api [nfc]: Extract Conversation.isSameAs

This will make it easier to support comparing the conversations
between subclasses of MessageBase.

The message list tests on displayRecipient are now mostly exercising the
logic on Conversation.isSameAs, which makes it reasonable to move the
tests.  Keep them here for now since this logic is more relevant to
message lists than it is to the rest of the app.

Dropped the comment on _equalIdSequences since it is now private
in the short body of DmConversation.

Author: PIG208

lib/api/model/model.dart

@@ -614,7 +614,10 @@ extension type const TopicName(String _value) {
 /// Different from [MessageDestination], this information comes from
 /// [getMessages] or [getEvents], identifying the conversation that contains a
 /// message.
-sealed class Conversation {}
+sealed class Conversation {
+  /// Whether [this] and [other] refer to the same Zulip conversation.
+  bool isSameAs(Conversation other);
+}
 
 /// The conversation a stream message is in.
 @JsonSerializable(fieldRename: FieldRename.snake, createToJson: false)
@@ -640,6 +643,13 @@ class StreamConversation extends Conversation {
 
   factory StreamConversation.fromJson(Map<String, dynamic> json) =>
     _$StreamConversationFromJson(json);
+
+  @override
+  bool isSameAs(Conversation other) {
+    return other is StreamConversation
+      && streamId == other.streamId
+      && topic.isSameAs(other.topic);
+  }
 }
 
 /// The conversation a DM message is in.
@@ -653,6 +663,21 @@ class DmConversation extends Conversation {
 
   DmConversation({required this.allRecipientIds})
     : assert(isSortedWithoutDuplicates(allRecipientIds.toList()));
+
+  bool _equalIdSequences(Iterable<int> xs, Iterable<int> ys) {
+    if (xs.length != ys.length) return false;
+    final xs_ = xs.iterator; final ys_ = ys.iterator;
+    while (xs_.moveNext() && ys_.moveNext()) {
+      if (xs_.current != ys_.current) return false;
+    }
+    return true;
+  }
+
+  @override
+  bool isSameAs(Conversation other) {
+    if (other is! DmConversation) return false;
+    return _equalIdSequences(allRecipientIds, other.allRecipientIds);
+  }
 }
 
 /// A message or message-like object, for showing in a message list.

lib/model/message_list.dart

@@ -351,26 +351,7 @@ mixin _MessageSequence {
 
 @visibleForTesting
 bool haveSameRecipient(Message prevMessage, Message message) {
-  if (prevMessage is StreamMessage && message is StreamMessage) {
-    if (prevMessage.streamId != message.streamId) return false;
-    if (prevMessage.topic.canonicalize() != message.topic.canonicalize()) return false;
-  } else if (prevMessage is DmMessage && message is DmMessage) {
-    if (!_equalIdSequences(prevMessage.allRecipientIds, message.allRecipientIds)) {
-      return false;
-    }
-  } else {
-    return false;
-  }
-  return true;
-
-  // switch ((prevMessage, message)) {
-  //   case (StreamMessage(), StreamMessage()):
-  //     // TODO(dart-3): this doesn't type-narrow prevMessage and message
-  //   case (DmMessage(), DmMessage()):
-  //     // …
-  //   default:
-  //     return false;
-  // }
+  return prevMessage.conversation.isSameAs(message.conversation);
 }
 
 @visibleForTesting
@@ -382,16 +363,6 @@ bool messagesSameDay(Message prevMessage, Message message) {
   return true;
 }
 
-// Intended for [Message.allRecipientIds].  Assumes efficient `length`.
-bool _equalIdSequences(Iterable<int> xs, Iterable<int> ys) {
-  if (xs.length != ys.length) return false;
-  final xs_ = xs.iterator; final ys_ = ys.iterator;
-  while (xs_.moveNext() && ys_.moveNext()) {
-    if (xs_.current != ys_.current) return false;
-  }
-  return true;
-}
-
 bool _sameDay(DateTime date1, DateTime date2) {
   if (date1.year != date2.year) return false;
   if (date1.month != date2.month) return false;