Advertise only spellable paths, and name what a choice's variants share

README.md

@@ -141,8 +141,8 @@ 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). It is exactly the
-set `Fake` accepts.
+dotted fields and folder segments (what the CLI's `-list` prints). Every path it
+lists renders.
 
 A `*Fakes` is **not** safe for concurrent use — create one per goroutine.
 
@@ -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 — `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.
+*through* a choice only where every variant carries the rest of it — every
+`currency` variant carries `symbol`, so `currency.symbol` resolves — which keeps a
+path from rendering on one call and failing on the next.
 
 ### Performance
 

edge_test.go

@@ -97,6 +97,7 @@ func TestPathThroughChoice(t *testing.T) {
 	dir := writeData(t, map[string]string{
 		"every":  `[{"format":"{f}","f":["1"]},{"format":"{f}","f":["2"]}]`,
 		"notall": `[{"format":"{f}","f":["1"]},["plain"]]`,
+		"some":   `[{"format":"{f}","f":["1"]},{"format":"{f}","f":["2"],"extra":["x"]}]`,
 	})
 	f := newFakes(t, dir, WithSeed(1))
 	for i := 0; i < 200; i++ {
@@ -104,6 +105,10 @@ func TestPathThroughChoice(t *testing.T) {
 			t.Fatalf("every.f = %q, want 1 or 2", got)
 		}
 	}
+	// A path only some variants carry is reported against what all of them carry.
+	if _, err := f.Fake("some.extra"); err == nil || !strings.Contains(err.Error(), "all carry [f]") {
+		t.Errorf("Fake(some.extra) = %v, want it to name what every variant carries", err)
+	}
 	var first string
 	for i := 0; i < 200; i++ {
 		_, err := f.Fake("notall.f")

fakes.go

@@ -21,8 +21,8 @@ import (
 	"encoding/binary"
 	"fmt"
 	"math/rand/v2"
-	"slices"
 	"sort"
+	"strings"
 )
 
 // Fakes generates fake data from a loaded namespace tree. Create one with [New].
@@ -90,7 +90,15 @@ func (f *Fakes) List() []string {
 		}
 	}
 	sort.Strings(out)
-	return slices.Compact(out)
+	return out
+}
+
+// addressable reports whether a name can be one segment of a dot path. A dot would
+// split it into two segments and an empty name into none, so a path through such a
+// name cannot be spelled — List must not offer one, and it must not count towards
+// what a choice's variants share.
+func addressable(name string) bool {
+	return name != "" && !strings.Contains(name, ".")
 }
 
 // paths lists the dot paths addressable from n, relative to it, where "" is n
@@ -100,6 +108,9 @@ func paths(n node) []string {
 	case *group:
 		var out []string
 		for _, name := range sortedNames(n.children) {
+			if !addressable(name) {
+				continue
+			}
 			for _, p := range paths(n.children[name]) {
 				out = append(out, join(name, p))
 			}
@@ -108,7 +119,7 @@ func paths(n node) []string {
 	case *template:
 		out := []string{""}
 		for _, name := range sortedNames(n.fields) {
-			if isRef(name) { // a bound {..path} reference, not an authored field
+			if isRef(name) || !addressable(name) { // a binding, or unreachable by path
 				continue
 			}
 			for _, p := range paths(n.fields[name]) {
@@ -132,29 +143,35 @@ 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. An item is counted once per path, since
-// one item offering a path twice does not make it shared.
+// through a multi-variant choice to reach. It intersects, bailing as soon as the set
+// is empty, which is immediate for a choice of plain strings.
 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 == "" || seen[p] {
-				continue
+	shared := subPaths(items[0])
+	for _, it := range items[1:] {
+		if len(shared) == 0 {
+			return nil
 		}
-			seen[p] = true
-			count[p]++
+		next := subPaths(it)
+		for p := range shared {
+			if !next[p] {
+				delete(shared, p)
 			}
 		}
-	shared := map[string]bool{}
-	for p, n := range count {
-		if n == len(items) {
-			shared[p] = true
-		}
 	}
 	return shared
 }
 
+// subPaths is paths(n) as a set, without the empty path that means n itself.
+func subPaths(n node) map[string]bool {
+	out := map[string]bool{}
+	for _, p := range paths(n) {
+		if p != "" {
+			out[p] = true
+		}
+	}
+	return out
+}
+
 func join(prefix, name string) string {
 	switch {
 	case prefix == "":

node.go

@@ -91,7 +91,11 @@ func compileChoice(items []any) (node, error) {
 		}
 		c.cum = cum
 	}
+	if len(c.items) > 1 {
+		// Safe to precompute: a choice's items come from one file, so no group can
+		// appear inside one, and neither mergeChildren nor linkRefs can reach in.
 		c.shared = sharedPaths(c.items)
+	}
 	return c, nil
 }
 

render.go

@@ -2,7 +2,6 @@ package fakes
 
 import (
 	"fmt"
-	"slices"
 	"sort"
 	"strings"
 )
@@ -52,10 +51,11 @@ func descend(s *session, n node, segments []string) (node, error) {
 		}
 		return descend(s, child, segments[1:])
 	case *choice:
-		want := strings.Join(segments, ".")
-		if !n.shared[want] {
+		if len(n.items) > 1 {
+			if want := strings.Join(segments, "."); !n.shared[want] {
 				return nil, unreachableInChoice(n, want)
 			}
+		}
 		return descend(s, pick(s, n), segments)
 	case literal:
 		return nil, fmt.Errorf("a plain string has no field %q", segments[0])
@@ -64,17 +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.
+// unreachableInChoice reports that a path cannot step through this choice, listing
+// what every variant does carry. It reads the precomputed set, so a failing path
+// costs no more than a rendering one.
 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)
+	if len(c.shared) == 0 {
+		return fmt.Errorf("no variant of this %d-way choice carries %q", len(c.items), want)
 	}
+	offered := make([]string, 0, len(c.shared))
+	for p := range c.shared {
+		offered = append(offered, p)
 	}
-	}
-	return fmt.Errorf("cannot reach %q", want)
+	sort.Strings(offered)
+	return fmt.Errorf("not every variant of this %d-way choice carries %q; all carry %v", len(c.items), want, offered)
 }
 
 // render evaluates a compiled node to a string. compile validates every node up