format)
-			}
-			s, err := f.resolve(string(rs[i+1:end]), fields)
-			if err != nil {
-				return "", err
-			}
-			b.WriteString(s)
 			i = end
 		default:
 			b.WriteRune(c)
 		}
 	}
-	return b.String(), nil
+	return b.String()
 }
 
-// resolve evaluates a "{token}" body: one or more field names separated by '|',
+// resolve renders a "{token}" body: one or more field names separated by '|',
-// of which one is chosen at random and rendered.
+// of which one is chosen at random. checkTokens guarantees every name exists.
-func (f *Fakes) resolve(token string, fields map[string]node) (string, error) {
+func resolve(r rng, token string, fields map[string]node) string {
 	names := strings.Split(token, "|")
-	name := names[f.intn(len(names))]
+	return render(r, fields[names[r.IntN(len(names))]])
-	child, ok := fields[name]
-	if !ok {
-		return "", fmt.Errorf("token {%s}: no field %q", token, name)
-	}
-	return f.render(child)
 }
 
 // compile converts parsed JSON into a node tree, validating structure up front.
@@ -183,12 +180,18 @@ func compile(v any) (node, error) {
 }
 
 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 := weightOf(raw)
+		w, err := weightOf(raw)
+		if err != nil {
+			return nil, err
+		}
 		if w != 1 {
 			weighted = true
 		}
@@ -201,6 +204,9 @@ func compileChoice(items []any) (node, error) {
 		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
@@ -211,9 +217,19 @@ func compileTemplate(m map[string]any) (node, error) {
 	if !ok {
 		return nil, fmt.Errorf("template object missing string \"format\"")
 	}
-	t := &template{format: format, fields: make(map[string]node, len(m))}
+	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" {
+		if k == "format" || k == "weight" || k == "repeat" || k == "separator" {
 			continue
 		}
 		n, err := compile(v)
@@ -222,15 +238,75 @@ func compileTemplate(m map[string]any) (node, error) {
 		}
 		t.fields[k] = n
 	}
+	if err := checkTokens(format, t.fields); err != nil {
+		return nil, err
+	}
 	return t, nil
 }
 
-// weightOf reads a node's "weight" (default 1) from its raw JSON form.
+// checkTokens validates a format string the way expand scans it, so every
-func weightOf(raw any) float64 {
+// "{token}" is balanced and names an existing field. This makes a typo'd or
-	if m, ok := raw.(map[string]any); ok {
+// dangling reference a New-time error, never a random render-time one.
-		if w, ok := m["weight"].(float64); ok {
+func checkTokens(format string, fields map[string]node) error {
-			return w
+	rs := []rune(format)
+	for i := 0; i < len(rs); i++ {
+		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])
+			for _, name := range strings.Split(body, "|") {
+				if _, ok := fields[name]; !ok {
+					return fmt.Errorf("token {%s}: no field %q", body, name)
+				}
+			}
+			i = end
 		}
 	}
-	return 1
+	return 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)
+	}
+	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
 }

template_test.go

@@ -30,18 +30,14 @@ func compiled(t *testing.T, s string) node {
 	return n
 }
 
-func render(t *testing.T, f *Fakes, s string) string {
+func mustRender(t *testing.T, f *Fakes, s string) string {
 	t.Helper()
-	out, err := f.render(compiled(t, s))
+	return render(f.rand, compiled(t, s))
-	if err != nil {
-		t.Fatalf("render %q: %v", s, err)
-	}
-	return out
 }
 
 func TestLiteralStringIsVerbatim(t *testing.T) {
 	// A bare string is a literal, never formatted: 'a'/'A' must survive.
-	if got := render(t, engine(1), `"Malmö"`); got != "Malmö" {
+	if got := mustRender(t, engine(1), `"Malmö"`); got != "Malmö" {
 		t.Fatalf("literal = %q, want Malmö", got)
 	}
 }
@@ -56,7 +52,7 @@ func TestCharacterClasses(t *testing.T) {
 	f := engine(7)
 	for tmpl, re := range cases {
 		for i := 0; i < 100; i++ {
-			if got := render(t, f, tmpl); !re.MatchString(got) {
+			if got := mustRender(t, f, tmpl); !re.MatchString(got) {
 				t.Fatalf("%s produced %q, want %s", tmpl, got, re)
 			}
 		}
@@ -65,13 +61,13 @@ func TestCharacterClasses(t *testing.T) {
 
 func TestEscapeAndLiteralChars(t *testing.T) {
 	// '#' escapes the next char; non-class chars (7, x, -) are literal.
-	if got := render(t, engine(1), `{"format":"#0#1#A#a## x7-z"}`); got != "01Aa# x7-z" {
+	if got := mustRender(t, engine(1), `{"format":"#0#1#A#a## x7-z"}`); got != "01Aa# x7-z" {
 		t.Fatalf("escape = %q, want \"01Aa# x7-z\"", got)
 	}
 }
 
 func TestTokenSubstitution(t *testing.T) {
-	if got := render(t, engine(1), `{"format":"{x}sson","x":["Erik"]}`); got != "Eriksson" {
+	if got := mustRender(t, engine(1), `{"format":"{x}sson","x":["Erik"]}`); got != "Eriksson" {
 		t.Fatalf("token = %q, want Eriksson", got)
 	}
 }
@@ -80,7 +76,7 @@ func TestAlternationPicksOneField(t *testing.T) {
 	f := engine(3)
 	seen := map[string]bool{}
 	for i := 0; i < 100; i++ {
-		seen[render(t, f, `{"format":"{a|b}","a":["A"],"b":["B"]}`)] = true
+		seen[mustRender(t, f, `{"format":"{a|b}","a":["A"],"b":["B"]}`)] = true
 	}
 	if !seen["A"] || !seen["B"] || len(seen) != 2 {
 		t.Fatalf("alternation produced %v, want both A and B", seen)
@@ -90,7 +86,7 @@ func TestAlternationPicksOneField(t *testing.T) {
 func TestWeightZeroNeverChosen(t *testing.T) {
 	f := engine(5)
 	for i := 0; i < 200; i++ {
-		if got := render(t, f, `[{"format":"X","weight":0},"Y"]`); got != "Y" {
+		if got := mustRender(t, f, `[{"format":"X","weight":0},"Y"]`); got != "Y" {
 			t.Fatalf("weight-0 variant chosen: %q", got)
 		}
 	}
@@ -100,7 +96,7 @@ func TestWeightSkewsDistribution(t *testing.T) {
 	f := engine(9)
 	heavy := 0
 	for i := 0; i < 1000; i++ {
-		if render(t, f, `[{"format":"H","weight":10},{"format":"L"}]`) == "H" {
+		if mustRender(t, f, `[{"format":"H","weight":10},{"format":"L"}]`) == "H" {
 			heavy++
 		}
 	}
@@ -109,24 +105,63 @@ func TestWeightSkewsDistribution(t *testing.T) {
 	}
 }
 
+func TestRepeatRendersFormatNTimes(t *testing.T) {
+	// repeat N concatenates N independent renders of the format ('x','y' are
+	// literal, not character classes).
+	if got := mustRender(t, engine(1), `{"format":"xy","repeat":3}`); got != "xyxyxy" {
+		t.Fatalf("repeat literal = %q, want xyxyxy", got)
+	}
+	// separator joins renders (N-1 of them, no trailing one).
+	if got := mustRender(t, engine(1), `{"format":"xy","repeat":3,"separator":"-"}`); got != "xy-xy-xy" {
+		t.Fatalf("repeat with separator = %q, want xy-xy-xy", got)
+	}
+	f, re := engine(7), regexp.MustCompile(`^[0-9]{4}$`)
+	seen := map[string]bool{}
+	for i := 0; i < 50; i++ {
+		got := mustRender(t, f, `{"format":"0","repeat":4}`)
+		if !re.MatchString(got) {
+			t.Fatalf("repeat-4 = %q, want 4 digits", got)
+		}
+		seen[got] = true
+	}
+	if len(seen) < 2 {
+		t.Fatalf("repeat never varied across renders: %v", seen)
+	}
+}
+
 func TestRecursionHasNoDepthLimit(t *testing.T) {
 	// Build {format:{a}, a:[{format:{a}, a:[ ... "deep" ]]}} 50 levels deep.
 	tmpl := `"deep"`
 	for i := 0; i < 50; i++ {
 		tmpl = `{"format":"{a}","a":[` + tmpl + `]}`
 	}
-	if got := render(t, engine(1), tmpl); got != "deep" {
+	if got := mustRender(t, engine(1), tmpl); got != "deep" {
 		t.Fatalf("deep recursion = %q, want deep", got)
 	}
 }
 
 func TestCompileErrors(t *testing.T) {
-	// Structural problems are caught up front, at compile/New time.
+	// Every structural problem is caught up front, at compile/New time, never
+	// deferred to a random render that happens to hit the bad branch.
 	for _, bad := range []string{
-		`{"x":["Q"]}`,            // object without "format"
+		`{"x":["Q"]}`,                 // object without "format"
-		`{"format":"{y}","x":1}`, // a field is a bare number
+		`{"format":"{y}","x":1}`,      // a field is a bare number
-		`[1, 2]`,                 // a choice of numbers
+		`[1, 2]`,                      // a choice of numbers
-		`5`,                      // unsupported node type
+		`5`,                           // unsupported node type
+		`[]`,                          // empty choice
+		`{"format":"{x"}`,             // unterminated brace
+		`{"format":"{y}","x":["Q"]}`,  // token names a missing field
+		`{"format":"{}"}`,             // empty token name
+		`{"format":"{a|}","a":["Q"]}`, // empty alternation segment
+		`[{"format":"A","weight":-1},{"format":"B"}]`,                   // negative weight
+		`[{"format":"A","weight":0},{"format":"B","weight":0}]`,         // weights sum to zero
+		`[{"format":"A","weight":1e308},{"format":"B","weight":1e308}]`, // weights overflow to +Inf
+		`[{"format":"A","weight":"heavy"}]`,                             // non-numeric weight
+		`{"format":"x","repeat":0}`,                                     // repeat below 1
+		`{"format":"x","repeat":-2}`,                                    // negative repeat
+		`{"format":"x","repeat":1.5}`,                                   // non-integer repeat
+		`{"format":"x","repeat":"two"}`,                                 // non-numeric repeat
+		`{"format":"x","repeat":2,"separator":5}`,                       // non-string separator
 	} {
 		if _, err := compile(parse(t, bad)); err == nil {
 			t.Errorf("compile(%s) = nil error, want error", bad)
@@ -134,20 +169,6 @@ func TestCompileErrors(t *testing.T) {
 	}
 }
 
-func TestRenderErrors(t *testing.T) {
-	// These compile, but fail when rendered.
-	f := engine(1)
-	for _, bad := range []string{
-		`{"format":"{x"}`,            // unterminated brace
-		`{"format":"{y}","x":["Q"]}`, // token names a missing field
-		`[]`,                         // empty choice
-	} {
-		if _, err := f.render(compiled(t, bad)); err == nil {
-			t.Errorf("render(%s) = nil error, want error", bad)
-		}
-	}
-}
-
 func TestFakePathNavigation(t *testing.T) {
 	f := engine(1)
 	f.categories = map[string]node{