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

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

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

@@ -0,0 +1,70 @@
+/**
+ * @license
+ * Copyright 2025 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+/**
+ * Implements an in-memory cache for file search results.
+ * This cache optimizes subsequent searches by leveraging previously computed results.
+ */
+export class ResultCache {
+  private readonly cache: Map<string, string[]>;
+  private hits = 0;
+  private misses = 0;
+
+  constructor(
+    private readonly allFiles: string[],
+    private readonly absoluteDir: string,
+  ) {
+    this.cache = new Map();
+  }
+
+  /**
+   * Retrieves cached search results for a given query, or provides a base set
+   * of files to search from.
+   * @param query The search query pattern.
+   * @returns An object containing the files to search and a boolean indicating
+   *          if the result is an exact cache hit.
+   */
+  async get(
+    query: string,
+  ): Promise<{ files: string[]; isExactMatch: boolean }> {
+    const isCacheHit = this.cache.has(query);
+
+    if (isCacheHit) {
+      this.hits++;
+      return { files: this.cache.get(query)!, isExactMatch: true };
+    }
+
+    this.misses++;
+
+    // This is the core optimization of the memory cache.
+    // If a user first searches for "foo", and then for "foobar",
+    // we don't need to search through all files again. We can start
+    // from the results of the "foo" search.
+    // This finds the most specific, already-cached query that is a prefix
+    // of the current query.
+    let bestBaseQuery = '';
+    for (const key of this.cache?.keys?.() ?? []) {
+      if (query.startsWith(key) && key.length > bestBaseQuery.length) {
+        bestBaseQuery = key;
+      }
+    }
+
+    const filesToSearch = bestBaseQuery
+      ? this.cache.get(bestBaseQuery)!
+      : this.allFiles;
+
+    return { files: filesToSearch, isExactMatch: false };
+  }
+
+  /**
+   * Stores search results in the cache.
+   * @param query The search query pattern.
+   * @param results The matching file paths to cache.
+   */
+  set(query: string, results: string[]): void {
+    this.cache.set(query, results);
+  }
+}