Add undiff tests

reconcile-js/src/index.test.ts

@@ -1,4 +1,9 @@
-import { reconcile, reconcileWithHistory } from './index';
+import { reconcile, reconcileWithHistory, diff, undiff } from './index';
+import * as fs from 'fs';
+import * as path from 'path';
+import { fileURLToPath } from 'url';
+
+const __dirname = path.dirname(fileURLToPath(import.meta.url));
 
 describe('reconcile', () => {
   it('call reconcile without cursors', () => {
@@ -44,3 +49,35 @@ describe('reconcile', () => {
     expect(result.history.length).toBeGreaterThan(0);
   });
 });
+
+describe('test_merge_files_without_panic', () => {
+  const resourcesPath = path.join(__dirname, '../../tests/resources');
+
+  const readFileSlice = (fileName: string, start: number, end: number): string => {
+    const filePath = path.join(resourcesPath, fileName);
+    const content = fs.readFileSync(filePath, 'utf-8');
+    const chars = Array.from(content); // Handle unicode properly
+    return chars.slice(start, Math.min(end, chars.length)).join('');
+  };
+
+  const files = ['pride_and_prejudice.txt', 'room_with_a_view.txt', 'blns.txt'];
+
+  const ranges = [{ start: 0, end: 50000 }];
+
+  files.forEach((file1) => {
+    files.forEach((file2) => {
+      ranges.forEach((range1) => {
+        ranges.forEach((range2) => {
+          it(`should diff & undiff ${file1}[${range1.start}..${range1.end}], ${file2}[${range2.start}..${range2.end}] without panic`, () => {
+            const content1 = readFileSlice(file1, range1.start, range1.end);
+            const content2 = readFileSlice(file2, range2.start, range2.end);
+
+            const changes = diff(content1, content2);
+            const actual = undiff(content1, changes);
+            expect(actual).toEqual(content2);
+          });
+        });
+      });
+    });
+  });
+});