String _convert()

in lib/src/percent/encoder.dart [58:107]
31 lines of code
15 McCabe index (conditional complexity)

String _convert(List<int> bytes, int start, int end) {
  var buffer = StringBuffer();

  // A bitwise OR of all bytes in [bytes]. This allows us to check for
  // out-of-range bytes without adding more branches than necessary to the
  // core loop.
  var byteOr = 0;
  for (var i = start; i < end; i++) {
    var byte = bytes[i];
    byteOr |= byte;

    // If the byte is an uppercase letter, convert it to lowercase to check if
    // it's unreserved. This works because uppercase letters in ASCII are
    // exactly `0b100000 = 0x20` less than lowercase letters, so if we ensure
    // that that bit is 1 we ensure that the letter is lowercase.
    var letter = 0x20 | byte;
    if ((letter >= $a && letter <= $z) ||
        (byte >= $0 && byte <= $9) ||
        byte == $dash ||
        byte == $dot ||
        byte == $underscore ||
        byte == $tilde) {
      // Unreserved characters are safe to write as-is.
      buffer.writeCharCode(byte);
      continue;
    }

    buffer.writeCharCode($percent);

    // The bitwise arithmetic here is equivalent to `byte ~/ 16` and `byte % 16`
    // for valid byte values, but is easier for dart2js to optimize given that
    // it can't prove that [byte] will always be positive.
    buffer.writeCharCode(_codeUnitForDigit((byte & 0xF0) >> 4));
    buffer.writeCharCode(_codeUnitForDigit(byte & 0x0F));
  }

  if (byteOr >= 0 && byteOr <= 255) return buffer.toString();

  // If there was an invalid byte, find it and throw an exception.
  for (var i = start; i < end; i++) {
    var byte = bytes[i];
    if (byte >= 0 && byte <= 0xff) continue;
    throw FormatException(
        "Invalid byte ${byte < 0 ? "-" : ""}0x${byte.abs().toRadixString(16)}.",
        bytes,
        i);
  }

  throw 'unreachable';
}