From 0f5161164fa0dd9454bead4128b8b5baeda21012 Mon Sep 17 00:00:00 2001
From: Codex <codex@local>
Date: Wed, 2 Sep 2026 22:31:28 +0300
Subject: [PATCH] test: prove the drift gate itself can fail (#422)

User-Visible: no
Issue: #422
---
 scripts/mutation-gate.mjs              | 24 +++++++++++++
 test/capture-determinism-gate.test.mjs | 50 ++++++++++++++++++++++++++
 2 files changed, 74 insertions(+)
 create mode 100644 test/capture-determinism-gate.test.mjs

diff --git a/scripts/mutation-gate.mjs b/scripts/mutation-gate.mjs
index ac4c15c0..c593df10 100644
--- a/scripts/mutation-gate.mjs
+++ b/scripts/mutation-gate.mjs
@@ -956,6 +956,30 @@ const MUTANT_DEFINITIONS = [
       replace: '',
     }],
   },
+  {
+    id: 'capture-gate-forgives-a-missing-frame',
+    guard: 'node --test test/capture-determinism-gate.test.mjs',
+    because: 'comparing only the frames both runs happen to have lets a run that '
+      + 'produced nine frames pass against a run that produced ten — «совпало» then '
+      + 'means nothing, which is the opposite of what the gate is for (#422)',
+    patches: [{
+      file: 'scripts/capture-determinism.mjs',
+      find: '  const names = [...new Set([...Object.keys(first), ...Object.keys(second)])].sort();',
+      replace: '  const names = Object.keys(first).filter((name) => name in second).sort();',
+    }],
+  },
+  {
+    id: 'capture-gate-hashes-everything-in-the-folder',
+    guard: 'node --test test/capture-determinism-gate.test.mjs',
+    because: 'hashing every file in docs/images drags the manifest into the comparison, '
+      + 'and the manifest legitimately changes between runs — the gate would then cry '
+      + 'wolf on every capture and be switched off within a week (#422)',
+    patches: [{
+      file: 'scripts/capture-determinism.mjs',
+      find: "    if (!name.endsWith('.png')) continue;\n",
+      replace: '',
+    }],
+  },
   {
     id: 'capture-drifts-between-runs',
     guard: 'node --test test/capture-clip.test.mjs',
diff --git a/test/capture-determinism-gate.test.mjs b/test/capture-determinism-gate.test.mjs
new file mode 100644
index 00000000..6ea379b2
--- /dev/null
+++ b/test/capture-determinism-gate.test.mjs
@@ -0,0 +1,50 @@
+import test from 'node:test';
+import assert from 'node:assert/strict';
+
+import { driftBetweenRuns, frameHashes } from '../scripts/capture-determinism.mjs';
+
+// #422. Гейт сравнивает два прогона съёмки. Его собственная логика должна
+// уметь падать не меньше, чем то, что он охраняет: разошедшийся кадр, кадр
+// пропавший и кадр появившийся — всё это расхождения, потому что набор обязан
+// быть одним и тем же, иначе «совпало» ничего не значит.
+
+const files = { 'a.png': 'AAA', 'b.png': 'BBB', 'notes.txt': 'ignore me' };
+const fakeList = () => Object.keys(files);
+const fakeRead = (path) => Buffer.from(files[path.split('/').pop()]);
+
+test('хешируются только кадры, посторонние файлы не участвуют', () => {
+  const hashes = frameHashes('/nowhere', fakeRead, fakeList);
+  assert.deepEqual(Object.keys(hashes), ['a.png', 'b.png']);
+  assert.match(hashes['a.png'], /^[0-9a-f]{64}$/);
+  assert.notEqual(hashes['a.png'], hashes['b.png']);
+});
+
+test('одинаковые прогоны расхождений не дают', () => {
+  const first = frameHashes('/nowhere', fakeRead, fakeList);
+  assert.deepEqual(driftBetweenRuns(first, { ...first }), []);
+});
+
+test('разошедшийся кадр назван по имени', () => {
+  const drift = driftBetweenRuns({ 'a.png': '1', 'b.png': '2' }, { 'a.png': '1', 'b.png': '9' });
+  assert.deepEqual(drift.map((item) => item.name), ['b.png']);
+  assert.equal(drift[0].first, '2');
+  assert.equal(drift[0].second, '9');
+});
+
+test('пропавший и появившийся кадр — тоже расхождение', () => {
+  const lost = driftBetweenRuns({ 'a.png': '1', 'b.png': '2' }, { 'a.png': '1' });
+  assert.deepEqual(lost.map((item) => item.name), ['b.png']);
+  assert.equal(lost[0].second, '(нет кадра)');
+
+  const appeared = driftBetweenRuns({ 'a.png': '1' }, { 'a.png': '1', 'c.png': '3' });
+  assert.deepEqual(appeared.map((item) => item.name), ['c.png']);
+  assert.equal(appeared[0].first, '(нет кадра)');
+});
+
+test('расхождения перечисляются в устойчивом порядке', () => {
+  const drift = driftBetweenRuns(
+    { 'c.png': '1', 'a.png': '1', 'b.png': '1' },
+    { 'c.png': '9', 'a.png': '9', 'b.png': '9' },
+  );
+  assert.deepEqual(drift.map((item) => item.name), ['a.png', 'b.png', 'c.png']);
+});