isWideGrapheme function

bool isWideGrapheme(String grapheme)

Returns true if the given grapheme cluster is a double-width (wide) character.

Optimization Note - Zero-Allocation Fast-Path:

1. For single BMP characters (length == 1), checks if the code unit is >= 0x1100 (the lowest wide character in Unicode is Hangul Jamo U+1100). If < 0x1100, exits in a single CPU comparison without any heap allocations.
2. For multi-code-unit graphemes, scans code units directly for Variation Selector-16 (0xFE0F), Combining Enclosing Keycap (0x20E3), or Zero-Width Joiner (0x200D), which force 2-column emoji presentation.
3. For SMP characters (surrogate pairs), decodes the first code point via bitwise math. This implementation achieves zero Runes/Iterator allocations in hot render loops.

Implementation

bool isWideGrapheme(String grapheme) {
  final len = grapheme.length;
  if (len == 0) return false;

  final cu0 = grapheme.codeUnitAt(0);
  // Fast-path: single BMP character
  if (len == 1) {
    return cu0 >= 0x1100 && isWideCodePoint(cu0);
  }

  // Extract first code point (decode surrogate pair if SMP)
  var firstRune = cu0;
  if (cu0 >= 0xD800 && cu0 <= 0xDBFF && len >= 2) {
    final cu1 = grapheme.codeUnitAt(1);
    if (cu1 >= 0xDC00 && cu1 <= 0xDFFF) {
      firstRune = 0x10000 + ((cu0 & 0x3FF) << 10) + (cu1 & 0x3FF);
    }
  }

  // Multi-code-unit cluster:
  // 1. Combining Enclosing Keycap (0x20E3): makes keycap sequences (e.g. 1️⃣, #️⃣) wide (2 cells).
  // 2. Variation Selector-16 (0xFE0F): forces emoji presentation (wide) for symbols/pictographs (firstRune >= 0x2000).
  // 3. Zero-Width Joiner (0x200D): joins emoji sequences (e.g. 👩‍💻, 👨‍👩‍👧‍👦, 🐱‍👤) into a single wide emoji.
  //    Only applies to emoji sequences where firstRune >= 0x2000 (not Latin letters with ZWJ).
  for (var i = 1; i < len; i++) {
    final cu = grapheme.codeUnitAt(i);
    if (cu == 0x20E3) return true;
    if ((cu == 0xFE0F || cu == 0x200D) && firstRune >= 0x2000) return true;
  }

  return isWideCodePoint(firstRune);
}