feat(web): add sortDmChannels utility with unread-first sorting

packages/web/src/utils/dmSorting.test.ts

@@ -0,0 +1,84 @@
+import { describe, it, expect } from 'vitest';
+import { sortDmChannels } from './dmSorting';
+import type { DmChannel } from '@backspace/shared';
+
+function makeDm(id: string, lastMessageCreatedAt: number | null, createdAt = 1000): DmChannel {
+  return {
+    id,
+    createdAt,
+    members: [],
+    lastMessage: lastMessageCreatedAt != null
+      ? { id: `msg-${id}`, dmChannelId: id, userId: 'u1', content: 'test', createdAt: lastMessageCreatedAt }
+      : null,
+  };
+}
+
+describe('sortDmChannels', () => {
+  it('sorts by recency when no unreads', () => {
+    const dms = [
+      makeDm('old', 100),
+      makeDm('new', 300),
+      makeDm('mid', 200),
+    ];
+    const sorted = sortDmChannels(dms, new Set(), null);
+    expect(sorted.map(d => d.id)).toEqual(['new', 'mid', 'old']);
+  });
+
+  it('puts unread channels first', () => {
+    const dms = [
+      makeDm('read-new', 300),
+      makeDm('unread-old', 100),
+      makeDm('read-mid', 200),
+    ];
+    const sorted = sortDmChannels(dms, new Set(['unread-old']), null);
+    expect(sorted.map(d => d.id)).toEqual(['unread-old', 'read-new', 'read-mid']);
+  });
+
+  it('sorts within unread group by recency', () => {
+    const dms = [
+      makeDm('unread-old', 100),
+      makeDm('unread-new', 300),
+      makeDm('read', 200),
+    ];
+    const sorted = sortDmChannels(dms, new Set(['unread-old', 'unread-new']), null);
+    expect(sorted.map(d => d.id)).toEqual(['unread-new', 'unread-old', 'read']);
+  });
+
+  it('sorts within read group by recency', () => {
+    const dms = [
+      makeDm('read-old', 100),
+      makeDm('read-new', 300),
+      makeDm('unread', 200),
+    ];
+    const sorted = sortDmChannels(dms, new Set(['unread']), null);
+    expect(sorted.map(d => d.id)).toEqual(['unread', 'read-new', 'read-old']);
+  });
+
+  it('excludes current channel from unread group', () => {
+    const dms = [
+      makeDm('current-unread', 100),
+      makeDm('other-unread', 200),
+      makeDm('read', 300),
+    ];
+    // current-unread is in unreadChannels but is the active channel — treat as read
+    const sorted = sortDmChannels(dms, new Set(['current-unread', 'other-unread']), 'current-unread');
+    expect(sorted.map(d => d.id)).toEqual(['other-unread', 'read', 'current-unread']);
+  });
+
+  it('falls back to createdAt when no lastMessage', () => {
+    const dms = [
+      makeDm('no-msg-old', null, 100),
+      makeDm('no-msg-new', null, 300),
+      makeDm('has-msg', 200),
+    ];
+    const sorted = sortDmChannels(dms, new Set(), null);
+    expect(sorted.map(d => d.id)).toEqual(['no-msg-new', 'has-msg', 'no-msg-old']);
+  });
+
+  it('does not mutate the input array', () => {
+    const dms = [makeDm('b', 100), makeDm('a', 200)];
+    const original = [...dms];
+    sortDmChannels(dms, new Set(), null);
+    expect(dms).toEqual(original);
+  });
+});

packages/web/src/utils/dmSorting.ts

@@ -0,0 +1,35 @@
+import type { DmChannel } from '@backspace/shared';
+
+/**
+ * Sort DM channels: unread first (by recency), then read (by recency).
+ * Does not mutate the input array.
+ *
+ * @param dmChannels - The DM channels to sort
+ * @param unreadChannels - Set of channel IDs that have unread messages
+ * @param currentChannelId - The currently active channel (excluded from unread group)
+ */
+export function sortDmChannels(
+  dmChannels: DmChannel[],
+  unreadChannels: Set<string>,
+  currentChannelId: string | null,
+): DmChannel[] {
+  const getTime = (dm: DmChannel) => dm.lastMessage?.createdAt ?? dm.createdAt;
+  const isUnread = (dm: DmChannel) => unreadChannels.has(dm.id) && dm.id !== currentChannelId;
+
+  const unread: DmChannel[] = [];
+  const read: DmChannel[] = [];
+
+  for (const dm of dmChannels) {
+    if (isUnread(dm)) {
+      unread.push(dm);
+    } else {
+      read.push(dm);
+    }
+  }
+
+  const byRecency = (a: DmChannel, b: DmChannel) => getTime(b) - getTime(a);
+  unread.sort(byRecency);
+  read.sort(byRecency);
+
+  return [...unread, ...read];
+}