docs: def number_of_bytes_to_modify(buf_len, fuzz_factor):
      """Calculate number of bytes to modify.
  
      :param buf_len: len of data buffer to fuzz.
      :param fuzz_factor: degree of fuzzing.
      :return: number of bytes to change.
      """
      return random.randrange(math.ceil((float(buf_len) / fuzz_factor))) + 1