wip prioritize prefix among popular

lib/model/emoji.dart

@@ -374,22 +374,38 @@ class EmojiAutocompleteView extends AutocompleteView<EmojiAutocompleteQuery, Emo
     return match == null ? null : EmojiAutocompleteResult(match, candidate);
   }
 
-  static const _numResultRanks = 6;
-
+  static const _numResultRanks = 7;
+
+  // Compare sort_emojis in Zulip web:shared/src/typeahead.ts .
+  // Differences in behavior include:
+  //  * Web ranks each name of a Unicode emoji separately.
+  //  * Among popular emoji with non-exact matches,
+  //    web doesn't prioritize by quality of match; we do.
+  //  * OTOH web only counts an emoji as "popular" for ranking if the query
+  //    is a prefix of a word in the emoji's name or alias.
+  //    TODO reconcile that; requiring word boundary may be helpful;
+  //      rejecting query that spans words seems unhelpful.
+  //  * TODO web ranks matches starting at a word boundary ahead of other
+  //      non-prefix matches; we don't yet.
+  //  * Web distinguishes prefix matches by case-sensitive vs. not; we don't.
+  //    That doesn't seem helpful 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.
+  //    TODO(web): that's a bug
   static int _rankResult(EmojiAutocompleteResult result) {
     if (result.matchQuality == EmojiMatchQuality.exact) {
       return 0;
     }
     final candidate = result.candidate;
-    if (_isPopularEmoji(candidate)) {
-      return 1;
-    }
+    final isPopular = _isPopularEmoji(candidate);
     final isRealmEmoji = (candidate.emojiType == ReactionType.realmEmoji);
     return switch (result.matchQuality) {
       EmojiMatchQuality.exact    => throw Error(), // handled above
-      EmojiMatchQuality.prefix   => isRealmEmoji ? 2 : 3,
+      EmojiMatchQuality.prefix   => isPopular ? 1 : isRealmEmoji ? 3 : 4,
       // TODO word-boundary vs. not
-      EmojiMatchQuality.internal => isRealmEmoji ? 4 : 5,
+      EmojiMatchQuality.internal => isPopular ? 2 : isRealmEmoji ? 5 : 6,
     };
   }