perf: aggregate reaction summaries without copying user arrays

resources/js/lib/reactions.ts

@@ -87,19 +87,20 @@ export function summarizeReactions(
     const byEmoji = new Map<string, ReactionSummary>();
 
     for (const reaction of reactions ?? []) {
-        const summary = byEmoji.get(reaction.emoji) ?? {
-            emoji: reaction.emoji,
-            count: 0,
-            isMine: false,
-            userIds: [],
-        };
-
-        byEmoji.set(reaction.emoji, {
-            ...summary,
-            count: summary.count + 1,
-            isMine: summary.isMine || reaction.user_id === userId,
-            userIds: [...summary.userIds, reaction.user_id],
-        });
+        const summary = byEmoji.get(reaction.emoji);
+
+        if (summary) {
+            summary.count++;
+            summary.isMine ||= reaction.user_id === userId;
+            summary.userIds.push(reaction.user_id);
+        } else {
+            byEmoji.set(reaction.emoji, {
+                emoji: reaction.emoji,
+                count: 1,
+                isMine: reaction.user_id === userId,
+                userIds: [reaction.user_id],
+            });
+        }
     }
 
     return [...byEmoji.values()];

tests/Unit/Reactions.test.ts

@@ -102,3 +102,22 @@ await test('takes two quick taps close together as a double tap', () => {
     assert.equal(isDoubleTap(first, { x: 160, y: 200, time: 1_100 }), false);
     assert.equal(isDoubleTap(null, first), false);
 });
+
+await test('summarizes repeated reactions without changing their order', () => {
+    const reactions = Array.from({ length: 120 }, (_, index) => ({
+        user_id: String(index),
+        emoji: index % 2 === 0 ? '🚀' : '❤️',
+    }));
+    const summaries = summarizeReactions(reactions, '63');
+
+    assert.deepEqual(summaries.map(({ emoji, count, isMine }) => ({
+        emoji,
+        count,
+        isMine,
+    })), [
+        { emoji: '🚀', count: 60, isMine: false },
+        { emoji: '❤️', count: 60, isMine: true },
+    ]);
+    assert.equal(summaries[0].userIds[0], '0');
+    assert.equal(summaries[0].userIds.at(-1), '118');
+});