fastrand.c

#include "fastrand.h"

static pcg32_random_t pcg32_global = { 0x853c49e6748fea9bULL, 0xda3e39cb94b95bdbULL };

static inline uint32_t pcg32_random_r(pcg32_random_t* rng) {

    uint64_t oldstate = rng->state;
    rng->state = oldstate * 6364136223846793005ULL + rng->inc;
    uint32_t xorshifted = (uint32_t)(((oldstate >> 18u) ^ oldstate) >> 27u);
    uint32_t rot = oldstate >> 59u;
    return (xorshifted >> rot) | (xorshifted << ((-rot) & 31));

}

static inline uint32_t pcg32_random(void) {
    return pcg32_random_r(&pcg32_global);
}

void pcg32_init_state(uint32_t state) {
    pcg32_global.state = state;
}

static inline void pcg32_init_inc(uint32_t inc) {
    pcg32_global.inc = inc | 1;
}

uint32_t pcg32_random_bounded_divisionless(uint32_t range) {
    
    uint64_t random32bit, multiresult;
    uint32_t leftover;
    uint32_t threshold;
    random32bit =  pcg32_random();
    multiresult = random32bit * range;
    leftover = (uint32_t) multiresult;
    if(leftover < range ) {
        threshold = -range % range ;
        while (leftover < threshold) {
            random32bit =  pcg32_random();
            multiresult = random32bit * range;
            leftover = (uint32_t) multiresult;
        }
    }
    return multiresult >> 32; // [0, range)

}