private static String unsignedIntToString(int x, int radix)
    {
        long asLong = x & INT_MASK;
        return Long.toString(asLong, radix);
    }
Returns a string representation of {@code x} for the given radix, where {@code x} is treated as unsigned.

@param x
           the value to convert to a string.
@param radix
           the radix to use while working with {@code x}
@throws IllegalArgumentException
            if {@code radix} is not between {@link Character#MIN_RADIX} and {@link Character#MAX_RADIX}.
protostuff-api/src/main/java/io/protostuff/UnsignedNumberUtil.java