Add support for .geminiignore file (#757)

packages/cli/src/utils/loadIgnorePatterns.ts

@@ -0,0 +1,71 @@
+/**
+ * @license
+ * Copyright 2025 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import * as fs from 'node:fs';
+import * as path from 'node:path';
+
+const GEMINI_IGNORE_FILE_NAME = '.geminiignore';
+
+/**
+ * Loads and parses a .geminiignore file from the given workspace root.
+ * The .geminiignore file follows a format similar to .gitignore:
+ * - Each line specifies a glob pattern.
+ * - Lines are trimmed of leading and trailing whitespace.
+ * - Blank lines (after trimming) are ignored.
+ * - Lines starting with a pound sign (#) (after trimming) are treated as comments and ignored.
+ * - Patterns are case-sensitive and follow standard glob syntax.
+ * - If a # character appears elsewhere in a line (not at the start after trimming),
+ *   it is considered part of the glob pattern.
+ *
+ * @param workspaceRoot The absolute path to the workspace root where the .geminiignore file is expected.
+ * @returns An array of glob patterns extracted from the .geminiignore file. Returns an empty array
+ *          if the file does not exist or contains no valid patterns.
+ */
+export function loadGeminiIgnorePatterns(workspaceRoot: string): string[] {
+  const ignoreFilePath = path.join(workspaceRoot, GEMINI_IGNORE_FILE_NAME);
+  const patterns: string[] = [];
+
+  try {
+    const fileContent = fs.readFileSync(ignoreFilePath, 'utf-8');
+    const lines = fileContent.split(/\r?\n/);
+
+    for (const line of lines) {
+      const trimmedLine = line.trim();
+      if (trimmedLine && !trimmedLine.startsWith('#')) {
+        patterns.push(trimmedLine);
+      }
+    }
+    if (patterns.length > 0) {
+      console.log(
+        `[INFO] Loaded ${patterns.length} patterns from .geminiignore`,
+      );
+    }
+  } catch (error: unknown) {
+    if (
+      error instanceof Error &&
+      'code' in error &&
+      typeof error.code === 'string'
+    ) {
+      if (error.code === 'ENOENT') {
+        // .geminiignore not found, which is fine.
+        console.log(
+          '[INFO] No .geminiignore file found. Proceeding without custom ignore patterns.',
+        );
+      } else {
+        // Other error reading the file (e.g., permissions)
+        console.warn(
+          `[WARN] Could not read .geminiignore file at ${ignoreFilePath}: ${error.message}`,
+        );
+      }
+    } else {
+      // For other types of errors, or if code is not available
+      console.warn(
+        `[WARN] An unexpected error occurred while trying to read ${ignoreFilePath}: ${String(error)}`,
+      );
+    }
+  }
+  return patterns;
+}