/* Fast pseudo-random bits based on clock_gettime.

   Copyright (C) 2019 Free Software Foundation, Inc.

   This file is part of the GNU C Library.

   The GNU C Library is free software; you can redistribute it and/or

   modify it under the terms of the GNU Lesser General Public

   License as published by the Free Software Foundation; either

   version 2.1 of the License, or (at your option) any later version.

   The GNU C Library is distributed in the hope that it will be useful,

   but WITHOUT ANY WARRANTY; without even the implied warranty of

   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU

   Lesser General Public License for more details.

   You should have received a copy of the GNU Lesser General Public

   License along with the GNU C Library; if not, see

   <http://www.gnu.org/licenses/>.  */

#ifndef _RANDOM_BITS_H

# define _RANDOM_BITS_H

#include <time.h>

#include <stdint.h>

/* Provides fast pseudo-random bits through clock_gettime.  It has unspecified

   starting time, nano-second accuracy, its randomness is significantly better

   than gettimeofday, and for mostly architectures it is implemented through

   vDSO instead of a syscall.  Since the source is a system clock, the upper

   bits will have less entropy. */

static inline uint32_t

random_bits (void)

{

  struct timespec tv;

  __clock_gettime (CLOCK_MONOTONIC, &tv;;

  /* Shuffle the lower bits to minimize the clock bias.  */

  uint32_t ret = tv.tv_nsec ^ tv.tv_sec;

  ret ^= (ret << 24) | (ret >> 8);

  return ret;

}

#endif