encode static method

List<int> encode(String s)

Implementation

static List<int> encode(String s) {
  final int length = s.length;

  int utf8Length = 0;

  for (int i = 0; i < length; i++) {
    int cu = s.codeUnitAt(i);

    if (cu >= 0xD800 && cu <= 0xDBFF) {
      if (i + 1 < length) {
        int c2 = s.codeUnitAt(i + 1);
        if (c2 >= 0xDC00 && c2 <= 0xDFFF) {
          cu = 0x10000 + ((cu - 0xD800) << 10) + (c2 - 0xDC00);
          i++;
        } else {
          cu = 0xFFFD; // replacement char
        }
      } else {
        cu = 0xFFFD; // lone high surrogate
      }
    } else if (cu >= 0xDC00 && cu <= 0xDFFF) {
      cu = 0xFFFD; // lone low surrogate
    }

    // Count UTF-8 bytes
    if (cu <= 0x7F) {
      utf8Length += 1;
    } else if (cu <= 0x7FF) {
      utf8Length += 2;
    } else if (cu <= 0xFFFF) {
      utf8Length += 3;
    } else {
      utf8Length += 4;
    }
  }

  // ---------- SECOND PASS: write bytes ----------
  final out = List<int>.filled(utf8Length, 0);
  int o = 0;

  for (int i = 0; i < length; i++) {
    int cu = s.codeUnitAt(i);

    // Same surrogate logic (MUST EXACTLY match first pass)
    if (cu >= 0xD800 && cu <= 0xDBFF) {
      if (i + 1 < length) {
        int c2 = s.codeUnitAt(i + 1);
        if (c2 >= 0xDC00 && c2 <= 0xDFFF) {
          cu = 0x10000 + ((cu - 0xD800) << 10) + (c2 - 0xDC00);
          i++;
        } else {
          cu = 0xFFFD;
        }
      } else {
        cu = 0xFFFD;
      }
    } else if (cu >= 0xDC00 && cu <= 0xDFFF) {
      cu = 0xFFFD;
    }

    // Encode
    if (cu <= 0x7F) {
      out[o++] = cu;
    } else if (cu <= 0x7FF) {
      out[o++] = 0xC0 | (cu >> 6);
      out[o++] = 0x80 | (cu & 0x3F);
    } else if (cu <= 0xFFFF) {
      out[o++] = 0xE0 | (cu >> 12);
      out[o++] = 0x80 | ((cu >> 6) & 0x3F);
      out[o++] = 0x80 | (cu & 0x3F);
    } else {
      out[o++] = 0xF0 | (cu >> 18);
      out[o++] = 0x80 | ((cu >> 12) & 0x3F);
      out[o++] = 0x80 | ((cu >> 6) & 0x3F);
      out[o++] = 0x80 | (cu & 0x3F);
    }
  }

  return out;
}