implemented enlarged emoji rendering for chat messages containing up to three emojis

lib/utils/emoji_detection.dart

@@ -0,0 +1,86 @@
+import 'package:characters/characters.dart';
+
+/// Detects chat messages that consist solely of emoji so the UI can render
+/// them enlarged (à la WhatsApp/iMessage).
+///
+/// Returns the font size to use for such a message, or `null` when [text]
+/// contains any non-emoji, non-whitespace character (i.e. render normally).
+
+const int _zwj = 0x200D;
+const int _vs15 = 0xFE0E;
+const int _vs16 = 0xFE0F;
+const int _keycap = 0x20E3;
+
+/// Codepoints that only ever glue an emoji sequence together and never stand
+/// on their own — they don't disqualify a cluster from counting as emoji.
+bool _isModifier(int r) =>
+    r == _zwj ||
+    r == _vs15 ||
+    r == _vs16 ||
+    r == _keycap ||
+    (r >= 0x1F3FB && r <= 0x1F3FF) || // skin-tone modifiers
+    (r >= 0xE0020 && r <= 0xE007F); // tag characters (flag sequences)
+
+/// The digits, `#` and `*` that become emoji only when combined with the
+/// keycap combining mark.
+bool _isKeycapBase(int r) =>
+    r == 0x23 || r == 0x2A || (r >= 0x30 && r <= 0x39);
+
+bool _isEmojiRune(int r) {
+  // Bulk of modern emoji live in the astral pictograph block: emoticons,
+  // transport, supplemental symbols, extended-A, regional indicators, …
+  if (r >= 0x1F000 && r <= 0x1FAFF) return true;
+  // Scattered BMP emoji ranges (misc symbols, dingbats, arrows, stars, …).
+  if (r >= 0x2600 && r <= 0x27BF) return true;
+  if (r >= 0x2300 && r <= 0x23FF) return true;
+  if (r >= 0x2B00 && r <= 0x2BFF) return true;
+  if (r >= 0x2190 && r <= 0x21FF) return true;
+  if (r >= 0x25A0 && r <= 0x25FF) return true;
+  const singles = {
+    0x00A9, 0x00AE, 0x203C, 0x2049, 0x2122, 0x2139,
+    0x2934, 0x2935, 0x3030, 0x303D, 0x3297, 0x3299, 0x24C2,
+  };
+  return singles.contains(r);
+}
+
+bool _isEmojiCluster(String cluster) {
+  final runes = cluster.runes.toList(growable: false);
+  final hasKeycap = runes.contains(_keycap);
+  var hasEmoji = false;
+  for (final r in runes) {
+    if (_isModifier(r)) continue;
+    if (_isEmojiRune(r)) {
+      hasEmoji = true;
+      continue;
+    }
+    if (hasKeycap && _isKeycapBase(r)) {
+      hasEmoji = true;
+      continue;
+    }
+    return false; // a plain text character → not an emoji cluster
+  }
+  return hasEmoji;
+}
+
+/// Maximum number of emojis a message may contain to still be enlarged; longer
+/// emoji strings would turn into a wall of giant glyphs and render normally.
+const int _maxEnlargedEmojis = 3;
+
+/// Font size for an emoji-only [text] of at most [_maxEnlargedEmojis] emojis.
+/// `null` means the message is not emoji-only (or has too many emojis) and
+/// should render at the normal body size.
+double? standaloneEmojiFontSize(String text) {
+  final trimmed = text.trim();
+  if (trimmed.isEmpty) return null;
+
+  var count = 0;
+  for (final cluster in trimmed.characters) {
+    if (cluster.trim().isEmpty) continue; // whitespace between emojis
+    if (!_isEmojiCluster(cluster)) return null;
+    count++;
+    if (count > _maxEnlargedEmojis) return null;
+  }
+
+  if (count == 0) return null;
+  return 34;
+}