compose: Introduce fallbackMarkdownLink function

Author: sm-sayedi

lib/model/compose.dart

@@ -185,6 +185,56 @@ String wildcardMention(WildcardMentionOption wildcardOption, {
 String userGroupMention(String userGroupName, {bool silent = false}) =>
   '@${silent ? '_' : ''}*$userGroupName*';
 
+// Corresponds to `topic_link_util.escape_invalid_stream_topic_characters`
+// in Zulip web:
+//   https://github.com/zulip/zulip/blob/b42d3e77e/web/src/topic_link_util.ts#L15-L34
+const _channelTopicFaultyCharsReplacements = {
+  '`': '`',
+  '>': '>',
+  '*': '*',
+  '&': '&',
+  '[': '[',
+  ']': ']',
+  r'$$': '$$',
+};
+
+final _channelTopicFaultyCharsRegex = RegExp(r'[`>*&[\]]|(?:\$\$)');
+
+/// Markdown link for channel, topic, or message when the channel or topic name
+/// includes characters that will break normal markdown rendering.
+///
+/// Refer to [_channelTopicFaultyCharsReplacements] for a complete list of
+/// these characters.
+// Corresponds to `topic_link_util.get_fallback_markdown_link` in Zulip web;
+//   https://github.com/zulip/zulip/blob/b42d3e77e/web/src/topic_link_util.ts#L96-L108
+String fallbackMarkdownLink({
+  required PerAccountStore store,
+  required ZulipStream channel,
+  TopicName? topic,
+  int? nearMessageId,
+}) {
+  assert(nearMessageId == null || topic != null);
+
+  String replaceFaultyChars(String str) {
+    return str.replaceAllMapped(_channelTopicFaultyCharsRegex,
+      (match) => _channelTopicFaultyCharsReplacements[match[0]]!);
+  }
+
+  final text = StringBuffer(replaceFaultyChars(channel.name));
+  if (topic != null) {
+    text.write(' > ${replaceFaultyChars(topic.displayName ?? store.realmEmptyTopicDisplayName)}');
+  }
+  if (nearMessageId != null) {
+    text.write(' @ 💬');
+  }
+
+  final narrow = topic == null
+    ? ChannelNarrow(channel.streamId) : TopicNarrow(channel.streamId, topic);
+  final linkFragment = narrowLinkFragment(store, narrow, nearMessageId: nearMessageId);
+
+  return inlineLink(text.toString(), '#$linkFragment');
+}
+
 /// https://spec.commonmark.org/0.30/#inline-link
 ///
 /// The "link text" is made by enclosing [visibleText] in square brackets.

test/model/compose_test.dart

@@ -1,6 +1,7 @@
 import 'package:checks/checks.dart';
 import 'package:test/scaffolding.dart';
 import 'package:zulip/api/model/events.dart';
+import 'package:zulip/api/model/model.dart';
 import 'package:zulip/model/compose.dart';
 import 'package:zulip/model/localizations.dart';
 import 'package:zulip/model/store.dart';
@@ -334,6 +335,38 @@ hello
     });
   });
 
+  test('fallbackMarkdownLink', () async {
+    final store = eg.store();
+    final channels = [
+      eg.stream(streamId: 1, name: '`code`'),
+      eg.stream(streamId: 2, name: 'score > 90'),
+      eg.stream(streamId: 3, name: 'A*'),
+      eg.stream(streamId: 4, name: 'R&D'),
+      eg.stream(streamId: 5, name: 'UI [v2]'),
+      eg.stream(streamId: 6, name: r'Save $$'),
+    ];
+    await store.addStreams(channels);
+
+    check(fallbackMarkdownLink(store: store,
+        channel: channels[1 - 1]))
+      .equals('[`code`](#narrow/channel/1-.60code.60)');
+    check(fallbackMarkdownLink(store: store,
+        channel: channels[2 - 1], topic: TopicName('topic')))
+      .equals('[score > 90 > topic](#narrow/channel/2-score-.3E-90/topic/topic)');
+    check(fallbackMarkdownLink(store: store,
+        channel: channels[3 - 1], topic: TopicName('R&D')))
+      .equals('[A* > R&D](#narrow/channel/3-A*/topic/R.26D)');
+    check(fallbackMarkdownLink(store: store,
+        channel: channels[4 - 1], topic: TopicName('topic'), nearMessageId: 10))
+      .equals('[R&D > topic @ 💬](#narrow/channel/4-R.26D/topic/topic/near/10)');
+    check(fallbackMarkdownLink(store: store,
+        channel: channels[5 - 1], topic: TopicName(r'Save $$'), nearMessageId: 10))
+      .equals('[UI [v2] > Save $$ @ 💬](#narrow/channel/5-UI-.5Bv2.5D/topic/Save.20.24.24/near/10)');
+    check(() => fallbackMarkdownLink(store: store,
+        channel: channels[6 - 1], nearMessageId: 10))
+      .throws<AssertionError>();
+  });
+
   test('inlineLink', () {
     check(inlineLink('CZO', 'https://chat.zulip.org/')).equals('[CZO](https://chat.zulip.org/)');
     check(inlineLink('Uploading file.txt…', '')).equals('[Uploading file.txt…]()');