autocomplete: In user-mention autocomplete results give priority to users in DM conversations

lib/model/autocomplete.dart

@@ -125,23 +125,11 @@ class AutocompleteViewManager {
     assert(removed);
   }
 
-  void handleRealmUserAddEvent(RealmUserAddEvent event) {
-    for (final view in _mentionAutocompleteViews) {
-      view.refreshStaleUserResults();
-    }
-  }
-
   void handleRealmUserRemoveEvent(RealmUserRemoveEvent event) {
-    for (final view in _mentionAutocompleteViews) {
-      view.refreshStaleUserResults();
-    }
     autocompleteDataCache.invalidateUser(event.userId);
   }
 
   void handleRealmUserUpdateEvent(RealmUserUpdateEvent event) {
-    for (final view in _mentionAutocompleteViews) {
-      view.refreshStaleUserResults();
-    }
     autocompleteDataCache.invalidateUser(event.userId);
   }
 
@@ -176,17 +164,54 @@ class AutocompleteViewManager {
 ///  * When the object will no longer be used, call [dispose] to free
 ///    resources on the [PerAccountStore].
 class MentionAutocompleteView extends ChangeNotifier {
-  MentionAutocompleteView._({required this.store, required this.narrow});
+  MentionAutocompleteView._({
+    required this.store,
+    required this.narrow,
+    required this.sortedUsers,
+  });
 
   factory MentionAutocompleteView.init({
     required PerAccountStore store,
     required Narrow narrow,
   }) {
-    final view = MentionAutocompleteView._(store: store, narrow: narrow);
+    final view = MentionAutocompleteView._(
+      store: store,
+      narrow: narrow,
+      sortedUsers: _usersByRelevance(store: store, narrow: narrow),
+    );
     store.autocompleteViewManager.registerMentionAutocomplete(view);
     return view;
   }
 
+  static List<User> _usersByRelevance({
+    required PerAccountStore store,
+    required Narrow narrow,
+  }) {
+    assert(narrow is! CombinedFeedNarrow);
+    return store.users.values.toList()
+      ..sort((userA, userB) => compareByDms(userA, userB, store: store));
+  }
+
+  /// Determines which of the two users is more recent in DM conversations.
+  ///
+  /// Returns a negative number if [userA] is more recent than [userB],
+  /// returns a positive number if [userB] is more recent than [userA],
+  /// and returns `0` if both [userA] and [userB] are equally recent
+  /// or there is no DM exchanged with them whatsoever.
+  @visibleForTesting
+  static int compareByDms(User userA, User userB, {required PerAccountStore store}) {
+    final recentDms = store.recentDmConversationsView;
+    final aLatestMessageId = recentDms.latestMessagesByRecipient[userA.userId];
+    final bLatestMessageId = recentDms.latestMessagesByRecipient[userB.userId];
+
+    return switch((aLatestMessageId, bLatestMessageId)) {
+      (int a, int b) => -a.compareTo(b),
+      (int(),     _) => -1,
+      (_,     int()) => 1,
+      _              => 0,
+    };
+  }
+
   @override
   void dispose() {
     store.autocompleteViewManager.unregisterMentionAutocomplete(this);
@@ -198,6 +223,7 @@ class MentionAutocompleteView extends ChangeNotifier {
 
   final PerAccountStore store;
   final Narrow narrow;
+  final List<User> sortedUsers;
 
   MentionAutocompleteQuery? get query => _query;
   MentionAutocompleteQuery? _query;
@@ -208,15 +234,6 @@ class MentionAutocompleteView extends ChangeNotifier {
     }
   }
 
-  /// Recompute user results for the current query, if any.
-  ///
-  /// Called in particular when we get a [RealmUserEvent].
-  void refreshStaleUserResults() {
-    if (_query != null) {
-      _startSearch(_query!);
-    }
-  }
-
   /// Called when the app is reassembled during debugging, e.g. for hot reload.
   ///
   /// This will redo the search from scratch for the current query, if any.
@@ -230,18 +247,7 @@ class MentionAutocompleteView extends ChangeNotifier {
   List<MentionAutocompleteResult> _results = [];
 
   Future<void> _startSearch(MentionAutocompleteQuery query) async {
-    List<MentionAutocompleteResult>? newResults;
-
-    while (true) {
-      try {
-        newResults = await _computeResults(query);
-        break;
-      } on ConcurrentModificationError {
-        // Retry
-        // TODO backoff?
-      }
-    }
-
+    final newResults = await _computeResults(query);
     if (newResults == null) {
       // Query was old; new search is in progress. Or, no listeners to notify.
       return;
@@ -253,9 +259,7 @@ class MentionAutocompleteView extends ChangeNotifier {
 
   Future<List<MentionAutocompleteResult>?> _computeResults(MentionAutocompleteQuery query) async {
     final List<MentionAutocompleteResult> results = [];
-    final Iterable<User> users = store.users.values;
-
-    final iterator = users.iterator;
+    final iterator = sortedUsers.iterator;
     bool isDone = false;
     while (!isDone) {
       // CPU perf: End this task; enqueue a new one for resuming this work
@@ -266,7 +270,7 @@ class MentionAutocompleteView extends ChangeNotifier {
       }
 
       for (int i = 0; i < 1000; i++) {
-        if (!iterator.moveNext()) { // Can throw ConcurrentModificationError
+        if (!iterator.moveNext()) {
           isDone = true;
           break;
         }
@@ -277,7 +281,7 @@ class MentionAutocompleteView extends ChangeNotifier {
         }
       }
     }
-    return results; // TODO(#228) sort for most relevant first
+    return results;
   }
 }
 

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 +21,29 @@ class RecentDmConversationsView extends ChangeNotifier {
         DmNarrow.ofRecentDmConversation(conversation, selfUserId: selfUserId),
         conversation.maxMessageId,
       )).toList()..sort((a, b) => -a.value.compareTo(b.value));
+
+    final latestMessagesByRecipient = <int, int>{};
+    for (final entry in entries) {
+      final dmNarrow = entry.key;
+      final maxMessageId = entry.value;
+      for (final userId in dmNarrow.otherRecipientIds) {
+        // Only take the latest message of a user across all the conversations.
+        latestMessagesByRecipient.putIfAbsent(userId, () => maxMessageId);
+      }
+    }
+
     return RecentDmConversationsView._(
       map: Map.fromEntries(entries),
       sorted: QueueList.from(entries.map((e) => e.key)),
+      latestMessagesByRecipient: latestMessagesByRecipient,
       selfUserId: selfUserId,
     );
   }
 
   RecentDmConversationsView._({
     required this.map,
     required this.sorted,
+    required this.latestMessagesByRecipient,
     required this.selfUserId,
   });
 
@@ -38,6 +53,15 @@ 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 +82,7 @@ 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. 🤷
@@ -89,6 +113,8 @@ class RecentDmConversationsView extends ChangeNotifier {
       return;
     }
     final key = DmNarrow.ofMessage(message, selfUserId: selfUserId);
+
+    // Update [map] and [sorted].
     final prev = map[key];
     if (prev == null) {
       // The conversation is new.  Add to both `map` and `sorted`.
@@ -117,6 +143,16 @@ class RecentDmConversationsView extends ChangeNotifier {
         _insertSorted(key, message.id);
       }
     }
+
+    // Update [latestMessagesByRecipient].
+    for (final recipient in key.otherRecipientIds) {
+      latestMessagesByRecipient.update(
+        recipient,
+        (latestMessageId) => max(message.id, latestMessageId),
+        ifAbsent: () => message.id,
+      );
+    }
+
     notifyListeners();
   }
 

lib/model/store.dart

@@ -418,7 +418,6 @@ class PerAccountStore extends ChangeNotifier with StreamStore, MessageStore {
     } else if (event is RealmUserAddEvent) {
       assert(debugLog("server event: realm_user/add"));
       users[event.person.userId] = event.person;
-      autocompleteViewManager.handleRealmUserAddEvent(event);
       notifyListeners();
     } else if (event is RealmUserRemoveEvent) {
       assert(debugLog("server event: realm_user/remove"));