/* Jenkins Small Fast -- "A small noncryptographic PRNG"
 * http://burtleburtle.net/bob/rand/smallprng.html
 * https://www.pcg-random.org/posts/some-prng-implementations.html */

#include <stdint.h>

#include "jsf.h"

#define rot32(x, k) (((x) << (k)) | ((x) >> (32 - (k))))

struct ranctx {
	uint32_t a;
	uint32_t b;
	uint32_t c;
	uint32_t d;
};

static struct ranctx ctx;

static uint32_t ranval(void);

uint32_t
jsf_below(uint32_t upper_bound)
{
	return ((uint64_t) ranval() * (uint64_t) upper_bound) >> 32;
}

int
jsf_coinflip(void)
{
	return ranval() & 1;
}

void
jsf_init(uint32_t seed)
{
	uint32_t i;
	ctx.a = 0xf1ea5eed, ctx.b = ctx.c = ctx.d = seed;
	for (i = 0; i < 20; ++i)
		ranval();
}

inline static uint32_t
ranval(void)
{
	uint32_t e = ctx.a - rot32(ctx.b, 27);
	ctx.a      = ctx.b ^ rot32(ctx.c, 17);
	ctx.b      = ctx.c + ctx.d;
	ctx.c      = ctx.d + e;
	ctx.d      = e + ctx.a;
	return ctx.d;
}