wip emoji: Rank "popular" > custom > other emoji; TODO test

lib/model/emoji.dart

@@ -376,23 +376,67 @@ class EmojiAutocompleteView extends AutocompleteView<EmojiAutocompleteQuery, Emo
   EmojiAutocompleteResult? _testCandidate(EmojiAutocompleteQuery query, EmojiCandidate candidate) {
     final match = query.match(candidate);
     if (match == null) return null;
-    return EmojiAutocompleteResult(_rankResult(match), candidate);
+    return EmojiAutocompleteResult(_rankResult(match, candidate), candidate);
   }
 
   /// A measure of the result's quality in the context of the query,
   /// ranked from 0 (best) to one less than [_numResultRanks].
-  static int _rankResult(EmojiMatchQuality matchQuality) {
+  static int _rankResult(EmojiMatchQuality matchQuality, EmojiCandidate candidate) {
     // Compare sort_emojis in Zulip web:shared/src/typeahead.ts .
-    // TODO(#1068): rank emoji results also by popular, realm, other
+    //
+    // Behavior differences we might copy, TODO:
+    //  * Web ranks matches starting at a word boundary ahead of
+    //    other non-prefix matches; we don't yet.
+    //  * Relatedly, web favors popular emoji only upon a word-aligned match.
+    //  * Web ranks each name of a Unicode emoji separately.
+    //
+    // Behavior differences that web should probably fix, TODO(web):
+    //  * Among popular emoji with non-exact matches,
+    //    web doesn't prioritize prefix over word-aligned; we do.
+    //    (This affects just one case: for query "o",
+    //    we put :octopus: before :working_on_it:.)
+    //  * Web only counts an emoji as "popular" for ranking if the query
+    //    is a prefix of a single word in the name; so "thumbs_" or "working_on_i"
+    //    lose the ranking boost for :thumbs_up: and :working_on_it: respectively.
+    //  * Web starts with only case-sensitive exact matches ("perfect matches"),
+    //    and puts case-insensitive exact matches just ahead of prefix matches;
+    //    it also distinguishes prefix matches by case-sensitive vs. not.
+    //    We use case-insensitive matches throughout;
+    //    case seems unhelpful for emoji search.
+    //  * Web suppresses Unicode emoji names shadowed by a realm emoji
+    //    only if the latter is also a match for the query.  That mostly works,
+    //    because emoji with the same name will mostly both match or both not;
+    //    but it breaks if the Unicode emoji was a literal match.
+
+    final isPopular = _isPopularEmoji(candidate);
+    final isCustomEmoji = switch (candidate.emojiType) {
+      // The web implementation calls this condition `is_realm_emoji`,
+      // but its actual semantics is it's true for the Zulip extra emoji too.
+      // See `zulip_emoji` in web:src/emoji.ts .
+      ReactionType.realmEmoji || ReactionType.zulipExtraEmoji => true,
+      ReactionType.unicodeEmoji => false,
+    };
     return switch (matchQuality) {
       EmojiMatchQuality.exact  => 0,
-      EmojiMatchQuality.prefix => 1,
-      EmojiMatchQuality.other  => 2,
+      EmojiMatchQuality.prefix => isPopular ? 1 : isCustomEmoji ? 3 : 4,
+      // TODO word-boundary vs. not
+      EmojiMatchQuality.other  => isPopular ? 2 : isCustomEmoji ? 5 : 6,
     };
   }
 
   /// The number of possible values returned by [_rankResult].
-  static const _numResultRanks = 3;
+  static const _numResultRanks = 7;
+
+  static bool _isPopularEmoji(EmojiCandidate candidate) {
+    return candidate.emojiType == ReactionType.unicodeEmoji
+      && _popularEmojiSet.contains(candidate.emojiCode);
+  }
+
+  static final _popularEmojiSet = (() {
+    assert(zulipPopularEmojis.every((c) =>
+      c.emojiType == ReactionType.unicodeEmoji));
+    return Set.of(zulipPopularEmojis.map((c) => c.emojiCode));
+  })();
 }
 
 class EmojiAutocompleteQuery extends ComposeAutocompleteQuery {