Add FakeTemplate: compile an inline template and render it against the loaded tree

graph.go

@@ -10,29 +10,36 @@ import (
 // visiting keys in sorted order so which of several broken nodes gets reported does
 // not depend on map iteration.
 func walkNodes(root map[string]node, fn func(path string, n node) error) error {
-	seen := map[node]bool{}
-	var visit func(string, node) error
-	visit = func(path string, n node) error {
-		if n == nil || seen[n] {
-			return nil
-		}
-		seen[n] = true
-		if err := fn(path, n); err != nil {
+	for _, name := range sortedNames(root) {
+		if err := eachNode(root[name], name, fn); err != nil {
 			return err
 		}
-		for _, c := range contained(n) {
+	}
+	return nil
+}
+
+// eachNode visits n and every node contained within it once, passing the dot path
+// that reaches each. It never crosses a reference edge — a bound {/path} field is
+// skipped, as in walkNodes — so a single inline node is walked on its own.
+func eachNode(n node, path string, fn func(path string, n node) error) error {
+	seen := map[node]bool{}
+	var visit func(string, node) error
+	visit = func(path string, m node) error {
+		if m == nil || seen[m] {
+			return nil
+		}
+		seen[m] = true
+		if err := fn(path, m); err != nil {
+			return err
+		}
+		for _, c := range contained(m) {
 			if err := visit(join(path, c.name), c.node); err != nil {
 				return err
 			}
 		}
 		return nil
 	}
-	for _, name := range sortedNames(root) {
-		if err := visit(name, root[name]); err != nil {
-			return err
-		}
-	}
-	return nil
+	return visit(path, n)
 }
 
 // namedNode is a contained child and the segment reaching it; a choice's items carry
@@ -162,30 +169,46 @@ func pathLeaves(n node, tail []string) []node {
 // path, so nested repeats cannot build what one repeat may not. It runs after
 // checkNoCycles, whose guarantee is what lets the walk terminate.
 func checkRepeatReach(root map[string]node) error {
-	reach := map[node]int{}
-	var of func(n node) int
-	of = func(n node) int {
-		if r, done := reach[n]; done {
-			return r
-		}
-		r := 1
-		for _, e := range renderEdges(n) {
-			if c := of(e.to); c > r {
-				r = c
-			}
-		}
-		if t, ok := n.(*template); ok {
-			r *= t.repeat
-		}
-		reach[n] = r
+	mem := reachMemo{}
+	return walkNodes(root, func(path string, n node) error {
+		return repeatCheck(path, n, mem)
+	})
+}
+
+// checkNodeRepeatReach is checkRepeatReach for one inline node: each template in
+// it is bounded, following reference edges into the already-validated tree the same
+// way a repeat through a reference multiplies along a path.
+func checkNodeRepeatReach(n node) error {
+	mem := reachMemo{}
+	return eachNode(n, "template", func(path string, m node) error {
+		return repeatCheck(path, m, mem)
+	})
+}
+
+type reachMemo map[node]int
+
+func (m reachMemo) of(n node) int {
+	if r, done := m[n]; done {
 		return r
 	}
-	return walkNodes(root, func(path string, n node) error {
-		if t, ok := n.(*template); ok && t.repeat > 1 && of(n) > MaxRepeat {
-			return fmt.Errorf("%s: repeat %d multiplies to %d renders along one path, above the maximum %d", path, t.repeat, of(n), MaxRepeat)
+	r := 1
+	for _, e := range renderEdges(n) {
+		if c := m.of(e.to); c > r {
+			r = c
 		}
-		return nil
-	})
+	}
+	if t, ok := n.(*template); ok {
+		r *= t.repeat
+	}
+	m[n] = r
+	return r
+}
+
+func repeatCheck(path string, n node, mem reachMemo) error {
+	if t, ok := n.(*template); ok && t.repeat > 1 && mem.of(n) > MaxRepeat {
+		return fmt.Errorf("%s: repeat %d multiplies to %d renders along one path, above the maximum %d", path, t.repeat, mem.of(n), MaxRepeat)
+	}
+	return nil
 }
 
 // checkNoCycles rejects a reference cycle: a node whose rendering can reach itself

hold.go

@@ -15,20 +15,32 @@ import (
 // It runs after checkNoCycles, whose guarantee is what lets the walk terminate.
 func checkBoundLevelsHeld(root map[string]node) error {
 	return walkNodes(root, func(path string, n node) error {
-		t, ok := n.(*template)
-		if !ok || len(t.held) == 0 {
-			return nil
-		}
-		readers := boundReaders(t.format, t.bound, t.refs)
-		for _, head := range heldHeads(t) {
-			if err := checkHeadHeld(t, head, readers); err != nil {
-				return fmt.Errorf("%s: %w", path, err)
-			}
-		}
-		return nil
+		return heldCheck(path, n)
 	})
 }
 
+// checkNodeBoundLevelsHeld is checkBoundLevelsHeld for one inline node: the same
+// held fence, over its own templates and the shared tree they reference.
+func checkNodeBoundLevelsHeld(n node) error {
+	return eachNode(n, "template", func(path string, m node) error {
+		return heldCheck(path, m)
+	})
+}
+
+func heldCheck(path string, n node) error {
+	t, ok := n.(*template)
+	if !ok || len(t.held) == 0 {
+		return nil
+	}
+	readers := boundReaders(t.format, t.bound, t.refs)
+	for _, head := range heldHeads(t) {
+		if err := checkHeadHeld(t, head, readers); err != nil {
+			return fmt.Errorf("%s: %w", path, err)
+		}
+	}
+	return nil
+}
+
 // heldHeads lists a template's held names, operand heads first, then paths, each
 // in name order: a level read both ways is reported by the operand's fence, and
 // which overlap is reported does not vary.

node.go

@@ -122,6 +122,18 @@ func (t *template) compileFormat() error {
 	return checkNoRepeatedRead(t.format, c, t.refs)
 }
 
+// compileInput compiles an inline template: a JSON value, or a bare format string
+// when the input is not JSON. A format string is a template with no fields, so a
+// string that is itself valid JSON (a JSON string literal) and a bare string
+// compile alike.
+func compileInput(input string) (node, error) {
+	var raw any
+	if err := json.Unmarshal([]byte(input), &raw); err != nil {
+		return compile(input)
+	}
+	return compile(raw)
+}
+
 func compileChoice(items []any) (node, error) {
 	if len(items) == 0 {
 		return nil, fmt.Errorf("empty choice")

reference.go

@@ -72,34 +72,54 @@ func refSegments(name string, folder []string) ([]string, error) {
 // error, never a random render-time one.
 func linkRefs(root map[string]node) error {
 	return eachTemplate(root, func(folder []string, path string, t *template) error {
-		names := refTokens(t.format)
-		if len(names) == 0 {
+		return linkTemplateRefs(folder, path, t, root)
+	})
+}
+
+// linkTemplateRefs binds one template's references against root. A template with
+// none is left untouched, so an inline format that references nothing costs only
+// the refTokens scan.
+func linkTemplateRefs(folder []string, path string, t *template, root map[string]node) error {
+	names := refTokens(t.format)
+	if len(names) == 0 {
+		return nil
+	}
+	if t.fields == nil {
+		t.fields = map[string]node{}
+	}
+	t.refs = make(map[string]refBinding, len(names))
+	for _, name := range names {
+		segments, err := refSegments(name, folder)
+		if err != nil {
+			return fmt.Errorf("%s: reference {%s}: %w", path, name, err)
+		}
+		head, target, tail, err := resolveRef(root, segments)
+		if err != nil {
+			return fmt.Errorf("%s: reference {%s}: %w", path, name, err)
+		}
+		key := "/" + strings.Join(head, ".")
+		if err := checkPath(target, tail, key); err != nil {
+			return fmt.Errorf("%s: reference {%s}: %w", path, name, err)
+		}
+		t.fields[key] = target
+		t.refs[name] = refBinding{key, tail}
+	}
+	if err := t.compileFormat(); err != nil {
+		return fmt.Errorf("%s: %w", path, err)
+	}
+	return nil
+}
+
+// linkNodeRefs binds the references in an inline node's templates against the
+// loaded tree. The node has no folder of its own, so every sigil is root-relative:
+// / and . mean the root, .. has no folder above it.
+func linkNodeRefs(n node, root map[string]node) error {
+	return eachNode(n, "template", func(path string, m node) error {
+		t, ok := m.(*template)
+		if !ok {
 			return nil
 		}
-		if t.fields == nil {
-			t.fields = map[string]node{}
-		}
-		t.refs = make(map[string]refBinding, len(names))
-		for _, name := range names {
-			segments, err := refSegments(name, folder)
-			if err != nil {
-				return fmt.Errorf("%s: reference {%s}: %w", path, name, err)
-			}
-			head, target, tail, err := resolveRef(root, segments)
-			if err != nil {
-				return fmt.Errorf("%s: reference {%s}: %w", path, name, err)
-			}
-			key := "/" + strings.Join(head, ".")
-			if err := checkPath(target, tail, key); err != nil {
-				return fmt.Errorf("%s: reference {%s}: %w", path, name, err)
-			}
-			t.fields[key] = target
-			t.refs[name] = refBinding{key, tail}
-		}
-		if err := t.compileFormat(); err != nil {
-			return fmt.Errorf("%s: %w", path, err)
-		}
-		return nil
+		return linkTemplateRefs(nil, path, t, root)
 	})
 }
 

render.go

@@ -30,6 +30,32 @@ func (f *Generator) Fake(path string) (string, error) {
 	return render(f.rand, n), nil
 }
 
+// FakeTemplate renders an inline template — a format string or a JSON value — the
+// way a category's data is compiled and rendered, references reaching the loaded
+// tree with {/path}. It shares [Fake]'s lock: an inline node is compiled and
+// referenced per draw, so a seeded sequence is reproducible when drawn from one
+// goroutine.
+func (f *Generator) FakeTemplate(input string) (string, error) {
+	f.mu.Lock()
+	defer f.mu.Unlock()
+	n, err := compileInput(input)
+	if err != nil {
+		return "", fmt.Errorf("fejkdata: %w", err)
+	}
+	if err := linkNodeRefs(n, f.categories); err != nil {
+		return "", fmt.Errorf("fejkdata: %w", err)
+	}
+	// No cycle is possible: a reference binds only into the loaded tree, which has
+	// no path into this node, so rendering it cannot reach itself.
+	if err := checkNodeRepeatReach(n); err != nil {
+		return "", fmt.Errorf("fejkdata: %w", err)
+	}
+	if err := checkNodeBoundLevelsHeld(n); err != nil {
+		return "", fmt.Errorf("fejkdata: %w", err)
+	}
+	return render(f.rand, n), nil
+}
+
 // descend walks named fields to the node a path names. It is the one render-side
 // step that can fail, because the path comes from the caller and may name a field
 // that does not exist. A choice consumes no segment, so the rest of the path must