if (digit < 0 || digit >= radix) {
      return null;
    }
    long accum = -digit;

    long cap = Long.MIN_VALUE / radix;

    while (index < string.length()) {
      digit = AsciiDigits.digit(string.charAt(index++));
      if (digit < 0 || digit >= radix || accum < cap) {
        return null;
      }
      accum *= radix;
      if (accum < Long.MIN_VALUE + digit) {
        return null;
      }