xorshift rng

random.cpp

#include <stdint.h>
typedef uint64_t uint64;

// Random number generators
static inline uint64 _splitmix64(uint64* x)
{
	*x += UINT64_C(0x9E3779B97F4A7C15);
	uint64 z = *x;
	z = (z ^ (z >> 30)) * UINT64_C(0xBF58476D1CE4E5B9);
	z = (z ^ (z >> 27)) * UINT64_C(0x94D049BB133111EB);
	return z ^ (z >> 31);	
}

static inline uint64 _rotate_left(const uint64 t, int64 k)
{
	return (t << k) | (t >> (64 - k));
}

struct Random
{
	uint64 x, y;
};

#define Random_Max (UINT64_MAX)
uint64 next_random_uint64(Random* r)
{
	uint64 a = r->x;
	uint64 b = r->y;
	uint64 result = a + b;

	b ^= a;
	r->x = _rotate_left(a, 55) ^ b ^ (b << 14);
	r->y = _rotate_left(b, 36);
	return result;
}

void init_random(Random* r, uint64 seed) 
{
	_splitmix64(&seed);
	r->x = _splitmix64(&seed);
	r->y = _splitmix64(&seed);
	next_random_uint64(r);
}