feat: implement file system reversion utilities for rewind (#15715)

packages/core/src/utils/fileDiffUtils.ts

@@ -0,0 +1,50 @@
+/**
+ * @license
+ * Copyright 2026 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import type { FileDiff } from '../tools/tools.js';
+import type { ToolCallRecord } from '../services/chatRecordingService.js';
+
+/**
+ * Safely extracts the FileDiff object from a tool call's resultDisplay.
+ * This helper performs runtime checks to ensure the object conforms to the FileDiff structure.
+ * @param resultDisplay The resultDisplay property of a ToolCallRecord.
+ * @returns The FileDiff object if found and valid, otherwise undefined.
+ */
+export function getFileDiffFromResultDisplay(
+  resultDisplay: ToolCallRecord['resultDisplay'],
+): FileDiff | undefined {
+  if (
+    resultDisplay &&
+    typeof resultDisplay === 'object' &&
+    'diffStat' in resultDisplay &&
+    typeof resultDisplay.diffStat === 'object' &&
+    resultDisplay.diffStat !== null
+  ) {
+    const diffStat = resultDisplay.diffStat as FileDiff['diffStat'];
+    if (diffStat) {
+      return resultDisplay;
+    }
+  }
+  return undefined;
+}
+
+export function computeAddedAndRemovedLines(
+  stats: FileDiff['diffStat'] | undefined,
+): {
+  addedLines: number;
+  removedLines: number;
+} {
+  if (!stats) {
+    return {
+      addedLines: 0,
+      removedLines: 0,
+    };
+  }
+  return {
+    addedLines: stats.model_added_lines + stats.user_added_lines,
+    removedLines: stats.model_removed_lines + stats.user_removed_lines,
+  };
+}