unreads [nfc]: Introduce _reverseStreamsLookup data structure

This one is needed for enhancing performance in _removeAllInStreams,
_isPresentInStreams which will be done in the upcoming commits.

Additionally this will be helpful for efficiently checking for
@-mention in subscription_list

Author: Khader-1

lib/model/unreads.dart

@@ -42,13 +42,17 @@ class Unreads extends ChangeNotifier {
     required ChannelStore channelStore,
   }) {
     final streams = <int, Map<String, QueueList<int>>>{};
+    final reverseStreamsLookup = <int, ({int streamId, String topic})>{};
     final dms = <DmNarrow, QueueList<int>>{};
     final mentions = Set.of(initial.mentions);
 
     for (final unreadChannelSnapshot in initial.channels) {
       final streamId = unreadChannelSnapshot.streamId;
       final topic = unreadChannelSnapshot.topic;
       (streams[streamId] ??= {})[topic] = QueueList.from(unreadChannelSnapshot.unreadMessageIds);
+      final messageInfo = (streamId: streamId, topic: topic);
+      reverseStreamsLookup.addEntries(
+        unreadChannelSnapshot.unreadMessageIds.map((messageId) => MapEntry(messageId, messageInfo)));
     }
 
     for (final unreadDmSnapshot in initial.dms) {
@@ -65,6 +69,7 @@ class Unreads extends ChangeNotifier {
     return Unreads._(
       channelStore: channelStore,
       streams: streams,
+      reverseStreamsLookup: reverseStreamsLookup,
       dms: dms,
       mentions: mentions,
       oldUnreadsMissing: initial.oldUnreadsMissing,
@@ -75,11 +80,12 @@ class Unreads extends ChangeNotifier {
   Unreads._({
     required this.channelStore,
     required this.streams,
+    required Map<int, ({int streamId, String topic})> reverseStreamsLookup,
     required this.dms,
     required this.mentions,
     required this.oldUnreadsMissing,
     required this.selfUserId,
-  });
+  }) : _reverseStreamsLookup = reverseStreamsLookup;
 
   final ChannelStore channelStore;
 
@@ -89,6 +95,9 @@ class Unreads extends ChangeNotifier {
   /// Unread stream messages, as: stream ID → topic → message IDs (sorted).
   final Map<int, Map<String, QueueList<int>>> streams;
 
+  /// Maps the message id to the (stream ID, topic) we stored it under in streams.
+  final Map<int, ({int streamId, String topic})> _reverseStreamsLookup;
+
   /// Unread DM messages, as: DM narrow → message IDs (sorted).
   final Map<DmNarrow, QueueList<int>> dms;
 
@@ -341,6 +350,7 @@ class Unreads extends ChangeNotifier {
               streams.clear();
               dms.clear();
               mentions.clear();
+              _reverseStreamsLookup.clear();
               oldUnreadsMissing = false;
             } else {
               final messageIdsSet = Set.of(event.messages);
@@ -435,6 +445,7 @@ class Unreads extends ChangeNotifier {
 
   void _addLastInStreamTopic(int messageId, int streamId, String topic) {
     ((streams[streamId] ??= {})[topic] ??= QueueList()).addLast(messageId);
+    _reverseStreamsLookup[messageId] = (streamId: streamId, topic: topic);
   }
 
   // [messageIds] must be sorted ascending and without duplicates.
@@ -447,6 +458,9 @@ class Unreads extends ChangeNotifier {
       // TODO(server-6) remove 6.0 comment
       (existing) => setUnion(existing, messageIds),
     );
+    final messageInfo = (streamId: streamId, topic: topic);
+    _reverseStreamsLookup.addEntries(
+      messageIds.map((messageId) => MapEntry(messageId, messageInfo)));
   }
 
   // TODO use efficient model lookups
@@ -470,6 +484,9 @@ class Unreads extends ChangeNotifier {
     for (final streamId in newlyEmptyStreams) {
       streams.remove(streamId);
     }
+    for (var messageId in idsToRemove) {
+      _reverseStreamsLookup.remove(messageId);
+    }
   }
 
   void _removeAllInStreamTopic(Set<int> incomingMessageIds, int streamId, String topic) {
@@ -486,6 +503,9 @@ class Unreads extends ChangeNotifier {
         streams.remove(streamId);
       }
     }
+    for (var messageId in incomingMessageIds) {
+      _reverseStreamsLookup.remove(messageId);
+    }
   }
 
   // TODO use efficient model lookups