unreads: Add helpers to provide unread counts for narrows

These helpers do not account for muted topics and streams,
those are left for #346.

Author: sirpengi

Diff for: lib/model/unreads.dart

@@ -123,8 +123,54 @@ class Unreads extends ChangeNotifier {
 
   final int selfUserId;
 
+  // TODO(#346): account for muted topics and streams
+  // TODO(#370): maintain this count incrementally, rather than recomputing from scratch
+  int countInAllMessagesNarrow() {
+    int c = 0;
+    for (final messageIds in dms.values) {
+      c = c + messageIds.length;
+    }
+    for (final topics in streams.values) {
+      for (final messageIds in topics.values) {
+        c = c + messageIds.length;
+      }
+    }
+    return c;
+  }
+
+  // TODO(#346): account for muted topics and streams
+  // TODO(#370): maintain this count incrementally, rather than recomputing from scratch
+  int countInStreamNarrow(int streamId) {
+    final topics = streams[streamId];
+    if (topics == null) return 0;
+    int c = 0;
+    for (final messageIds in topics.values) {
+      c = c + messageIds.length;
+    }
+    return c;
+  }
+
+  // TODO(#346): account for muted topics and streams
+  int countInTopicNarrow(int streamId, String topic) {
+    final topics = streams[streamId];
+    return topics?[topic]?.length ?? 0;
+  }
+
   int countInDmNarrow(DmNarrow narrow) => dms[narrow]?.length ?? 0;
 
+  int countInNarrow(Narrow narrow) {
+    switch (narrow) {
+      case AllMessagesNarrow():
+        return countInAllMessagesNarrow();
+      case StreamNarrow():
+        return countInStreamNarrow(narrow.streamId);
+      case TopicNarrow():
+        return countInTopicNarrow(narrow.streamId, narrow.topic);
+      case DmNarrow():
+        return countInDmNarrow(narrow);
+    }
+  }
+
   void handleMessageEvent(MessageEvent event) {
     final message = event.message;
     if (message.flags.contains(MessageFlag.read)) {

Diff for: test/model/unreads_test.dart

@@ -146,6 +146,53 @@ void main() {
     });
   });
 
+  group('count helpers', () {
+    test('countInAllMessagesNarrow', () {
+      final stream1 = eg.stream();
+      final stream2 = eg.stream();
+      prepare();
+      fillWithMessages([
+        eg.streamMessage(stream: stream1, topic: 'a', flags: []),
+        eg.streamMessage(stream: stream1, topic: 'b', flags: []),
+        eg.streamMessage(stream: stream1, topic: 'b', flags: []),
+        eg.streamMessage(stream: stream2, topic: 'c', flags: []),
+        eg.dmMessage(from: eg.otherUser, to: [eg.selfUser], flags: []),
+        eg.dmMessage(from: eg.thirdUser, to: [eg.selfUser], flags: []),
+      ]);
+      check(model.countInAllMessagesNarrow()).equals(6);
+    });
+
+    test('countInStreamNarrow', () {
+      final stream = eg.stream();
+      prepare();
+      fillWithMessages([
+        eg.streamMessage(stream: stream, topic: 'a', flags: []),
+        eg.streamMessage(stream: stream, topic: 'a', flags: []),
+        eg.streamMessage(stream: stream, topic: 'b', flags: []),
+        eg.streamMessage(stream: stream, topic: 'b', flags: []),
+        eg.streamMessage(stream: stream, topic: 'b', flags: []),
+      ]);
+      check(model.countInStreamNarrow(stream.streamId)).equals(5);
+    });
+
+    test('countInTopicNarrow', () {
+      final stream = eg.stream();
+      prepare();
+      fillWithMessages(List.generate(7, (i) => eg.streamMessage(
+        stream: stream, topic: 'a', flags: [])));
+      check(model.countInTopicNarrow(stream.streamId, 'a')).equals(7);
+    });
+
+    test('countInDmNarrow', () {
+      prepare();
+      fillWithMessages(List.generate(5, (i) => eg.dmMessage(
+        from: eg.otherUser, to: [eg.selfUser], flags: [])));
+      final narrow = DmNarrow.withUser(
+        eg.otherUser.userId, selfUserId: eg.selfUser.userId);
+      check(model.countInDmNarrow(narrow)).equals(5);
+    });
+  });
+
   group('handleMessageEvent', () {
     for (final (isUnread, isStream, isDirectMentioned, isWildcardMentioned) in [
       (true,  true,  true,  true ),