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

@@ -634,6 +634,9 @@ sealed class Message {
   final String contentType;
 
   // final List<MessageEditHistory> editHistory; // TODO handle
+  @JsonKey(readValue: MessageEditState.readFromMessage, fromJson: Message._messageEditStateFromJson)
+  MessageEditState messageEditState;
+
   final int id;
   bool isMeMessage;
   int? lastEditTimestamp;
@@ -658,6 +661,12 @@ sealed class Message {
   final String? matchContent;
   final String? matchSubject;
 
+  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;
@@ -676,6 +685,7 @@ sealed class Message {
     required this.client,
     required this.content,
     required this.contentType,
+    required this.messageEditState,
     required this.id,
     required this.isMeMessage,
     required this.lastEditTimestamp,
@@ -741,6 +751,7 @@ class StreamMessage extends Message {
     required super.client,
     required super.content,
     required super.contentType,
+    required super.messageEditState,
     required super.id,
     required super.isMeMessage,
     required super.lastEditTimestamp,
@@ -843,6 +854,7 @@ class DmMessage extends Message {
     required super.client,
     required super.content,
     required super.contentType,
+    required super.messageEditState,
     required super.id,
     required super.isMeMessage,
     required super.lastEditTimestamp,
@@ -866,3 +878,80 @@ class DmMessage extends Message {
   @override
   Map<String, dynamic> toJson() => _$DmMessageToJson(this);
 }
+
+@visibleForTesting
+enum MessageEditState {
+  none,
+  edited,
+  moved;
+
+  static bool isTopicMoved(String topic, String prevTopic) {
+    // TODO(#744) Extract this to its own home to fully support mark as resolve
+
+    // 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.
+     */
+    // The class has the same characters as RESOLVED_TOPIC_PREFIX.
+    // It's designed to remove a weird "✔ ✔✔ " prefix, if present.
+    final RegExp resolvedTopicPrefixRe = RegExp('^✔ [ ✔]*');
+
+    // Normalize both topics so the resolved-topic prefix do not interfere.
+    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 Otherwise, editHistory has to be a list
+    final editHistory = (json['edit_history'] as List<dynamic>?);
+
+    if (editHistory == null) {
+      return (json['last_edit_timestamp'] != 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 = isTopicMoved(
+            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

@@ -564,6 +564,9 @@ void main() {
       );
       check(m2).flags.deepEquals([MessageFlag.read, MessageFlag.unknown]);
     });
+
+    // Code relevant to editHistory is tested separately in the MessageEditState
+    // group.
   });
 
   group('DmMessage', () {
@@ -628,4 +631,100 @@ 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
+      ).messageEditState).equals(MessageEditState.none);
+
+      check(Message.fromJson(baseJson()
+          ..['edit_history'] = null
+          ..['last_edit_timestamp'] = 1678139636).messageEditState
+      ).equals(MessageEditState.edited);
+    });
+
+    test('edit history exists', () {
+      // Only channel name changed: moved
+      check(Message.fromJson(baseJson()
+        ..['edit_history'] = [{
+          'prev_stream': 'old_stream',
+          'stream': 'new_stream',
+        }]
+      ).messageEditState).equals(MessageEditState.moved);
+
+      // Only topic name changed: moved
+      check(Message.fromJson(baseJson()
+        ..['edit_history'] = [{
+          'prev_topic': 'old_topic',
+          'topic': 'new_topic',
+        }]
+      ).messageEditState).equals(MessageEditState.moved);
+
+      // Both topic and content changed: edited
+      check(Message.fromJson(baseJson()
+        ..['edit_history'] = [{
+          'prev_topic': 'old_topic',
+          'topic': 'new_topic',
+        }, {
+          'prev_content': 'old_content',
+        }]
+      ).messageEditState).equals(MessageEditState.edited);
+
+      // Only content changed: edited
+      check(Message.fromJson(baseJson()
+        ..['edit_history'] = [{
+          'prev_content': 'old_content',
+        }]
+      ).messageEditState).equals(MessageEditState.edited);
+
+      // 'prev_topic' present without the 'topic' field: moved
+      check(Message.fromJson(baseJson()
+        ..['edit_history'] = [{
+          'prev_topic': 'old_topic',
+        }]
+      ).messageEditState).equals(MessageEditState.moved);
+    });
+
+    test('topic resolved in edit history', () {
+      // Topic was only resolved: none
+      check(Message.fromJson(baseJson()
+        ..['edit_history'] = [{
+          'prev_topic': 'old_topic',
+          'topic': '✔ old_topic',
+        }]
+      ).messageEditState).equals(MessageEditState.none);
+
+      // Topic was resolved but the content changed in the history: edited
+      check(Message.fromJson(baseJson()
+        ..['edit_history'] = [{
+          'prev_topic': 'old_topic',
+          'topic': '✔ old_topic',
+        }, {
+          'prev_content': 'old_content',
+        }]
+      ).messageEditState).equals(MessageEditState.edited);
+
+      // Topic was resolved but it also changed in the history: moved
+      check(Message.fromJson(baseJson()
+        ..['edit_history'] = [{
+          'prev_topic': '✔ old_topic',
+          'topic': 'old_topic',
+        }, {
+          'prev_topic': 'old_topic',
+          'topic': 'new_topic',
+        }]
+      ).messageEditState).equals(MessageEditState.moved);
+
+      // Unresolving topic with a weird prefix:
+      check(Message.fromJson(baseJson()
+        ..['edit_history'] = [{
+          'prev_topic': '✔ ✔old_topic',
+          'topic': 'old_topic',
+        }]
+      ).messageEditState).equals(MessageEditState.none);
+    });
+  });
 }