From c20ea4b9163d7bcc9e52dc812245c7726e55b558 Mon Sep 17 00:00:00 2001
From: Eric Rakestraw <eric@rakestraw.us>
Date: Fri, 9 Oct 2026 12:16:13 +0000
Subject: [PATCH] Index combined diff sensitivity

---
 docs/roadmap/implementation.md           |  2 +-
 internal/engine/diff_sensitivity.go      | 48 +++++++++++++
 internal/engine/diff_sensitivity_test.go | 90 ++++++++++++++++++++++++
 3 files changed, 139 insertions(+), 1 deletion(-)
 create mode 100644 internal/engine/diff_sensitivity.go
 create mode 100644 internal/engine/diff_sensitivity_test.go

diff --git a/docs/roadmap/implementation.md b/docs/roadmap/implementation.md
index 4c42d62..ada2f60 100644
--- a/docs/roadmap/implementation.md
+++ b/docs/roadmap/implementation.md
@@ -177,7 +177,7 @@ no consumer-facing operation exposes them yet.
 
 ## Stage 3 — Combine sensitivity into a local lookup
 
-Status: planned. Scope: classification lookup, independently of safe rendering.
+Status: complete. Scope: classification lookup, independently of safe rendering.
 
 1. Add `internal/engine/diff_sensitivity.go` and focused tests. Union true
    classifications from both inspections without mutating either map.
diff --git a/internal/engine/diff_sensitivity.go b/internal/engine/diff_sensitivity.go
new file mode 100644
index 0000000..802950f
--- /dev/null
+++ b/internal/engine/diff_sensitivity.go
@@ -0,0 +1,48 @@
+package engine
+
+type diffSensitivity struct {
+	prefixes pathPrefixIndex
+	pointers []string
+}
+
+func newDiffSensitivity(left, right map[string]bool) *diffSensitivity {
+	sensitivity := &diffSensitivity{}
+	seen := make(map[string]struct{}, len(left)+len(right))
+	for _, classifications := range []map[string]bool{left, right} {
+		for pointer, classified := range classifications {
+			if !classified {
+				continue
+			}
+			if _, found := seen[pointer]; found {
+				continue
+			}
+			seen[pointer] = struct{}{}
+			id := len(sensitivity.pointers)
+			sensitivity.pointers = append(sensitivity.pointers, pointer)
+			sensitivity.prefixes.insert(pointer, id)
+		}
+	}
+	return sensitivity
+}
+
+func (sensitivity *diffSensitivity) isSensitiveAtOrAbove(pointer string) bool {
+	if sensitivity == nil {
+		return false
+	}
+	found := false
+	sensitivity.prefixes.visitAncestors(pointer, func(int) { found = true })
+	return found
+}
+
+func (sensitivity *diffSensitivity) hasStrictDescendant(pointer string) bool {
+	if sensitivity == nil {
+		return false
+	}
+	found := false
+	sensitivity.prefixes.visitDescendants(pointer, func(id int) {
+		if sensitivity.pointers[id] != pointer {
+			found = true
+		}
+	})
+	return found
+}
diff --git a/internal/engine/diff_sensitivity_test.go b/internal/engine/diff_sensitivity_test.go
new file mode 100644
index 0000000..fe252b6
--- /dev/null
+++ b/internal/engine/diff_sensitivity_test.go
@@ -0,0 +1,90 @@
+package engine
+
+import (
+	"maps"
+	"testing"
+)
+
+func TestDiffSensitivityCombinesClassificationsWithoutMutation(t *testing.T) {
+	left := map[string]bool{
+		"/a":                  true,
+		"/a/b":                true,
+		"/historical/removed": true,
+		"/cross~1key":         true,
+		"/literal~01slash":    true,
+		"/items/01":           true,
+		"/items/+1":           false,
+		"/false-left":         false,
+	}
+	right := map[string]bool{
+		"/a/b":         true,
+		"/right-only":  true,
+		"/false-right": false,
+	}
+	leftBefore, rightBefore := maps.Clone(left), maps.Clone(right)
+	sensitivity := newDiffSensitivity(left, right)
+
+	for _, test := range []struct {
+		pointer string
+		want    bool
+	}{
+		{pointer: "/a", want: true},
+		{pointer: "/a/b", want: true},
+		{pointer: "/a/b/c", want: true},
+		{pointer: "/ab", want: false},
+		{pointer: "/ab/c", want: false},
+		{pointer: "/historical/removed", want: true},
+		{pointer: "/historical/removed/child", want: true},
+		{pointer: "/historical", want: false},
+		{pointer: "/right-only", want: true},
+		{pointer: "/false-left", want: false},
+		{pointer: "/false-right", want: false},
+		{pointer: "/cross~1key", want: true},
+		{pointer: "/cross/key", want: false},
+		{pointer: "/literal~01slash", want: true},
+		{pointer: "/literal~1slash", want: false},
+		{pointer: "/items/01", want: true},
+		{pointer: "/items/1", want: false},
+		{pointer: "/items/+1", want: false},
+	} {
+		if got := sensitivity.isSensitiveAtOrAbove(test.pointer); got != test.want {
+			t.Errorf("isSensitiveAtOrAbove(%q) = %v, want %v", test.pointer, got, test.want)
+		}
+	}
+
+	for _, test := range []struct {
+		pointer string
+		want    bool
+	}{
+		{pointer: "", want: true},
+		{pointer: "/a", want: true},
+		{pointer: "/a/b", want: false},
+		{pointer: "/ab", want: false},
+		{pointer: "/historical", want: true},
+		{pointer: "/right-only", want: false},
+		{pointer: "/cross~1key", want: false},
+	} {
+		if got := sensitivity.hasStrictDescendant(test.pointer); got != test.want {
+			t.Errorf("hasStrictDescendant(%q) = %v, want %v", test.pointer, got, test.want)
+		}
+	}
+
+	if !maps.Equal(left, leftBefore) || !maps.Equal(right, rightBefore) {
+		t.Fatal("classification maps were modified")
+	}
+}
+
+func TestDiffSensitivityDistinguishesRootAndEmptyKey(t *testing.T) {
+	root := newDiffSensitivity(map[string]bool{"": true}, nil)
+	if !root.isSensitiveAtOrAbove("") || !root.isSensitiveAtOrAbove("/empty") ||
+		root.hasStrictDescendant("") {
+		t.Fatal("root classification did not remain distinct from strict descendants")
+	}
+
+	emptyKey := newDiffSensitivity(map[string]bool{"/": true}, nil)
+	if emptyKey.isSensitiveAtOrAbove("") || !emptyKey.isSensitiveAtOrAbove("/") ||
+		emptyKey.isSensitiveAtOrAbove("/other") || !emptyKey.hasStrictDescendant("") ||
+		emptyKey.hasStrictDescendant("/") {
+		t.Fatal("empty-key classification was treated as the root or an unrelated path")
+	}
+}