// https://prng.di.unimi.it/xoroshiro64starstar.c

/*  Written in 2018 by David Blackman and Sebastiano Vigna (vigna@acm.org)

To the extent possible under law, the author has dedicated all copyright
and related and neighboring rights to this software to the public domain
worldwide.

Permission to use, copy, modify, and/or distribute this software for any
purpose with or without fee is hereby granted.

THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR
IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE. */

#include <stdint.h>

/* This is xoroshiro64** 1.0, our 32-bit all-purpose, rock-solid,
   small-state generator. It is extremely fast and it passes all tests we
   are aware of, but its state space is not large enough for any parallel
   application.

   For generating just single-precision (i.e., 32-bit) floating-point
   numbers, xoroshiro64* is even faster.

   The state must be seeded so that it is not everywhere zero. */


static inline uint32_t rotl(const uint32_t x, int k) {
	return (x << k) | (x >> (32 - k));
}


static uint32_t s[2];

uint32_t next(void) {
	const uint32_t s0 = s[0];
	uint32_t s1 = s[1];
	const uint32_t result = rotl(s0 * 0x9E3779BB, 5) * 5;

	s1 ^= s0;
	s[0] = rotl(s0, 26) ^ s1 ^ (s1 << 9); // a, b
	s[1] = rotl(s1, 13); // c

	return result;
}
