decode method

String decode(Uint8List input)

Decodes a valid UTF-8 octet sequence in input directly to a String.

This is a faster equivalent of String.fromCharCodes(convert(...)) that emits UTF-16 code units in a single pass. Throws a FormatException if input is not a valid UTF-8 octet sequence.

Implementation

String decode(Uint8List input) {
  int len = input.length;
  if (len == 0) return '';

  int x, y, n = 0, p = 0;
  var units = Uint16List(len);
  while (p < len) {
    x = input[p];
    // Case: 1-byte ASCII run
    if (x <= 0x7F) {
      units[n++] = x;
      p++;
    }
    // Case: 2-bytes
    else if ((x & 0xE0) == 0xC0) {
      units[n++] = _decode2(input, len, p, x);
      p += 2;
    }
    // Case: 3-bytes
    else if ((x & 0xF0) == 0xE0) {
      units[n++] = _decode3(input, len, p, x);
      p += 3;
    }
    // Case: 4-bytes UTF-16 surrogate pair
    else if ((x & 0xF8) == 0xF0) {
      y = _decode4(input, len, p, x) - 0x10000;
      units[n++] = 0xD800 | (y >> 10);
      units[n++] = 0xDC00 | (y & 0x3FF);
      p += 4;
    }
    // Case: 5 or more bytes
    else {
      throw FormatException('Invalid code $x at $p');
    }
  }
  return String.fromCharCodes(units, 0, n);
}