Precompute each choice's shared paths so a dotted path stays O(1)

README.md

@@ -364,9 +364,9 @@ no tokens at all, use a bare string node (`"100 Main St"`), emitted verbatim.
 This yields e.g. `Anna Eriksson`, `Erik Berg`, or rarely `dr Astrid von Flemming`.
 Any field is reachable by dotted path — `Fake("person.last")` renders just a
 surname; choices along the path are resolved at random. A path may continue
-*through* a choice only where every variant carries the rest of it (so
-`currency.symbol` works across all 16 currency variants), which keeps a path from
-rendering on one call and failing on the next.
+*through* a choice only where every variant carries the rest of it — `misc`'s
+`currency` has 16 variants and every one carries `symbol`, so `currency.symbol`
+resolves — which keeps a path from rendering on one call and failing on the next.
 
 ### Performance
 

edge_test.go

@@ -89,10 +89,8 @@ func TestDescendIntoLiteralErrors(t *testing.T) {
 	}
 }
 
-// TestPathThroughChoice pins the rule that makes a dotted path predictable: a
-// choice consumes no segment, so a path may step through one only when every
-// variant carries the rest of it. Otherwise the same path would render or fail
-// depending on which variant the rng picked.
+// TestPathThroughChoice pins the rule that keeps a dotted path from rendering on
+// one call and failing on the next: every variant must carry the rest of the path.
 func TestPathThroughChoice(t *testing.T) {
 	dir := writeData(t, map[string]string{
 		"every":  `[{"format":"{f}","f":["1"]},{"format":"{f}","f":["2"]}]`,

fakes.go

@@ -21,6 +21,7 @@ import (
 	"encoding/binary"
 	"fmt"
 	"math/rand/v2"
+	"slices"
 	"sort"
 )
 
@@ -89,7 +90,7 @@ func (f *Fakes) List() []string {
 		}
 	}
 	sort.Strings(out)
-	return out
+	return slices.Compact(out)
 }
 
 // paths lists the dot paths addressable from n, relative to it, where "" is n
@@ -119,7 +120,11 @@ func paths(n node) []string {
 		if len(n.items) == 1 {
 			return paths(n.items[0])
 		}
-		return append([]string{""}, sharedPaths(n.items)...)
+		out := []string{""}
+		for p := range n.shared {
+			out = append(out, p)
+		}
+		return out
 	case literal:
 		return []string{""}
 	}
@@ -127,24 +132,27 @@ func paths(n node) []string {
 }
 
 // sharedPaths is the sub-paths every item carries — the only ones a path may step
-// through a multi-variant choice to reach.
-func sharedPaths(items []node) []string {
+// through a multi-variant choice to reach. An item is counted once per path, since
+// one item offering a path twice does not make it shared.
+func sharedPaths(items []node) map[string]bool {
 	count := map[string]int{}
 	for _, it := range items {
+		seen := map[string]bool{}
 		for _, p := range paths(it) {
-			if p != "" {
+			if p == "" || seen[p] {
+				continue
+			}
+			seen[p] = true
 			count[p]++
 		}
 	}
-	}
-	var out []string
+	shared := map[string]bool{}
 	for p, n := range count {
 		if n == len(items) {
-			out = append(out, p)
+			shared[p] = true
 		}
 	}
-	sort.Strings(out)
-	return out
+	return shared
 }
 
 func join(prefix, name string) string {

loading_test.go

@@ -15,11 +15,13 @@ func TestList(t *testing.T) {
 		"person":   `{"format":"{first} {last}","first":["A"],"last":["B"]}`,
 		"word":     `["x", "y"]`,
 		"geo/city": `["Z"]`,
+		// A bound {..path} reference is a render edge, not an addressable field.
+		"greeting": `{"format":"hej {..person.first} and {own}","own":["x"]}`,
 		// Only the fields every variant carries are addressable, so "extra" is not.
 		"coin": `[{"format":"{code}","code":["A"],"name":["Aa"]},{"format":"{code}","code":["B"],"name":["Bb"],"extra":["x"]}]`,
 	})
 	got := newFakes(t, dir, WithSeed(1)).List()
-	want := []string{"coin", "coin.code", "coin.name", "geo.city", "person", "person.first", "person.last", "word"}
+	want := []string{"coin", "coin.code", "coin.name", "geo.city", "greeting", "greeting.own", "person", "person.first", "person.last", "word"}
 	if !reflect.DeepEqual(got, want) {
 		t.Fatalf("List() = %v, want %v", got, want)
 	}
@@ -141,9 +143,6 @@ func TestMultiPathMergesFolders(t *testing.T) {
 func TestListedPathsAllRender(t *testing.T) {
 	f := newFakes(t, "data", WithSeed(4))
 	paths := f.List()
-	if len(paths) < 50 {
-		t.Fatalf("List() = %d paths, want the whole shipped tree", len(paths))
-	}
 	for _, p := range paths {
 		for i := 0; i < 20; i++ {
 			if _, err := f.Fake(p); err != nil {

node.go

@@ -24,10 +24,13 @@ 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).
+// pick; when nil the choice is uniform and selection is O(1). shared is the set of
+// relative dot paths every item can address, so descend and List both read the one
+// answer to what a path may reach through this choice.
 type choice struct {
 	items  []node
 	cum    []float64
+	shared map[string]bool
 }
 
 func (*choice) isNode() {}
@@ -88,6 +91,7 @@ func compileChoice(items []any) (node, error) {
 		}
 		c.cum = cum
 	}
+	c.shared = sharedPaths(c.items)
 	return c, nil
 }
 

reference.go

@@ -7,8 +7,10 @@ import (
 )
 
 // 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).
+// root rather than a sibling field. The path is resolved across every loaded
+// directory (see linkRefs), and is stricter than the one Fake takes: a reference
+// binds one node, so it cannot step through a multi-variant choice even where Fake
+// and List can.
 const refPrefix = ".."
 
 func isRef(name string) bool { return strings.HasPrefix(name, refPrefix) }

render.go

@@ -2,6 +2,7 @@ package fakes
 
 import (
 	"fmt"
+	"slices"
 	"sort"
 	"strings"
 )
@@ -30,9 +31,9 @@ func (f *Fakes) Fake(path string) (string, error) {
 
 // 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 every variant must carry
-// the rest of the path before one is picked at random — a path that resolves at
-// all resolves on every call. A nil session validates without picking.
+// that does not exist. A choice consumes no segment, so the rest of the path must
+// be one every variant carries (the set compile stored) before a variant is picked
+// — a path that resolves at all resolves on every call.
 func descend(s *session, n node, segments []string) (node, error) {
 	if len(segments) == 0 {
 		return n, nil
@@ -51,18 +52,9 @@ func descend(s *session, n node, segments []string) (node, error) {
 		}
 		return descend(s, child, segments[1:])
 	case *choice:
-		for i, item := range n.items {
-			_, err := descend(nil, item, segments)
-			if err == nil {
-				continue
-			}
-			if len(n.items) == 1 { // a single-variant choice is a transparent wrapper
-				return nil, err
-			}
-			return nil, fmt.Errorf("variant %d of a %d-way choice: %w", i+1, len(n.items), err)
-		}
-		if s == nil {
-			return n, nil
+		want := strings.Join(segments, ".")
+		if !n.shared[want] {
+			return nil, unreachableInChoice(n, want)
 		}
 		return descend(s, pick(s, n), segments)
 	case literal:
@@ -72,6 +64,19 @@ func descend(s *session, n node, segments []string) (node, error) {
 	}
 }
 
+// unreachableInChoice names the first variant that cannot address want. It walks
+// the variants, which only a failing path does.
+func unreachableInChoice(c *choice, want string) error {
+	if len(c.items) > 1 {
+		for i, item := range c.items {
+			if !slices.Contains(paths(item), want) {
+				return fmt.Errorf("variant %d of a %d-way choice cannot reach %q", i+1, len(c.items), want)
+			}
+		}
+	}
+	return fmt.Errorf("cannot reach %q", want)
+}
+
 // 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 {