fix(popular-posts): rank by time-decayed popularity instead of raw reply count

src/hooks/use-popular-posts.ts

@@ -5,9 +5,24 @@ import { getCommentMediaInfo, getHasThumbnail } from '../lib/utils/media-utils';
 const MAX_POSTS = 8;
 const MAX_PER_SUB = 3;
 
+// Activity relevance halves every 3 days
+const HALF_LIFE_SECONDS = 72 * 3600;
+
 /**
- * Ranked by replyCount instead of a static threshold so the box
- * adapts to both low- and high-activity periods.
+ * Time-decayed popularity: replyCount divided by age of latest
+ * activity so a stale post with many old replies loses to a newer
+ * post with a few recent replies.
+ */
+function popularityScore(post: Comment, nowSeconds: number): number {
+  const lastActivity = post.lastReplyTimestamp ?? post.timestamp ?? 0;
+  const ageSeconds = Math.max(0, nowSeconds - lastActivity);
+  const replies = post.replyCount ?? 0;
+  return Math.max(replies, 0.1) / (1 + ageSeconds / HALF_LIFE_SECONDS);
+}
+
+/**
+ * Ranked by time-decayed popularity so the box surfaces posts with
+ * recent engagement rather than stale all-time reply leaders.
  *
  * Grow-only commit: once a post enters the grid it never shifts or
  * disappears — new posts fill remaining slots until the cap is reached.
@@ -33,6 +48,8 @@ const usePopularPosts = (subplebbits: Subplebbit[]) => {
   const candidates = useMemo(() => {
     if (committedRef.current.posts.length >= MAX_POSTS) return [];
 
+    const nowSeconds = Math.floor(Date.now() / 1000);
+
     try {
       const uniqueLinks = new Set<string>();
       const allPosts: Comment[] = [];
@@ -57,15 +74,11 @@ const usePopularPosts = (subplebbits: Subplebbit[]) => {
           }
         }
 
-        subPosts.sort((a, b) => (b.replyCount ?? 0) - (a.replyCount ?? 0));
+        subPosts.sort((a, b) => popularityScore(b, nowSeconds) - popularityScore(a, nowSeconds));
         allPosts.push(...subPosts.slice(0, MAX_PER_SUB));
       }
 
-      // Primary: most replies first. Tiebreaker: newest first.
-      allPosts.sort((a, b) => {
-        const diff = (b.replyCount ?? 0) - (a.replyCount ?? 0);
-        return diff !== 0 ? diff : (b.timestamp ?? 0) - (a.timestamp ?? 0);
-      });
+      allPosts.sort((a, b) => popularityScore(b, nowSeconds) - popularityScore(a, nowSeconds));
 
       return allPosts;
     } catch (err) {