api [nfc]: Add TopicName.interpretAsServer

The point of this helper is to replicate what a topic sent from the
client will become, after being processed by the server.

This important when trying to create a local copy of a stream message,
whose topic can get translated when it's delivered by the server.

Author: PIG208

lib/api/model/model.dart

@@ -532,6 +532,15 @@ String? tryParseEmojiCodeToUnicode(String emojiCode) {
   }
 }
 
+/// The topic servers understand to mean "there is no topic".
+///
+/// This should match
+///   https://github.com/zulip/zulip/blob/6.0/zerver/actions/message_edit.py#L940
+/// or similar logic at the latest `main`.
+// This is hardcoded in the server, and therefore untranslated; that's
+// zulip/zulip#3639.
+const String kNoTopicTopic = '(no topic)';
+
 /// The name of a Zulip topic.
 // TODO(dart): Can we forbid calling Object members on this extension type?
 //   (The lack of "implements Object" ought to do that, but doesn't.)
@@ -586,6 +595,30 @@ extension type const TopicName(String _value) {
   /// using [canonicalize].
   bool isSameAs(TopicName other) => canonicalize() == other.canonicalize();
 
+  /// Convert this topic to match how it would appear on a message object from
+  /// the server, assuming the topic is originally for a send-message request.
+  ///
+  /// For a client that does not support empty topics,
+  /// a modern server (FL>=334) would convert "(no topic)" and empty topics to
+  /// `store.realmEmptyTopicDisplayName`.
+  ///
+  /// See also: https://zulip.com/api/send-message#parameter-topic
+  TopicName interpretAsServer({
+    required int zulipFeatureLevel,
+    required String? realmEmptyTopicDisplayName,
+  }) {
+    if (zulipFeatureLevel < 334) {
+      assert(_value.isNotEmpty);
+      return this;
+    }
+    if (_value == kNoTopicTopic || _value.isEmpty) {
+      // TODO(#1250): this assumes that the 'support_empty_topics'
+      //   client_capability is false; update this when we set it to true
+      return TopicName(realmEmptyTopicDisplayName!);
+    }
+    return TopicName(_value);
+  }
+
   TopicName.fromJson(this._value);
 
   String toJson() => apiName;

lib/api/route/messages.dart

@@ -169,15 +169,6 @@ const int kMaxTopicLengthCodePoints = 60;
 // https://zulip.com/api/send-message#parameter-content
 const int kMaxMessageLengthCodePoints = 10000;
 
-/// The topic servers understand to mean "there is no topic".
-///
-/// This should match
-///   https://github.com/zulip/zulip/blob/6.0/zerver/actions/message_edit.py#L940
-/// or similar logic at the latest `main`.
-// This is hardcoded in the server, and therefore untranslated; that's
-// zulip/zulip#3639.
-const String kNoTopicTopic = '(no topic)';
-
 /// https://zulip.com/api/send-message
 Future<SendMessageResult> sendMessage(
   ApiConnection connection, {

test/api/model/model_test.dart

@@ -161,6 +161,27 @@ void main() {
 
       doCheck(eg.t('✔ a'),          eg.t('✔ b'),          false);
     });
+
+    test('interpretAsServer', () {
+      final emptyTopicDisplayName = eg.defaultRealmEmptyTopicDisplayName;
+      void doCheck(TopicName topicA, TopicName expected, int zulipFeatureLevel) {
+        check(topicA.interpretAsServer(
+          zulipFeatureLevel: zulipFeatureLevel,
+          realmEmptyTopicDisplayName: emptyTopicDisplayName),
+        ).equals(expected);
+      }
+
+      check(() => doCheck(eg.t(''),        eg.t(''),                    333))
+        .throws<void>();
+      doCheck(eg.t('(no topic)'),          eg.t('(no topic)'),          333);
+      doCheck(eg.t(emptyTopicDisplayName), eg.t(emptyTopicDisplayName), 333);
+      doCheck(eg.t('other topic'),         eg.t('other topic'),         333);
+
+      doCheck(eg.t(''),                    eg.t(emptyTopicDisplayName), 334);
+      doCheck(eg.t('(no topic)'),          eg.t(emptyTopicDisplayName), 334);
+      doCheck(eg.t(emptyTopicDisplayName), eg.t(emptyTopicDisplayName), 334);
+      doCheck(eg.t('other topic'),         eg.t('other topic'),         334);
+    });
   });
 
   group('DmMessage', () {