feat(catalog): add reply-count sorting mode

src/lib/utils/__tests__/catalog-sort.test.ts

@@ -0,0 +1,88 @@
+import { describe, it, expect } from 'vitest';
+import { sortCatalogFeedForDisplay, type CatalogPost } from '../catalog-sort';
+
+describe('sortCatalogFeedForDisplay', () => {
+  describe('replyCount sort', () => {
+    it('orders by replyCount descending', () => {
+      const posts: CatalogPost[] = [
+        { cid: 'a', replyCount: 5 },
+        { cid: 'b', replyCount: 20 },
+        { cid: 'c', replyCount: 10 },
+      ];
+      const result = sortCatalogFeedForDisplay(posts, 'replyCount');
+      expect(result.map((p) => p.cid)).toEqual(['b', 'c', 'a']);
+      expect(result.map((p) => p.replyCount)).toEqual([20, 10, 5]);
+    });
+
+    it('tie-breaks on bump order (lastReplyTimestamp, then timestamp)', () => {
+      const posts: CatalogPost[] = [
+        { cid: 'a', replyCount: 10, lastReplyTimestamp: 100, timestamp: 50 },
+        { cid: 'b', replyCount: 10, lastReplyTimestamp: 300, timestamp: 60 },
+        { cid: 'c', replyCount: 10, lastReplyTimestamp: 200, timestamp: 70 },
+      ];
+      const result = sortCatalogFeedForDisplay(posts, 'replyCount');
+      expect(result.map((p) => p.cid)).toEqual(['b', 'c', 'a']);
+    });
+
+    it('handles undefined replyCount as 0', () => {
+      const posts: CatalogPost[] = [{ cid: 'a', replyCount: 5 }, { cid: 'b' }, { cid: 'c', replyCount: undefined }, { cid: 'd', replyCount: null }];
+      const result = sortCatalogFeedForDisplay(posts, 'replyCount');
+      expect(result.map((p) => p.cid)).toEqual(['a', 'b', 'c', 'd']);
+    });
+
+    it('keeps pinned posts at top in input order', () => {
+      const posts: CatalogPost[] = [
+        { cid: 'mid', replyCount: 100 },
+        { cid: 'pinned1', replyCount: 1, pinned: true },
+        { cid: 'low', replyCount: 5 },
+        { cid: 'pinned2', replyCount: 50, pinned: true },
+      ];
+      const result = sortCatalogFeedForDisplay(posts, 'replyCount');
+      expect(result.map((p) => p.cid)).toEqual(['pinned1', 'pinned2', 'mid', 'low']);
+    });
+
+    it('produces deterministic ordering under full ties', () => {
+      const posts: CatalogPost[] = [
+        { cid: 'z', replyCount: 0 },
+        { cid: 'a', replyCount: 0 },
+        { cid: 'm', replyCount: 0 },
+      ];
+      const result = sortCatalogFeedForDisplay(posts, 'replyCount');
+      expect(result.map((p) => p.cid)).toEqual(['a', 'm', 'z']);
+    });
+
+    it('produces deterministic ordering under full ties (repeated runs)', () => {
+      const posts: CatalogPost[] = [
+        { cid: 'x', replyCount: 5, lastReplyTimestamp: 100, timestamp: 50 },
+        { cid: 'y', replyCount: 5, lastReplyTimestamp: 100, timestamp: 50 },
+      ];
+      const r1 = sortCatalogFeedForDisplay(posts, 'replyCount');
+      const r2 = sortCatalogFeedForDisplay(posts, 'replyCount');
+      expect(r1.map((p) => p.cid)).toEqual(r2.map((p) => p.cid));
+    });
+  });
+
+  describe('non-replyCount sorts', () => {
+    it('returns original order for active sort', () => {
+      const posts: CatalogPost[] = [
+        { cid: 'c', replyCount: 10 },
+        { cid: 'a', replyCount: 100 },
+        { cid: 'b', replyCount: 50 },
+      ];
+      const result = sortCatalogFeedForDisplay(posts, 'active');
+      expect(result).toBe(posts);
+      expect(result.map((p) => p.cid)).toEqual(['c', 'a', 'b']);
+    });
+
+    it('returns original order for new sort', () => {
+      const posts: CatalogPost[] = [
+        { cid: 'c', replyCount: 10 },
+        { cid: 'a', replyCount: 100 },
+        { cid: 'b', replyCount: 50 },
+      ];
+      const result = sortCatalogFeedForDisplay(posts, 'new');
+      expect(result).toBe(posts);
+      expect(result.map((p) => p.cid)).toEqual(['c', 'a', 'b']);
+    });
+  });
+});