From 373c60b52173e6ed8bcc01b3c4f1c6ac4734398a Mon Sep 17 00:00:00 2001
From: Hazelnoot <acomputerdog@gmail.com>
Date: Sat, 24 May 2025 18:28:43 -0400
Subject: [PATCH] add diff-arrays utility for efficient array diffs

---
 packages/backend/src/misc/diff-arrays.ts      | 56 +++++++++++++++++++
 .../backend/test/unit/misc/diff-arrays.ts     | 53 ++++++++++++++++++
 2 files changed, 109 insertions(+)
 create mode 100644 packages/backend/src/misc/diff-arrays.ts
 create mode 100644 packages/backend/test/unit/misc/diff-arrays.ts

diff --git a/packages/backend/src/misc/diff-arrays.ts b/packages/backend/src/misc/diff-arrays.ts
new file mode 100644
index 0000000000..1f6820481b
--- /dev/null
+++ b/packages/backend/src/misc/diff-arrays.ts
@@ -0,0 +1,56 @@
+/*
+ * SPDX-FileCopyrightText: hazelnoot and other Sharkey contributors
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+export interface DiffResult<T> {
+	added: T[];
+	removed: T[];
+}
+
+/**
+ * Calculates the difference between two snapshots of data.
+ * Null, undefined, and empty arrays are supported, and values do not have to be unique.
+ * Result sets are de-duplicated, and will be empty if no data was added or removed (respectively).
+ * @param dataBefore Array containing data before the change
+ * @param dataAfter Array containing data after the change
+ */
+export function diffArrays<T>(dataBefore: T[] | null | undefined, dataAfter: T[] | null | undefined): DiffResult<T> {
+	const before = dataBefore ? new Set(dataBefore) : null;
+	const after = dataAfter ? new Set(dataAfter) : null;
+
+	// data before AND after => changed
+	if (before?.size && after?.size) {
+		const added: T[] = [];
+		const removed: T[] = [];
+
+		for (const host of before) {
+			// before and NOT after => removed
+			if (!after.has(host)) {
+				removed.push(host);
+			}
+		}
+
+		for (const host of after) {
+			// after and NOT before => added
+			if (!before.has(host)) {
+				added.push(host);
+			}
+		}
+
+		return { added, removed };
+	}
+
+	// data ONLY before => all removed
+	if (before?.size) {
+		return { added: [], removed: Array.from(before) };
+	}
+
+	// data ONLY after => all added
+	if (after?.size) {
+		return { added: Array.from(after), removed: [] };
+	}
+
+	// data NEITHER before nor after => no change
+	return { added: [], removed: [] };
+}
diff --git a/packages/backend/test/unit/misc/diff-arrays.ts b/packages/backend/test/unit/misc/diff-arrays.ts
new file mode 100644
index 0000000000..3eed5a8913
--- /dev/null
+++ b/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']);
+	});
+});