code: private static String digits(long value, int digits) {
  
      long hi = 1L << (digits * 4);
      return Long.toHexString(hi | (value & (hi - 1))).substring(1);
    }
docs: Returns the given {@code value} represented by the specified number of hex {@code digits}.
  
  @param value is the number to format.
  @param digits are the number of digits requested.
  @return the given {@code value} as hex {@link String} with the given number of digits.