model: Add MessageEditState to Message.

This new field is computed from edit_history provided by the API. We
create a readValue function that processes the list of edits and
determine if message has been edited or moved.

Some special handling was needed because topic being marked as resolved
should not be considered "moved".

Signed-off-by: Zixuan James Li <zixuan@zulip.com>

Author: PIG208

lib/api/model/model.dart

@@ -464,6 +464,9 @@ sealed class Message {
   final String contentType;
 
   // final List<MessageEditHistory> editHistory; // TODO handle
+  @JsonKey(readValue: MessageEditState._readFromMessage, fromJson: Message._messageEditStateFromJson)
+  MessageEditState editState;
+
   final int id;
   bool isMeMessage;
   int? lastEditTimestamp;
@@ -490,6 +493,12 @@ sealed class Message {
   @JsonKey(name: 'match_subject')
   final String? matchTopic;
 
+  static MessageEditState _messageEditStateFromJson(dynamic json) {
+    // This is a no-op so that [MessageEditState._readFromMessage]
+    // can return the enum value directly.
+    return json as MessageEditState;
+  }
+
   static Reactions? _reactionsFromJson(dynamic json) {
     final list = (json as List<dynamic>);
     return list.isNotEmpty ? Reactions.fromJson(list) : null;
@@ -508,6 +517,7 @@ sealed class Message {
     required this.client,
     required this.content,
     required this.contentType,
+    required this.editState,
     required this.id,
     required this.isMeMessage,
     required this.lastEditTimestamp,
@@ -573,6 +583,7 @@ class StreamMessage extends Message {
     required super.client,
     required super.content,
     required super.contentType,
+    required super.editState,
     required super.id,
     required super.isMeMessage,
     required super.lastEditTimestamp,
@@ -675,6 +686,7 @@ class DmMessage extends Message {
     required super.client,
     required super.content,
     required super.contentType,
+    required super.editState,
     required super.id,
     required super.isMeMessage,
     required super.lastEditTimestamp,
@@ -698,3 +710,79 @@ class DmMessage extends Message {
   @override
   Map<String, dynamic> toJson() => _$DmMessageToJson(this);
 }
+
+enum MessageEditState {
+  none,
+  edited,
+  moved;
+
+  // Adapted from the shared code:
+  //   https://github.com/zulip/zulip/blob/1fac99733/web/shared/src/resolved_topic.ts
+  // The canonical resolved-topic prefix.
+  static const String _resolvedTopicPrefix = '✔ ';
+
+  /// Whether the two topics differs and only differs by whether one of them is
+  /// resolved or not.
+  ///
+  /// When a topic is resolved, the clients agree on adding a ✔ prefix to the
+  /// topic string. Topics whose only difference is the ✔ prefix are considered
+  /// the same. This helper can be helpful when checking if a message has been
+  /// moved.
+  static bool topicsDifferByResolvednessOnly(String topic, String prevTopic) {
+    if (topic.startsWith(_resolvedTopicPrefix)
+      && topic.substring(_resolvedTopicPrefix.length) == prevTopic) {
+      return true;
+    }
+
+    if (prevTopic.startsWith(_resolvedTopicPrefix)
+      && prevTopic.substring(_resolvedTopicPrefix.length) == topic) {
+      return true;
+    }
+
+    return false;
+  }
+
+  static MessageEditState _readFromMessage(Map<dynamic, dynamic> json, String key) {
+    // Adapted from `analyze_edit_history` in the web app:
+    //   https://github.com/zulip/zulip/blob/c31cebbf68a93927d41e9947427c2dd4d46503e3/web/src/message_list_view.js#L68-L118
+    final editHistory = json['edit_history'] as List<dynamic>?;
+    final lastEditTimestamp = json['last_edit_timestamp'] as int?;
+    if (editHistory == null) {
+      return (lastEditTimestamp != null)
+        ? MessageEditState.edited
+        : MessageEditState.none;
+    }
+
+    // Edit history should never be empty whenever it is present
+    assert(editHistory.isNotEmpty);
+
+    bool hasMoved = false;
+    for (final entry in editHistory) {
+      if (entry['prev_content'] != null) {
+        return MessageEditState.edited;
+      }
+
+      if (entry['prev_stream'] != null) {
+        hasMoved = true;
+        continue;
+      }
+
+      // TODO(server-5) prev_subject was the old name of prev_topic on pre-5.0 servers
+      final prevTopic = (entry['prev_topic'] ?? entry['prev_subject']) as String?;
+      final topic = entry['topic'] as String?;
+      if (prevTopic != null) {
+        // TODO(server-5) pre-5.0 servers do not have the 'topic' field
+        if (topic == null) {
+          hasMoved = true;
+        } else {
+          hasMoved |= !topicsDifferByResolvednessOnly(topic, prevTopic);
+        }
+      }
+    }
+
+    if (hasMoved) return MessageEditState.moved;
+
+    // This can happen when a topic is resolved but nothing else has been edited
+    return MessageEditState.none;
+  }
+}

lib/api/model/model.g.dart

@@ -34,6 +34,7 @@ extension MessageChecks on Subject<Message> {
   Subject<int> get id => has((e) => e.id, 'id');
   Subject<bool> get isMeMessage => has((e) => e.isMeMessage, 'isMeMessage');
   Subject<int?> get lastEditTimestamp => has((e) => e.lastEditTimestamp, 'lastEditTimestamp');
+  Subject<MessageEditState> get editState => has((e) => e.editState, 'editState');
   Subject<Reactions?> get reactions => has((e) => e.reactions, 'reactions');
   Subject<int> get recipientId => has((e) => e.recipientId, 'recipientId');
   Subject<String> get senderEmail => has((e) => e.senderEmail, 'senderEmail');

test/api/model/model_checks.dart

@@ -148,6 +148,9 @@ void main() {
       );
       check(m2).flags.deepEquals([MessageFlag.read, MessageFlag.unknown]);
     });
+
+    // Code relevant to messageEditState is tested separately in the
+    // MessageEditState group.
   });
 
   group('DmMessage', () {
@@ -212,4 +215,110 @@ void main() {
         .deepEquals([2, 3, 11]);
     });
   });
+
+  group('MessageEditState', () {
+    Map<String, dynamic> baseJson() => deepToJson(eg.streamMessage()) as Map<String, dynamic>;
+
+    group('Edit history is absent', () {
+      test('Message with no evidence of an edit history -> none', () {
+        check(Message.fromJson(baseJson()..['edit_history'] = null))
+          .editState.equals(MessageEditState.none);
+      });
+
+      test('Message without edit history has last edit timestamp -> edited', () {
+        check(Message.fromJson(baseJson()
+            ..['edit_history'] = null
+            ..['last_edit_timestamp'] = 1678139636))
+          .editState.equals(MessageEditState.edited);
+      });
+    });
+
+    void checkEditState(MessageEditState editState, List<Map<String, dynamic>> editHistory){
+      check(Message.fromJson(baseJson()..['edit_history'] = editHistory))
+        .editState.equals(editState);
+    }
+
+    group('edit history exists', () {
+      test('Moved message has last edit timestamp but no actual edits -> moved', () {
+        check(Message.fromJson(baseJson()
+            ..['edit_history'] = [{'prev_stream': 5, 'stream': 7}]
+            ..['last_edit_timestamp'] = 1678139636))
+          .editState.equals(MessageEditState.moved);
+      });
+
+      test('Channel change only -> moved', () {
+        checkEditState(MessageEditState.moved,
+          [{'prev_stream': 5, 'stream': 7}]);
+      });
+
+      test('Topic name change only -> moved', () {
+        checkEditState(MessageEditState.moved,
+          [{'prev_topic': 'old_topic', 'topic': 'new_topic'}]);
+      });
+
+      test('Both topic and content changed -> edited', () {
+        checkEditState(MessageEditState.edited, [
+          {'prev_topic': 'old_topic', 'topic': 'new_topic'},
+          {'prev_content': 'old_content'},
+        ]);
+        checkEditState(MessageEditState.edited, [
+          {'prev_content': 'old_content'},
+          {'prev_topic': 'old_topic', 'topic': 'new_topic'},
+        ]);
+      });
+
+      test('Both topic and content changed in a single edit -> edited', () {
+        checkEditState(MessageEditState.edited,
+          [{'prev_topic': 'old_topic', 'topic': 'new_topic', 'prev_content': 'old_content'}]);
+      });
+
+      test('Content change only -> edited', () {
+        checkEditState(MessageEditState.edited,
+          [{'prev_content': 'old_content'}]);
+      });
+
+      test("'prev_topic' present without the 'topic' field -> moved", () {
+        checkEditState(MessageEditState.moved,
+          [{'prev_topic': 'old_topic'}]);
+      });
+
+      test("'prev_subject' present from a pre-5.0 server -> moved", () {
+        checkEditState(MessageEditState.moved,
+          [{'prev_subject': 'old_topic'}]);
+      });
+    });
+
+    group('topic resolved in edit history', () {
+      test('Topic was only resolved -> none', () {
+        checkEditState(MessageEditState.none,
+          [{'prev_topic': 'old_topic', 'topic': '✔ old_topic'}]);
+      });
+
+      test('Topic was resolved but the content changed in the history -> edited', () {
+        checkEditState(MessageEditState.edited, [
+          {'prev_topic': 'old_topic', 'topic': '✔ old_topic'},
+          {'prev_content': 'old_content'},
+        ]);
+      });
+
+      test('Topic was resolved but it also moved in the history -> moved', () {
+        checkEditState(MessageEditState.moved, [
+          {'prev_topic': 'old_topic', 'topic': 'new_topic'},
+          {'prev_topic': '✔ old_topic', 'topic': 'old_topic'},
+        ]);
+      });
+
+      test('Topic was moved but it also was resolved in the history -> moved', () {
+        checkEditState(MessageEditState.moved, [
+          {'prev_topic': '✔ old_topic', 'topic': 'old_topic'},
+          {'prev_topic': 'old_topic', 'topic': 'new_topic'},
+        ]);
+      });
+
+      test('Unresolving topic with a weird prefix -> moved', () {
+          checkEditState(MessageEditState.moved,
+            [{'prev_topic': '✔ ✔old_topic', 'topic': 'old_topic'}]);
+      });
+    });
+  });
 }