compose: Add narrowLink helper, to write links to Narrows

We'll use this soon, for quote-and-reply zulip#116.

With the recent commit 4e11ea7, PerAccountStoreTestExtension now
has `addStream`, which we use in the tests.

lib/model/compose.dart

@@ -1,5 +1,9 @@
 import 'dart:math';
 
+import '../api/model/narrow.dart';
+import 'narrow.dart';
+import 'store.dart';
+
 //
 // Put functions for nontrivial message-content generation in this file.
 //
@@ -96,5 +100,61 @@ String wrapWithBacktickFence({required String content, String? infoString}) {
   return resultBuffer.toString();
 }
 
+const _hashReplacements = {
+  "%": ".",
+  "(": ".28",
+  ")": ".29",
+  ".": ".2E",
+};
+
+final _encodeHashComponentRegex = RegExp(r'[%().]');
+
+// Corresponds to encodeHashComponent in Zulip web;
+// see web/shared/src/internal_url.ts.
+String _encodeHashComponent(String str) {
+  return Uri.encodeComponent(str)
+    .replaceAllMapped(_encodeHashComponentRegex, (Match m) => _hashReplacements[m[0]!]!);
+}
+
+/// A URL to the given [Narrow], on `store`'s realm.
+Uri narrowLink(PerAccountStore store, Narrow narrow) {
+  final apiNarrow = narrow.apiEncode();
+  final fragment = StringBuffer('narrow');
+  for (ApiNarrowElement element in apiNarrow) {
+    fragment.write('/');
+    if (element.negated) {
+      fragment.write('-');
+    }
+
+    if (element is ApiNarrowDm) {
+      final supportsOperatorDm = store.connection.zulipFeatureLevel! >= 177; // TODO(server-7)
+      element = element.resolve(legacy: !supportsOperatorDm);
+    }
+
+    fragment.write('${element.operator}/');
+
+    switch (element) {
+      case ApiNarrowStream():
+        final streamId = element.operand;
+        final name = store.streams[streamId]?.name ?? 'unknown';
+        final slugifiedName = _encodeHashComponent(name.replaceAll(' ', '-'));
+        fragment.write('$streamId-$slugifiedName');
+      case ApiNarrowTopic():
+        fragment.write(_encodeHashComponent(element.operand));
+      case ApiNarrowDmModern():
+        final suffix = element.operand.length >= 3 ? 'group' : 'dm';
+        fragment.write('${element.operand.join(',')}-$suffix');
+      case ApiNarrowPmWith():
+        final suffix = element.operand.length >= 3 ? 'group' : 'pm';
+        fragment.write('${element.operand.join(',')}-$suffix');
+      case ApiNarrowDm():
+        assert(false, 'ApiNarrowDm should have been resolved');
+      case ApiNarrowMessageId():
+        fragment.write(element.operand.toString());
+    }
+  }
+  return store.account.realmUrl.replace(fragment: fragment.toString());
+}
+
 // TODO more, like /near links to messages in conversations
 //   (also to be used in quote-and-reply)

test/model/compose_test.dart

@@ -1,6 +1,10 @@
 import 'package:checks/checks.dart';
 import 'package:test/scaffolding.dart';
 import 'package:zulip/model/compose.dart';
+import 'package:zulip/model/narrow.dart';
+
+import '../example_data.dart' as eg;
+import 'test_store.dart';
 
 void main() {
   group('wrapWithBacktickFence', () {
@@ -215,4 +219,71 @@ hello
 ''');
     });
   });
+
+  group('narrowLink', () {
+    test('AllMessagesNarrow', () {
+      final store = eg.store();
+      check(narrowLink(store, const AllMessagesNarrow())).equals(store.account.realmUrl.resolve('#narrow'));
+    });
+
+    test('StreamNarrow / TopicNarrow', () {
+      void checkNarrow(String expectedFragment, {
+        required int streamId,
+        required String name,
+        String? topic,
+      }) {
+        assert(expectedFragment.startsWith('#'), 'wrong-looking expectedFragment');
+        final store = eg.store();
+        store.addStream(eg.stream(streamId: streamId, name: name));
+        final narrow = topic == null
+          ? StreamNarrow(streamId)
+          : TopicNarrow(streamId, topic);
+        check(narrowLink(store, narrow)).equals(store.account.realmUrl.resolve(expectedFragment));
+      }
+
+      checkNarrow(streamId: 1,   name: 'announce',       '#narrow/stream/1-announce');
+      checkNarrow(streamId: 378, name: 'api design',     '#narrow/stream/378-api-design');
+      checkNarrow(streamId: 391, name: 'Outreachy',      '#narrow/stream/391-Outreachy');
+      checkNarrow(streamId: 415, name: 'chat.zulip.org', '#narrow/stream/415-chat.2Ezulip.2Eorg');
+      checkNarrow(streamId: 419, name: 'français',       '#narrow/stream/419-fran.C3.A7ais');
+      checkNarrow(streamId: 403, name: 'Hshs[™~}(.',     '#narrow/stream/403-Hshs.5B.E2.84.A2~.7D.28.2E');
+
+      checkNarrow(streamId: 48,  name: 'mobile', topic: 'Welcome screen UI',
+                  '#narrow/stream/48-mobile/topic/Welcome.20screen.20UI');
+      checkNarrow(streamId: 243, name: 'mobile-team', topic: 'Podfile.lock clash #F92',
+                  '#narrow/stream/243-mobile-team/topic/Podfile.2Elock.20clash.20.23F92');
+      checkNarrow(streamId: 377, name: 'translation/zh_tw', topic: '翻譯 "stream"',
+                  '#narrow/stream/377-translation.2Fzh_tw/topic/.E7.BF.BB.E8.AD.AF.20.22stream.22');
+    });
+
+    test('DmNarrow', () {
+      void checkNarrow(String expectedFragment, String legacyExpectedFragment, {
+        required List<int> allRecipientIds,
+        required int selfUserId,
+      }) {
+        assert(expectedFragment.startsWith('#'), 'wrong-looking expectedFragment');
+        final store = eg.store();
+        final narrow = DmNarrow(allRecipientIds: allRecipientIds, selfUserId: selfUserId);
+        check(narrowLink(store, narrow)).equals(store.account.realmUrl.resolve(expectedFragment));
+        store.connection.zulipFeatureLevel = 176;
+        check(narrowLink(store, narrow)).equals(store.account.realmUrl.resolve(legacyExpectedFragment));
+      }
+
+      checkNarrow(allRecipientIds: [1], selfUserId: 1,
+        '#narrow/dm/1-dm',
+        '#narrow/pm-with/1-pm');
+      checkNarrow(allRecipientIds: [1, 2], selfUserId: 1,
+        '#narrow/dm/1,2-dm',
+        '#narrow/pm-with/1,2-pm');
+      checkNarrow(allRecipientIds: [1, 2, 3], selfUserId: 1,
+        '#narrow/dm/1,2,3-group',
+        '#narrow/pm-with/1,2,3-group');
+      checkNarrow(allRecipientIds: [1, 2, 3, 4], selfUserId: 4,
+        '#narrow/dm/1,2,3,4-group',
+        '#narrow/pm-with/1,2,3,4-group');
+    });
+
+    // TODO other Narrow subclasses as we add them:
+    //   starred, mentioned; searches; arbitrary
+  });
 }