autocomplete: Add view-model ChannelLinkAutocompleteView

As of this commit, it's not yet possible in the app to initiate a
channel link autocomplete interaction. So in the widgets code that would
consume the results of such an interaction, we just throw for now,
leaving that to be implemented in a later commit.

Author: sm-sayedi

lib/model/autocomplete.dart

@@ -1,15 +1,18 @@
 import 'dart:math';
 
+import 'package:collection/collection.dart';
 import 'package:flutter/foundation.dart';
 import 'package:flutter/services.dart';
 import 'package:unorm_dart/unorm_dart.dart' as unorm;
 
 import '../api/model/events.dart';
 import '../api/model/model.dart';
 import '../api/route/channels.dart';
+import '../basic.dart';
 import '../generated/l10n/zulip_localizations.dart';
 import '../widgets/compose_box.dart';
 import 'algorithms.dart';
+import 'channel.dart';
 import 'compose.dart';
 import 'emoji.dart';
 import 'narrow.dart';
@@ -209,6 +212,7 @@ class AutocompleteViewManager {
   final Set<MentionAutocompleteView> _mentionAutocompleteViews = {};
   final Set<TopicAutocompleteView> _topicAutocompleteViews = {};
   final Set<EmojiAutocompleteView> _emojiAutocompleteViews = {};
+  final Set<ChannelLinkAutocompleteView> _channelLinkAutocompleteViews = {};
 
   AutocompleteDataCache autocompleteDataCache = AutocompleteDataCache();
 
@@ -242,6 +246,16 @@ class AutocompleteViewManager {
     assert(removed);
   }
 
+  void registerChannelLinkAutocomplete(ChannelLinkAutocompleteView view) {
+    final added = _channelLinkAutocompleteViews.add(view);
+    assert(added);
+  }
+
+  void unregisterChannelLinkAutocomplete(ChannelLinkAutocompleteView view) {
+    final removed = _channelLinkAutocompleteViews.remove(view);
+    assert(removed);
+  }
+
   void handleRealmUserRemoveEvent(RealmUserRemoveEvent event) {
     autocompleteDataCache.invalidateUser(event.userId);
   }
@@ -258,6 +272,17 @@ class AutocompleteViewManager {
     autocompleteDataCache.invalidateUserGroup(event.groupId);
   }
 
+  void handleChannelDeleteEvent(ChannelDeleteEvent event) {
+    final channelIds = event.streamIds ?? event.streams!.map((e) => e.streamId);
+    for (final channelId in channelIds) {
+      autocompleteDataCache.invalidateChannel(channelId);
+    }
+  }
+
+  void handleChannelUpdateEvent(ChannelUpdateEvent event) {
+    autocompleteDataCache.invalidateChannel(event.streamId);
+  }
+
   /// Called when the app is reassembled during debugging, e.g. for hot reload.
   ///
   /// Calls [AutocompleteView.reassemble] for all that are registered.
@@ -1021,6 +1046,20 @@ class AutocompleteDataCache {
       ??= normalizedNameForUserGroup(userGroup).split(' ');
   }
 
+  final Map<int, String> _normalizedNamesByChannel = {};
+
+  String normalizedNameForChannel(ZulipStream channel) {
+    return _normalizedNamesByChannel[channel.streamId]
+      ??= AutocompleteQuery.lowercaseAndStripDiacritics(channel.name);
+  }
+
+  final Map<int, List<String>> _normalizedNameWordsByChannel = {};
+
+  List<String> normalizedNameWordsForChannel(ZulipStream channel) {
+    return _normalizedNameWordsByChannel[channel.streamId]
+      ?? normalizedNameForChannel(channel).split(' ');
+  }
+
   void invalidateUser(int userId) {
     _normalizedNamesByUser.remove(userId);
     _normalizedNameWordsByUser.remove(userId);
@@ -1031,6 +1070,11 @@ class AutocompleteDataCache {
     _normalizedNamesByUserGroup.remove(id);
     _normalizedNameWordsByUserGroup.remove(id);
   }
+
+  void invalidateChannel(int channelId) {
+    _normalizedNamesByChannel.remove(channelId);
+    _normalizedNameWordsByChannel.remove(channelId);
+  }
 }
 
 /// A result the user chose, or might choose, from an autocomplete interaction.
@@ -1238,3 +1282,243 @@ class TopicAutocompleteResult extends AutocompleteResult {
 
   TopicAutocompleteResult({required this.topic});
 }
+
+/// An [AutocompleteView] for a #channel autocomplete interaction,
+/// an example of a [ComposeAutocompleteView].
+class ChannelLinkAutocompleteView extends AutocompleteView<ChannelLinkAutocompleteQuery, ChannelLinkAutocompleteResult> {
+  ChannelLinkAutocompleteView._({
+    required super.store,
+    required super.query,
+    required this.narrow,
+    required this.sortedChannels,
+  });
+
+  factory ChannelLinkAutocompleteView.init({
+    required PerAccountStore store,
+    required Narrow narrow,
+    required ChannelLinkAutocompleteQuery query,
+  }) {
+    final view = ChannelLinkAutocompleteView._(
+      store: store,
+      query: query,
+      narrow: narrow,
+      sortedChannels: _channelsByRelevance(store: store, narrow: narrow),
+    );
+    store.autocompleteViewManager.registerChannelLinkAutocomplete(view);
+    return view;
+  }
+
+  final Narrow narrow;
+  final List<ZulipStream> sortedChannels;
+
+  static List<ZulipStream> _channelsByRelevance({
+    required PerAccountStore store,
+    required Narrow narrow,
+  }) {
+    return store.streams.values.sorted(_comparator(narrow: narrow));
+  }
+
+  static Comparator<ZulipStream> _comparator({required Narrow narrow}) {
+    // See also [ChannelLinkAutocompleteQuery._rankResult];
+    // that ranking takes precedence over this.
+
+    final channelId = switch (narrow) {
+      ChannelNarrow(:var streamId) || TopicNarrow(:var streamId) => streamId,
+      DmNarrow() => null,
+      CombinedFeedNarrow()
+        || MentionsNarrow()
+        || StarredMessagesNarrow()
+        || KeywordSearchNarrow() => () {
+             assert(false, 'No compose box, thus no autocomplete is available in ${narrow.runtimeType}.');
+             return null;
+           }(),
+    };
+    return (a, b) => _compareByRelevance(a, b, currentChannelId: channelId);
+  }
+
+  // Check `typeahead_helper.compare_by_activity` in Zulip web;
+  // We follow the behavior of Web but with a small difference in that Web
+  // compares "recent activity" only for subscribed channels, but we do it
+  // for unsubscribed ones too.
+  //   https://github.com/zulip/zulip/blob/c3fdee6ed/web/src/typeahead_helper.ts#L972-L988
+  static int _compareByRelevance(ZulipStream a, ZulipStream b, {
+    required int? currentChannelId,
+  }) {
+    // The order of each comparator element in the list is important; top having
+    // the highest priority and bottom having the least priority.
+    final comparators = [
+      if (currentChannelId != null)
+        () => compareByBeingCurrent(a, b, currentChannelId: currentChannelId),
+      () => compareByBeingSubscribed(a, b),
+      () => compareByRecentActivity(a, b),
+      () => compareByWeeklyTraffic(a, b),
+      () => ChannelStore.compareChannelsByName(a, b),
+    ];
+    return comparators.map((compare) => compare())
+      .firstWhere((result) => result != 0, orElse: () => 0);
+  }
+
+  /// Comparator that puts the channel being composed to, before other ones.
+  @visibleForTesting
+  static int compareByBeingCurrent(ZulipStream a, ZulipStream b, {
+    required int currentChannelId,
+  }) {
+    return switch ((a.streamId, b.streamId)) {
+      (int id, _) when id == currentChannelId => -1,
+      (_, int id) when id == currentChannelId =>  1,
+      _                                       =>  0,
+    };
+  }
+
+  /// Comparator that puts subscribed channels before unsubscribed ones.
+  ///
+  /// For subscribed channels, it puts them in the following way:
+  ///   pinned unmuted > unpinned unmuted > pinned muted > unpinned muted
+  @visibleForTesting
+  static int compareByBeingSubscribed(ZulipStream a, ZulipStream b) {
+    return switch((tryCast<Subscription>(a), tryCast<Subscription>(b))) {
+      (Subscription(), null) => -1,
+      (null, Subscription()) =>  1,
+      (Subscription(isMuted:  false), Subscription(isMuted:   true)) => -1,
+      (Subscription(isMuted:   true), Subscription(isMuted:  false)) =>  1,
+      (Subscription(pinToTop:  true), Subscription(pinToTop: false)) => -1,
+      (Subscription(pinToTop: false), Subscription(pinToTop:  true)) =>  1,
+      _                                                              =>  0,
+    };
+  }
+
+  /// Comparator that puts recently-active channels before inactive ones.
+  ///
+  /// A channel is recently active if there are messages sent to it recently,
+  /// which is determined by [ZulipStream.isRecentlyActive].
+  @visibleForTesting
+  static int compareByRecentActivity(ZulipStream a, ZulipStream b) {
+    return switch((a.isRecentlyActive, b.isRecentlyActive)) {
+      (true, false) => -1,
+      (false, true) =>  1,
+      // The combination of `null` and `bool` is not possible as they're both
+      // either `null` or `bool`, before or after server-10, respectively.
+      // TODO(server-10): remove the preceding comment
+      _             =>  0,
+    };
+  }
+
+  /// Comparator that puts channels with more weekly traffic first.
+  ///
+  /// A channel with undefined weekly traffic (`null`) is put after the channel
+  /// with a weekly traffic defined (even if it is zero).
+  ///
+  /// Weekly traffic is the average number of messages sent to the channel per
+  /// week, which is determined by [ZulipStream.streamWeeklyTraffic].
+  @visibleForTesting
+  static int compareByWeeklyTraffic(ZulipStream a, ZulipStream b) {
+    return switch((a.streamWeeklyTraffic, b.streamWeeklyTraffic)) {
+      (int a, int b) => -a.compareTo(b),
+      (int(),  null) => -1,
+      (null,  int()) =>  1,
+      _              =>  0,
+    };
+  }
+
+  @override
+  Future<List<ChannelLinkAutocompleteResult>?> computeResults() async {
+    final unsorted = <ChannelLinkAutocompleteResult>[];
+    if (await filterCandidates(filter: _testChannel,
+          candidates: sortedChannels, results: unsorted)) {
+      return null;
+    }
+
+    return bucketSort(unsorted,
+      (r) => r.rank, numBuckets: ChannelLinkAutocompleteQuery._numResultRanks);
+  }
+
+  ChannelLinkAutocompleteResult? _testChannel(ChannelLinkAutocompleteQuery query, ZulipStream channel) {
+    return query.testChannel(channel, store);
+  }
+
+  @override
+  void dispose() {
+    store.autocompleteViewManager.unregisterChannelLinkAutocomplete(this);
+    super.dispose();
+  }
+}
+
+/// A #channel autocomplete query, used by [ChannelLinkAutocompleteView].
+class ChannelLinkAutocompleteQuery extends ComposeAutocompleteQuery {
+  ChannelLinkAutocompleteQuery(super.raw);
+
+  @override
+  ChannelLinkAutocompleteView initViewModel({
+    required PerAccountStore store,
+    required ZulipLocalizations localizations,
+    required Narrow narrow,
+  }) {
+    return ChannelLinkAutocompleteView.init(store: store, query: this, narrow: narrow);
+  }
+
+  ChannelLinkAutocompleteResult? testChannel(ZulipStream channel, PerAccountStore store) {
+    final cache = store.autocompleteViewManager.autocompleteDataCache;
+    final matchQuality = _matchName(
+      normalizedName: cache.normalizedNameForChannel(channel),
+      normalizedNameWords: cache.normalizedNameWordsForChannel(channel));
+    if (matchQuality == null) return null;
+    return ChannelLinkAutocompleteResult(
+      channelId: channel.streamId, rank: _rankResult(matchQuality));
+  }
+
+  /// A measure of a channel result's quality in the context of the query,
+  /// from 0 (best) to one less than [_numResultRanks].
+  static int _rankResult(NameMatchQuality matchQuality) {
+    return switch(matchQuality) {
+      NameMatchQuality.exact        => 0,
+      NameMatchQuality.totalPrefix  => 1,
+      NameMatchQuality.wordPrefixes => 2,
+    };
+  }
+
+  /// The number of possible values returned by [_rankResult].
+  static const _numResultRanks = 3;
+
+  @override
+  String toString() {
+    return '${objectRuntimeType(this, 'ChannelLinkAutocompleteQuery')}($raw)';
+  }
+
+  @override
+  bool operator ==(Object other) {
+    return other is ChannelLinkAutocompleteQuery && other.raw == raw;
+  }
+
+  @override
+  int get hashCode => Object.hash('ChannelLinkAutocompleteQuery', raw);
+}
+
+/// An autocomplete result for a #channel.
+class ChannelLinkAutocompleteResult extends ComposeAutocompleteResult {
+  ChannelLinkAutocompleteResult({required this.channelId, required this.rank});
+
+  final int channelId;
+
+  /// A measure of the result's quality in the context of the query.
+  ///
+  /// Used internally by [ChannelLinkAutocompleteView] for ranking the results.
+  // See also [ChannelLinkAutocompleteView._channelsByRelevance];
+  // results with equal [rank] will appear in the order they were put in
+  // by that method.
+  //
+  // Compare sort_streams in Zulip web:
+  //   https://github.com/zulip/zulip/blob/a5d25826b/web/src/typeahead_helper.ts#L998-L1008
+  //
+  // Behavior we have that web doesn't and might like to follow:
+  // - A "word-prefixes" match quality on channel names:
+  //   see [NameMatchQuality.wordPrefixes], which we rank on.
+  //
+  // Behavior web has that seems undesired, which we don't plan to follow:
+  // - A "word-boundary" match quality on channel names:
+  //   special rank when the whole query appears contiguously
+  //   right after a word-boundary character.
+  //   Our [NameMatchQuality.wordPrefixes] seems smarter.
+  // - Ranking some case-sensitive matches differently from case-insensitive
+  //   matches. Users will expect a lowercase query to be adequate.
+  final int rank;
+}

lib/model/store.dart

@@ -827,6 +827,11 @@ class PerAccountStore extends PerAccountStoreBase with
       case ChannelEvent():
         assert(debugLog("server event: stream/${event.op}"));
         _channels.handleChannelEvent(event);
+        if (event is ChannelDeleteEvent) {
+          autocompleteViewManager.handleChannelDeleteEvent(event);
+        } else if (event is ChannelUpdateEvent) {
+          autocompleteViewManager.handleChannelUpdateEvent(event);
+        }
         notifyListeners();
 
       case SubscriptionEvent():

lib/widgets/autocomplete.dart

@@ -226,6 +226,8 @@ class ComposeAutocomplete extends AutocompleteField<ComposeAutocompleteQuery, Co
         // TODO(#1805) language-appropriate space character; check active keyboard?
         //   (maybe handle centrally in `controller`)
         replacementString = '${userGroupMention(userGroup.name, silent: query.silent)} ';
+      case ChannelLinkAutocompleteResult():
+        throw UnimplementedError(); // TODO(#124)
     }
 
     controller.value = intent.textEditingValue.replaced(
@@ -243,6 +245,7 @@ class ComposeAutocomplete extends AutocompleteField<ComposeAutocompleteQuery, Co
     final child = switch (option) {
       MentionAutocompleteResult() => MentionAutocompleteItem(
         option: option, narrow: narrow),
+      ChannelLinkAutocompleteResult() => throw UnimplementedError(), // TODO(#124)
       EmojiAutocompleteResult() => _EmojiAutocompleteItem(option: option),
     };
     return InkWell(