add diff-arrays utility for efficient array diffs

packages/backend/test/unit/misc/diff-arrays.ts

@@ -0,0 +1,53 @@
+/*
+ * SPDX-FileCopyrightText: hazelnoot and other Sharkey contributors
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+import { diffArrays } from '@/misc/diff-arrays.js';
+
+describe(diffArrays, () => {
+	it('should return empty result when both inputs are null', () => {
+		const result = diffArrays(null, null);
+		expect(result.added).toHaveLength(0);
+		expect(result.removed).toHaveLength(0);
+	});
+
+	it('should return empty result when both inputs are empty', () => {
+		const result = diffArrays([], []);
+		expect(result.added).toHaveLength(0);
+		expect(result.removed).toHaveLength(0);
+	});
+
+	it('should remove before when added is empty', () => {
+		const result = diffArrays([1, 2, 3], []);
+		expect(result.added).toHaveLength(0);
+		expect(result.removed).toEqual([1, 2, 3]);
+	});
+
+	it('should deduplicate before when added is empty', () => {
+		const result = diffArrays([1, 1, 2, 2, 3], []);
+		expect(result.added).toHaveLength(0);
+		expect(result.removed).toEqual([1, 2, 3]);
+	});
+
+	it('should remove after when before is empty', () => {
+		const result = diffArrays([], [1, 2, 3]);
+		expect(result.added).toEqual([1, 2, 3]);
+		expect(result.removed).toHaveLength(0);
+	});
+
+	it('should deduplicate after when before is empty', () => {
+		const result = diffArrays([], [1, 1, 2, 2, 3]);
+		expect(result.added).toEqual([1, 2, 3]);
+		expect(result.removed).toHaveLength(0);
+	});
+
+	it('should return diff when both have values', () => {
+		const result = diffArrays(
+			['a', 'b', 'c', 'd'],
+			['a', 'c', 'e', 'f'],
+		);
+		expect(result.added).toEqual(['e', 'f']);
+		expect(result.removed).toEqual(['b', 'd']);
+	});
+});