feat(lock-sprite): stable alias-set + source-file persistence across re-bakes

src/lock-sprite.js

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+// src/lock-sprite.js
+// Sprite-Atlas-Lock: stable alias-set + source-file mapping across
+// re-bakes. New aliases append; removed aliases surface in `removed[]`
+// so the caller can loud-warn.
+
+const fs = require('node:fs');
+
+function loadSpriteLock(lockPath) {
+    if (!lockPath || !fs.existsSync(lockPath)) return null;
+    const raw = fs.readFileSync(lockPath, 'utf8');
+    return JSON.parse(raw);
+}
+
+function mergeSpriteLock(existing, sources) {
+    const sourceMap = {};
+    const currentAliases = [];
+    for (const s of sources) {
+        sourceMap[s.alias] = s.sourceFile;
+        currentAliases.push(s.alias);
+    }
+    let mergedAliases;
+    let removed = [];
+    if (existing) {
+        const incoming = new Set(currentAliases);
+        // Preserve existing order; append new
+        mergedAliases = existing.aliases.filter(a => incoming.has(a));
+        for (const a of currentAliases) {
+            if (!existing.aliases.includes(a)) mergedAliases.push(a);
+        }
+        removed = existing.aliases.filter(a => !incoming.has(a));
+    } else {
+        mergedAliases = currentAliases;
+    }
+    return {
+        merged: {
+            atlas_id: existing ? existing.atlas_id : null,
+            version: 1,
+            aliases: mergedAliases,
+            sources: sourceMap,
+        },
+        removed,
+    };
+}
+
+function serializeSpriteLock(lock) {
+    return JSON.stringify(lock, null, 2) + '\n';
+}
+
+module.exports = { loadSpriteLock, mergeSpriteLock, serializeSpriteLock };

tests/lock-sprite.test.js

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+const { test } = require('node:test');
+const assert = require('node:assert');
+const fs = require('node:fs');
+const path = require('node:path');
+const os = require('node:os');
+const { loadSpriteLock, mergeSpriteLock, serializeSpriteLock } = require('../src/lock-sprite');
+
+test('loadSpriteLock: missing file -> null', () => {
+    const result = loadSpriteLock('/nonexistent/lock.json');
+    assert.strictEqual(result, null);
+});
+
+test('loadSpriteLock: reads valid lock', () => {
+    const tmp = fs.mkdtempSync(path.join(os.tmpdir(), 'lock-test-'));
+    const lockPath = path.join(tmp, 'lock.json');
+    fs.writeFileSync(lockPath, JSON.stringify({
+        atlas_id: 'tcbasics',
+        version: 1,
+        aliases: ['bed1', 'bench1'],
+        sources: { bed1: 'Bed1.png', bench1: 'Bench1.png' },
+    }));
+    const lock = loadSpriteLock(lockPath);
+    assert.strictEqual(lock.atlas_id, 'tcbasics');
+    assert.deepStrictEqual(lock.aliases, ['bed1', 'bench1']);
+});
+
+test('mergeSpriteLock: no existing lock -> all new', () => {
+    const sources = [
+        { alias: 'bed1',  sourceFile: 'Bed1.png' },
+        { alias: 'bench1',sourceFile: 'Bench1.png' },
+    ];
+    const { merged, removed } = mergeSpriteLock(null, sources);
+    assert.deepStrictEqual(merged.aliases, ['bed1', 'bench1']);
+    assert.deepStrictEqual(removed, []);
+});
+
+test('mergeSpriteLock: existing alias preserved; new appended', () => {
+    const existing = {
+        atlas_id: 'tcbasics',
+        version: 1,
+        aliases: ['bed1'],
+        sources: { bed1: 'Bed1.png' },
+    };
+    const sources = [
+        { alias: 'bed1',   sourceFile: 'Bed1.png' },
+        { alias: 'bench1', sourceFile: 'Bench1.png' },
+    ];
+    const { merged, removed } = mergeSpriteLock(existing, sources);
+    assert.deepStrictEqual(merged.aliases, ['bed1', 'bench1']);
+    assert.deepStrictEqual(removed, []);
+});
+
+test('mergeSpriteLock: removed alias appears in `removed`', () => {
+    const existing = {
+        atlas_id: 'tcbasics',
+        version: 1,
+        aliases: ['bed1', 'bench1'],
+        sources: { bed1: 'Bed1.png', bench1: 'Bench1.png' },
+    };
+    const sources = [
+        { alias: 'bed1', sourceFile: 'Bed1.png' },
+    ];
+    const { merged, removed } = mergeSpriteLock(existing, sources);
+    assert.deepStrictEqual(merged.aliases, ['bed1']);
+    assert.deepStrictEqual(removed, ['bench1']);
+});
+
+test('serializeSpriteLock: stable JSON shape', () => {
+    const lock = {
+        atlas_id: 'tcbasics',
+        version: 1,
+        aliases: ['bed1'],
+        sources: { bed1: 'Bed1.png' },
+    };
+    const json = serializeSpriteLock(lock);
+    const parsed = JSON.parse(json);
+    assert.deepStrictEqual(parsed, lock);
+    // Trailing newline for POSIX-friendliness
+    assert.ok(json.endsWith('\n'));
+});