minor optimization to diff-arrays

2 files changed, 7 insertions, 5 deletions

diff --git a/packages/backend/src/misc/diff-arrays.ts b/packages/backend/src/misc/diff-arrays.ts
index 1f6820481b..b3879cc996 100644
--- a/packages/backend/src/misc/diff-arrays.ts
+++ b/packages/backend/src/misc/diff-arrays.ts
@@ -10,8 +10,9 @@ export interface DiffResult<T> {
 
 /**
  * Calculates the difference between two snapshots of data.
- * Null, undefined, and empty arrays are supported, and values do not have to be unique.
+ * Null, undefined, and empty arrays are supported, and duplicate values are ignored.
  * Result sets are de-duplicated, and will be empty if no data was added or removed (respectively).
+ * The inputs are treated as un-ordered, so a re-ordering of the same data will NOT be considered a change.
  * @param dataBefore Array containing data before the change
  * @param dataAfter Array containing data after the change
  */
@@ -26,7 +27,8 @@ export function diffArrays<T>(dataBefore: T[] | null | undefined, dataAfter: T[]
 
 		for (const host of before) {
 			// before and NOT after => removed
-			if (!after.has(host)) {
+			// delete operation removes duplicates to speed up the "after" loop
+			if (!after.delete(host)) {
 				removed.push(host);
 			}
 		}
diff --git a/packages/backend/test/unit/misc/diff-arrays.ts b/packages/backend/test/unit/misc/diff-arrays.ts
index 3eed5a8913..a2dee50652 100644
--- a/packages/backend/test/unit/misc/diff-arrays.ts
+++ b/packages/backend/test/unit/misc/diff-arrays.ts
@@ -18,19 +18,19 @@ describe(diffArrays, () => {
 		expect(result.removed).toHaveLength(0);
 	});
 
-	it('should remove before when added is empty', () => {
+	it('should remove before when after 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', () => {
+	it('should deduplicate before when after 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', () => {
+	it('should add after when before is empty', () => {
 		const result = diffArrays([], [1, 2, 3]);
 		expect(result.added).toEqual([1, 2, 3]);
 		expect(result.removed).toHaveLength(0);