api [nfc]: Add/use topicsMatchModuloResolvePrefix helper, with tests

Author: chrisbobbe

lib/api/model/model.dart

@@ -813,19 +813,8 @@ enum MessageEditState {
   /// The Zulip "resolved topics" feature is implemented by renaming the topic;
   /// but for purposes of [Message.editState], we want to ignore such renames.
   /// This method identifies topic moves that should be ignored in that context.
-  static bool topicMoveWasResolveOrUnresolve(String topic, String prevTopic) {
-    if (topic.startsWith(kResolvedTopicPrefix)
-        && topic.substring(kResolvedTopicPrefix.length) == prevTopic) {
-      return true;
-    }
-
-    if (prevTopic.startsWith(kResolvedTopicPrefix)
-        && prevTopic.substring(kResolvedTopicPrefix.length) == topic) {
-      return true;
-    }
-
-    return false;
-  }
+  static bool topicMoveWasResolveOrUnresolve(String topic, String prevTopic) =>
+    topic != prevTopic && topicsMatchModuloResolvePrefix(topic, prevTopic);
 
   static MessageEditState _readFromMessage(Map<dynamic, dynamic> json, String key) {
     // Adapted from `analyze_edit_history` in the web app:

lib/api/route/messages.dart

@@ -163,6 +163,14 @@ const int kMaxTopicLength = 60;
 //   https://github.com/zulip/zulip/blob/1fac99733/web/shared/src/resolved_topic.ts
 const String kResolvedTopicPrefix = '✔ ';
 
+String stripResolvePrefixIfPresent(String topic) =>
+  topic.startsWith(kResolvedTopicPrefix)
+    ? topic.substring(kResolvedTopicPrefix.length)
+    : topic;
+
+bool topicsMatchModuloResolvePrefix(String topicA, String topicB) =>
+  stripResolvePrefixIfPresent(topicA) == stripResolvePrefixIfPresent(topicB);
+
 // https://zulip.com/api/send-message#parameter-content
 const int kMaxMessageLengthCodePoints = 10000;
 

test/api/route/messages_test.dart

@@ -297,6 +297,42 @@ void main() {
     });
   });
 
+  group('resolve/unresolve with kResolvedTopicPrefix', () {
+    group('stripResolvePrefixIfPresent', () {
+      void doTest(String description, String input, String expected) {
+        test(description, () {
+          check(stripResolvePrefixIfPresent(input)).equals(expected);
+        });
+      }
+      doTest('not present', 'topic', 'topic');
+      doTest('present once', '✔ topic', 'topic');
+      doTest('present twice', '✔ ✔ topic', '✔ topic');
+      doTest('present in middle', 'some ✔ topic', 'some ✔ topic');
+    });
+
+    group('topicsMatchModuloResolvePrefix', () {
+      void doTest(String description, String topicA, String topicB, bool expected) {
+        test(description, () {
+          check(topicsMatchModuloResolvePrefix(topicA, topicB)).equals(expected);
+        });
+      }
+      doTest('same-named, both unresolved', 'topic', 'topic', true);
+      doTest('same-named, topicA resolved', '✔ topic', 'topic', true);
+      doTest('same-named, topicB resolved', 'topic', '✔ topic', true);
+      doTest('same-named, both resolved', '✔ topic', '✔ topic', true);
+
+      doTest('different-named, both unresolved', 'this', 'other', false);
+      doTest('different-named, topicA resolved', '✔ this', 'other', false);
+      doTest('different-named, topicB resolved', 'this', '✔ other', false);
+      doTest('different-named, both resolved', '✔ this', '✔ other', false);
+
+      doTest('different-named because one duplicates prefix',
+             '✔ topic', '✔ ✔ topic', false);
+      doTest("different-named but one's name is a prefix of the other",
+             'topic', 'topic zulip', false);
+    });
+  });
+
   group('sendMessage', () {
     const streamId = 123;
     const content = 'hello';