test: add incremental edge parity CI check

Four mutation scenarios verify that incremental rebuilds produce
identical edges to clean full builds: comment-only touch, body edit,
new export addition, and file deletion. Each scenario compares node
and edge identity between incremental and full builds.

Author: carlos-alm

tests/integration/incremental-edge-parity.test.js

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+/**
+ * Incremental edge parity CI check.
+ *
+ * Verifies that incremental rebuilds produce exactly the same edges as a
+ * clean full build, across multiple mutation scenarios:
+ *   1. Comment-only touch (no semantic change)
+ *   2. Body edit (change implementation, keep exports)
+ *   3. New export added (structural change)
+ *
+ * Uses the sample-project fixture (CJS, classes, cross-file calls) for
+ * broader edge coverage than the barrel-project fixture.
+ */
+
+import fs from 'node:fs';
+import os from 'node:os';
+import path from 'node:path';
+import Database from 'better-sqlite3';
+import { beforeAll, describe, expect, it } from 'vitest';
+import { buildGraph } from '../../src/domain/graph/builder.js';
+
+const FIXTURE_DIR = path.join(import.meta.dirname, '..', 'fixtures', 'sample-project');
+
+function copyDirSync(src, dest) {
+  fs.mkdirSync(dest, { recursive: true });
+  for (const entry of fs.readdirSync(src, { withFileTypes: true })) {
+    const s = path.join(src, entry.name);
+    const d = path.join(dest, entry.name);
+    if (entry.isDirectory()) copyDirSync(s, d);
+    else fs.copyFileSync(s, d);
+  }
+}
+
+function readEdges(dbPath) {
+  const db = new Database(dbPath, { readonly: true });
+  const edges = db
+    .prepare(
+      `SELECT n1.name AS source_name, n2.name AS target_name, e.kind
+       FROM edges e
+       JOIN nodes n1 ON e.source_id = n1.id
+       JOIN nodes n2 ON e.target_id = n2.id
+       ORDER BY n1.name, n2.name, e.kind`,
+    )
+    .all();
+  db.close();
+  return edges;
+}
+
+function readNodes(dbPath) {
+  const db = new Database(dbPath, { readonly: true });
+  const nodes = db.prepare('SELECT name, kind, file FROM nodes ORDER BY name, kind, file').all();
+  db.close();
+  return nodes;
+}
+
+function edgeKey(e) {
+  return `${e.source_name} -[${e.kind}]-> ${e.target_name}`;
+}
+
+/**
+ * Build a full-build copy and an incremental-build copy after applying
+ * the same mutation to both, then compare edges.
+ */
+async function buildAndCompare(fixtureDir, mutate) {
+  const tmpBase = fs.mkdtempSync(path.join(os.tmpdir(), 'codegraph-edge-parity-'));
+  const fullDir = path.join(tmpBase, 'full');
+  const incrDir = path.join(tmpBase, 'incr');
+
+  try {
+    copyDirSync(fixtureDir, fullDir);
+    copyDirSync(fixtureDir, incrDir);
+
+    // Initial full build on the incr copy (establishes baseline hashes)
+    await buildGraph(incrDir, { incremental: false, skipRegistry: true });
+
+    // Apply the mutation to both copies
+    mutate(fullDir);
+    mutate(incrDir);
+
+    // Full build on the full copy (clean, from scratch)
+    await buildGraph(fullDir, { incremental: false, skipRegistry: true });
+    // Incremental rebuild on the incr copy
+    await buildGraph(incrDir, { incremental: true, skipRegistry: true });
+
+    const fullEdges = readEdges(path.join(fullDir, '.codegraph', 'graph.db'));
+    const incrEdges = readEdges(path.join(incrDir, '.codegraph', 'graph.db'));
+    const fullNodes = readNodes(path.join(fullDir, '.codegraph', 'graph.db'));
+    const incrNodes = readNodes(path.join(incrDir, '.codegraph', 'graph.db'));
+
+    return { fullEdges, incrEdges, fullNodes, incrNodes };
+  } finally {
+    fs.rmSync(tmpBase, { recursive: true, force: true });
+  }
+}
+
+describe('Incremental edge parity (CI gate)', () => {
+  // Scenario 1: Comment-only touch — edges must be identical
+  describe('comment-only touch', () => {
+    let result;
+
+    beforeAll(async () => {
+      result = await buildAndCompare(FIXTURE_DIR, (dir) => {
+        const p = path.join(dir, 'math.js');
+        fs.appendFileSync(p, '\n// comment touch\n');
+      });
+    }, 60_000);
+
+    it('edge count matches', () => {
+      expect(result.incrEdges.length).toBe(result.fullEdges.length);
+    });
+
+    it('edges are identical', () => {
+      expect(result.incrEdges).toEqual(result.fullEdges);
+    });
+  });
+
+  // Scenario 2: Body edit — change function implementation, keep exports
+  describe('body edit (same exports)', () => {
+    let result;
+
+    beforeAll(async () => {
+      result = await buildAndCompare(FIXTURE_DIR, (dir) => {
+        const p = path.join(dir, 'math.js');
+        let src = fs.readFileSync(p, 'utf-8');
+        // Change add implementation but keep the same signature and exports
+        src = src.replace('return a + b;', 'return b + a;');
+        fs.writeFileSync(p, src);
+      });
+    }, 60_000);
+
+    it('edge count matches', () => {
+      expect(result.incrEdges.length).toBe(result.fullEdges.length);
+    });
+
+    it('edges are identical', () => {
+      expect(result.incrEdges).toEqual(result.fullEdges);
+    });
+  });
+
+  // Scenario 3: New export added — edges from consumers should resolve
+  describe('new export added', () => {
+    let result;
+
+    beforeAll(async () => {
+      result = await buildAndCompare(FIXTURE_DIR, (dir) => {
+        const mathPath = path.join(dir, 'math.js');
+        let src = fs.readFileSync(mathPath, 'utf-8');
+        // Add a new function before the module.exports line
+        src = src.replace(
+          'module.exports = { add, multiply, square };',
+          `function subtract(a, b) {\n  return a - b;\n}\n\nmodule.exports = { add, multiply, square, subtract };`,
+        );
+        fs.writeFileSync(mathPath, src);
+
+        // Have index.js import and call the new function
+        const indexPath = path.join(dir, 'index.js');
+        let indexSrc = fs.readFileSync(indexPath, 'utf-8');
+        indexSrc = indexSrc.replace(
+          "const { add } = require('./math');",
+          "const { add, subtract } = require('./math');",
+        );
+        indexSrc = indexSrc.replace(
+          'console.log(add(1, 2));',
+          'console.log(add(1, 2));\n  console.log(subtract(5, 3));',
+        );
+        fs.writeFileSync(indexPath, indexSrc);
+      });
+    }, 60_000);
+
+    it('node count matches', () => {
+      expect(result.incrNodes.length).toBe(result.fullNodes.length);
+    });
+
+    it('edge count matches', () => {
+      expect(result.incrEdges.length).toBe(result.fullEdges.length);
+    });
+
+    it('edges are identical', () => {
+      if (result.incrEdges.length !== result.fullEdges.length) {
+        // Diagnostic: show which edges differ
+        const fullSet = new Set(result.fullEdges.map(edgeKey));
+        const incrSet = new Set(result.incrEdges.map(edgeKey));
+        const missingInIncr = [...fullSet].filter((k) => !incrSet.has(k));
+        const extraInIncr = [...incrSet].filter((k) => !fullSet.has(k));
+        expect.fail(
+          `Edge mismatch:\n  Missing in incremental: ${missingInIncr.join(', ') || 'none'}\n  Extra in incremental: ${extraInIncr.join(', ') || 'none'}`,
+        );
+      }
+      expect(result.incrEdges).toEqual(result.fullEdges);
+    });
+  });
+
+  // Scenario 4: File deletion — stale edges must be purged
+  describe('file deletion', () => {
+    let result;
+
+    beforeAll(async () => {
+      result = await buildAndCompare(FIXTURE_DIR, (dir) => {
+        // Delete utils.js — edges involving sumOfSquares/Calculator should disappear
+        fs.unlinkSync(path.join(dir, 'utils.js'));
+        // Update index.js to remove the require
+        const indexPath = path.join(dir, 'index.js');
+        let src = fs.readFileSync(indexPath, 'utf-8');
+        src = src.replace("const { sumOfSquares, Calculator } = require('./utils');\n", '');
+        src = src.replace('  console.log(sumOfSquares(3, 4));\n', '');
+        src = src.replace('  const calc = new Calculator();\n', '');
+        src = src.replace('  console.log(calc.compute(5, 6));\n', '');
+        fs.writeFileSync(indexPath, src);
+      });
+    }, 60_000);
+
+    it('node count matches', () => {
+      expect(result.incrNodes.length).toBe(result.fullNodes.length);
+    });
+
+    it('edge count matches', () => {
+      expect(result.incrEdges.length).toBe(result.fullEdges.length);
+    });
+
+    it('edges are identical', () => {
+      expect(result.incrEdges).toEqual(result.fullEdges);
+    });
+  });
+});