Generates a random-enough double in the range [0, 1)

rand_double.c

#include <stdint.h>

static uint64_t simple_rnd_state = 0xcafef00dd15ea5e5u;

// returns a random 64-bit value
uint64_t simple_rng(void){
	uint64_t x = simple_rnd_state;
	simple_rnd_state = x * 6364136223846793005u;
	x ^= x >> 22;
	return x;
}

// initialize rng_state with a seed
void simple_rng_init(uint64_t seed){
	simple_rnd_state = 2 * seed + 1;
	(void)simple_rng();
}

// returns a random [0, 1) double
double rand_double(void){
	const uint64_t mask = 0x000FFFFFFFFFFFFFUL;
	const uint64_t sExp = 0x3FF0000000000000UL;
	union{
		uint64_t as_int;
		double as_double;
	}rnd = {
		.as_int = simple_rng()
	};
	rnd.as_int &= mask;
	rnd.as_int |= sExp;
	return rnd.as_double - 1;
}