module/zfs/vdev_draid_rand.c

   1 /*
   2  * Xorshift Pseudo Random Number Generator based on work by David Blackman
   3  * and Sebastiano Vigna (vigna@acm.org).
   4  *
   5  *   "Further scramblings of Marsaglia's xorshift generators"
   6  *   http://vigna.di.unimi.it/ftp/papers/xorshiftplus.pdf
   7  *   http://prng.di.unimi.it/xoroshiro128plusplus.c
   8  *
   9  * To the extent possible under law, the author has dedicated all copyright
  10  * and related and neighboring rights to this software to the public domain
  11  * worldwide. This software is distributed without any warranty.
  12  *
  13  * See <http://creativecommons.org/publicdomain/zero/1.0/>.
  14  *
  15  * This is xoroshiro128++ 1.0, one of our all-purpose, rock-solid,
  16  * small-state generators. It is extremely (sub-ns) fast and it passes all
  17  * tests we are aware of, but its state space is large enough only for
  18  * mild parallelism.
  19  */
  20
  21 #include <sys/vdev_draid.h>
  22
  23 static inline uint64_t rotl(const uint64_t x, int k)
  24 {
  25         return (x << k) | (x >> (64 - k));
  26 }
  27
  28 uint64_t
  29 vdev_draid_rand(uint64_t *s)
  30 {
  31         const uint64_t s0 = s[0];
  32         uint64_t s1 = s[1];
  33         const uint64_t result = rotl(s0 + s1, 17) + s0;
  34
  35         s1 ^= s0;
  36         s[0] = rotl(s0, 49) ^ s1 ^ (s1 << 21); // a, b
  37         s[1] = rotl(s1, 28); // c
  38
  39         return (result);
  40 }