recent-dm-conversations: Add latestMessagesByRecipient data structure

This data structure is used to keep track of the latest message of
each recipient in all DM conversations.

Author: sm-sayedi

lib/model/recent_dm_conversations.dart

@@ -1,3 +1,5 @@
+import 'dart:math';
+
 import 'package:collection/collection.dart';
 import 'package:flutter/foundation.dart';
 
@@ -19,16 +21,30 @@ class RecentDmConversationsView extends ChangeNotifier {
         DmNarrow.ofRecentDmConversation(conversation, selfUserId: selfUserId),
         conversation.maxMessageId,
       )).toList()..sort((a, b) => -a.value.compareTo(b.value));
+    final map = Map.fromEntries(entries);
+    final sorted = QueueList.from(entries.map((e) => e.key));
+
+    final msgsByUser = <int, int>{};
+    for (final entry in entries) {
+      final dmNarrow = entry.key;
+      final msg = entry.value;
+      for (final userId in dmNarrow.otherRecipientIds) {
+        // only take the latest message of a user among all the conversations.
+        msgsByUser.putIfAbsent(userId, () => msg);
+      }
+    }
     return RecentDmConversationsView._(
-      map: Map.fromEntries(entries),
-      sorted: QueueList.from(entries.map((e) => e.key)),
+      map: map,
+      sorted: sorted,
+      latestMessagesByRecipient: msgsByUser,
       selfUserId: selfUserId,
     );
   }
 
   RecentDmConversationsView._({
     required this.map,
     required this.sorted,
+    required this.latestMessagesByRecipient,
     required this.selfUserId,
   });
 
@@ -38,6 +54,15 @@ class RecentDmConversationsView extends ChangeNotifier {
   /// The [DmNarrow] keys of [map], sorted by latest message descending.
   final QueueList<DmNarrow> sorted;
 
+  /// Map from user ID to the latest message ID in any conversation with the user.
+  ///
+  /// Both 1:1 and group DM conversations are considered.
+  /// The self-user ID is excluded even if there is a self-DM conversation.
+  ///
+  /// (The identified message was not necessarily sent by the identified user;
+  /// it might have been sent by anyone in its conversation.)
+  final Map<int, int> latestMessagesByRecipient;
+
   final int selfUserId;
 
   /// Insert the key at the proper place in [sorted].
@@ -58,7 +83,7 @@ class RecentDmConversationsView extends ChangeNotifier {
     }
   }
 
-  /// Handle [MessageEvent], updating [map] and [sorted].
+  /// Handle [MessageEvent], updating [map], [sorted], and [latestMessagesByRecipient].
   ///
   /// Can take linear time in general.  That sounds inefficient...
   /// but it's what the webapp does, so must not be catastrophic. 🤷
@@ -117,6 +142,13 @@ class RecentDmConversationsView extends ChangeNotifier {
         _insertSorted(key, message.id);
       }
     }
+    for (final recipient in key.otherRecipientIds) {
+      latestMessagesByRecipient.update(
+        recipient,
+        (latestMessageId) => max(message.id, latestMessageId),
+        ifAbsent: () => message.id,
+      );
+    }
     notifyListeners();
   }
 

test/model/recent_dm_conversations_checks.dart

@@ -6,4 +6,6 @@ import 'package:zulip/model/recent_dm_conversations.dart';
 extension RecentDmConversationsViewChecks on Subject<RecentDmConversationsView> {
   Subject<Map<DmNarrow, int>> get map => has((v) => v.map, 'map');
   Subject<QueueList<DmNarrow>> get sorted => has((v) => v.sorted, 'sorted');
+  Subject<Map<int, int>> get latestMessagesByRecipient => has(
+    (v) => v.latestMessagesByRecipient, 'latestMessagesByRecipient');
 }

test/model/recent_dm_conversations_test.dart

@@ -22,7 +22,8 @@ void main() {
       check(RecentDmConversationsView(selfUserId: eg.selfUser.userId,
         initial: []))
           ..map.isEmpty()
-          ..sorted.isEmpty();
+          ..sorted.isEmpty()
+          ..latestMessagesByRecipient.isEmpty();
 
       check(RecentDmConversationsView(selfUserId: eg.selfUser.userId,
         initial: [
@@ -35,7 +36,8 @@ void main() {
             key([]):     200,
             key([1]):    100,
           })
-          ..sorted.deepEquals([key([1, 2]), key([]), key([1])]);
+          ..sorted.deepEquals([key([1, 2]), key([]), key([1])])
+          ..latestMessagesByRecipient.deepEquals({1: 300, 2: 300});
     });
 
     group('message event (new message)', () {
@@ -55,7 +57,8 @@ void main() {
             key([1]):    200,
             key([1, 2]): 100,
           })
-          ..sorted.deepEquals([key([1]), key([1, 2])]);
+          ..sorted.deepEquals([key([1]), key([1, 2])])
+          ..latestMessagesByRecipient.deepEquals({1: 200, 2: 100});
       });
 
       test('stream message -> do nothing', () {
@@ -65,7 +68,8 @@ void main() {
           ..addListener(() { listenersNotified = true; })
           ..handleMessageEvent(MessageEvent(id: 1, message: eg.streamMessage()))
         ) ..map.deepEquals(expected.map)
-          ..sorted.deepEquals(expected.sorted);
+          ..sorted.deepEquals(expected.sorted)
+          ..latestMessagesByRecipient.deepEquals(expected.latestMessagesByRecipient);
         check(listenersNotified).isFalse();
       });
 
@@ -80,7 +84,8 @@ void main() {
             key([1]):    200,
             key([1, 2]): 100,
           })
-          ..sorted.deepEquals([key([2]), key([1]), key([1, 2])]);
+          ..sorted.deepEquals([key([2]), key([1]), key([1, 2])])
+          ..latestMessagesByRecipient.deepEquals({1: 200, 2: 300});
         check(listenersNotified).isTrue();
       });
 
@@ -95,7 +100,8 @@ void main() {
             key([2]):    150,
             key([1, 2]): 100,
           })
-          ..sorted.deepEquals([key([1]), key([2]), key([1, 2])]);
+          ..sorted.deepEquals([key([1]), key([2]), key([1, 2])])
+          ..latestMessagesByRecipient.deepEquals({1: 200, 2: 150});
         check(listenersNotified).isTrue();
       });
 
@@ -110,7 +116,8 @@ void main() {
             key([1, 2]): 300,
             key([1]):    200,
           })
-          ..sorted.deepEquals([key([1, 2]), key([1])]);
+          ..sorted.deepEquals([key([1, 2]), key([1])])
+          ..latestMessagesByRecipient.deepEquals({1: 300, 2: 300});
         check(listenersNotified).isTrue();
       });
 
@@ -124,7 +131,8 @@ void main() {
             key([1]):    300,
             key([1, 2]): 100,
           })
-          ..sorted.deepEquals([key([1]), key([1, 2])]);
+          ..sorted.deepEquals([key([1]), key([1, 2])])
+          ..latestMessagesByRecipient.deepEquals({1: 300, 2: 100});
         check(listenersNotified).isTrue();
       });
 
@@ -135,7 +143,42 @@ void main() {
         check(setupView()
           ..handleMessageEvent(MessageEvent(id: 1, message: message))
         ) ..map.deepEquals(expected.map)
-          ..sorted.deepEquals(expected.sorted);
+          ..sorted.deepEquals(expected.sorted)
+          ..latestMessagesByRecipient.deepEquals(expected.latestMessagesByRecipient);
+      });
+
+      test('new conversation with one existing and one new user, newest message', () {
+        bool listenersNotified = false;
+        final message = eg.dmMessage(id: 300, from: eg.selfUser,
+          to: [eg.user(userId: 1), eg.user(userId: 3)]);
+        check(setupView()
+          ..addListener(() { listenersNotified = true; })
+          ..handleMessageEvent(MessageEvent(id: 1, message: message))
+        ) ..map.deepEquals({
+            key([1, 3]): 300,
+            key([1]):    200,
+            key([1, 2]): 100,
+          })
+          ..sorted.deepEquals([key([1, 3]), key([1]), key([1, 2])])
+          ..latestMessagesByRecipient.deepEquals({1: 300, 2: 100, 3: 300});
+        check(listenersNotified).isTrue();
+      });
+
+      test('new conversation with one existing and one new user, not newest message', () {
+        bool listenersNotified = false;
+        final message = eg.dmMessage(id: 150, from: eg.selfUser,
+          to: [eg.user(userId: 1), eg.user(userId: 3)]);
+        check(setupView()
+          ..addListener(() { listenersNotified = true; })
+          ..handleMessageEvent(MessageEvent(id: 1, message: message))
+        ) ..map.deepEquals({
+            key([1]):    200,
+            key([1, 3]): 150,
+            key([1, 2]): 100,
+          })
+          ..sorted.deepEquals([key([1]), key([1, 3]), key([1, 2])])
+          ..latestMessagesByRecipient.deepEquals({1: 200, 2: 100, 3: 150});
+        check(listenersNotified).isTrue();
       });
     });
   });