feat(core, cli): Introduce high-performance FileSearch engine (#5136)

Co-authored-by: Jacob Richman <jacob314@gmail.com>

packages/core/src/utils/filesearch/result-cache.test.ts

@@ -0,0 +1,56 @@
+/**
+ * @license
+ * Copyright 2025 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import path from 'node:path';
+import { test, expect } from 'vitest';
+import { ResultCache } from './result-cache.js';
+
+test('ResultCache basic usage', async () => {
+  const files = [
+    'foo.txt',
+    'bar.js',
+    'baz.md',
+    'subdir/file.txt',
+    'subdir/other.js',
+    'subdir/nested/file.md',
+  ];
+  const cache = new ResultCache(files, path.resolve('.'));
+  const { files: resultFiles, isExactMatch } = await cache.get('*.js');
+  expect(resultFiles).toEqual(files);
+  expect(isExactMatch).toBe(false);
+});
+
+test('ResultCache cache hit/miss', async () => {
+  const files = ['foo.txt', 'bar.js', 'baz.md'];
+  const cache = new ResultCache(files, path.resolve('.'));
+  // First call: miss
+  const { files: result1Files, isExactMatch: isExactMatch1 } =
+    await cache.get('*.js');
+  expect(result1Files).toEqual(files);
+  expect(isExactMatch1).toBe(false);
+
+  // Simulate FileSearch applying the filter and setting the result
+  cache.set('*.js', ['bar.js']);
+
+  // Second call: hit
+  const { files: result2Files, isExactMatch: isExactMatch2 } =
+    await cache.get('*.js');
+  expect(result2Files).toEqual(['bar.js']);
+  expect(isExactMatch2).toBe(true);
+});
+
+test('ResultCache best base query', async () => {
+  const files = ['foo.txt', 'foobar.js', 'baz.md'];
+  const cache = new ResultCache(files, path.resolve('.'));
+
+  // Cache a broader query
+  cache.set('foo', ['foo.txt', 'foobar.js']);
+
+  // Search for a more specific query that starts with the broader one
+  const { files: resultFiles, isExactMatch } = await cache.get('foobar');
+  expect(resultFiles).toEqual(['foo.txt', 'foobar.js']);
+  expect(isExactMatch).toBe(false);
+});