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) {
+    // The value passed here must be a MessageEditState already due to
+    // processing work done in [MessageEditState.readFromMessage].
+    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 +710,78 @@ class DmMessage extends Message {
   @override
   Map<String, dynamic> toJson() => _$DmMessageToJson(this);
 }
+
+enum MessageEditState {
+  none,
+  edited,
+  moved;
+
+  // Code adapted from the shared code: web/shared/src/resolve_topic.ts
+  // Pattern for an arbitrary resolved-topic prefix.
+  // These always begin with the canonical prefix, but can go on longer.
+  // It's designed to remove a weird "✔ ✔✔ " prefix, if present.
+  static RegExp resolvedTopicPrefixRe = RegExp('^✔ [ ✔]*');
+
+  /// Whether two topics are equal, ignoring any resolved-topic prefix.
+  ///
+  /// 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 areSameTopic(String topic, String prevTopic) {
+    // TODO(#744) Extract this to its own home to support "mark as resolve".
+    topic = topic.replaceFirst(resolvedTopicPrefixRe, '');
+    prevTopic = prevTopic.replaceFirst(resolvedTopicPrefixRe, '');
+
+    return topic == prevTopic;
+  }
+
+  static MessageEditState readFromMessage(Map<dynamic, dynamic> json, String key) {
+    // TODO refactor this into a helper that computes this from the serialized
+    // MessageEditHistory.
+    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 hasEditedContent = false;
+    bool hasMoved = false;
+    for (final entry in editHistory) {
+      if (entry['prev_content'] != null) {
+        hasEditedContent = true;
+      }
+
+      if (entry['prev_stream'] != null) {
+        hasMoved = true;
+      }
+
+      // TODO(server-5) prev_subject was the old name of prev_topic on pre-5.0 servers
+      if (entry['prev_topic'] != null || entry['prev_subject'] != null) {
+        // TODO(server-5) pre-5.0 servers do not have the 'topic' field
+        if (entry['topic'] == null) {
+          hasMoved = true;
+        } else {
+          hasMoved = !areSameTopic(
+            entry['topic'] as String,
+            // TODO(server-5) prev_subject was the old name of prev_topic on pre-5.0 servers
+            (entry['prev_topic'] ?? entry['prev_subject']) as String
+          );
+        }
+      }
+    }
+
+    // Prioritize the 'edited' state over 'moved' when they both apply
+    if (hasEditedContent) return MessageEditState.edited;
+
+    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 +215,85 @@ void main() {
         .deepEquals([2, 3, 11]);
     });
   });
+
+  group('MessageEditState', () {
+    Map<String, dynamic> baseJson() => deepToJson(eg.streamMessage()) as Map<String, dynamic>;
+
+    test('edit history is absent', () {
+      check(Message.fromJson(baseJson()..['edit_history'] = null))
+        .editState.equals(MessageEditState.none);
+
+      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('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'},
+        ]);
+      });
+
+      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'}]);
+      });
+    });
+
+    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 changed 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 -> none', () {
+          checkEditState(MessageEditState.none,
+            [{'prev_topic': '✔ ✔old_topic', 'topic': 'old_topic'}]);
+      });
+
+    });
+  });
 }