quickrnd.c

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#include <stdint.h>
#include <assert.h>
 
//  2^64 / phi:  Maximally irrational, with no obvious pattern of zeros and ones
static const uint64_t GOLDEN_RATIO = 0x9e3779b97f4a7c15ULL;
 
//  2^-52
static const double two_powm52  = 0x1.0p-52;
 
//  2^-53
static const double two_powm53 = 0x1.0p-53;
 
double u64_to_open01(const uint64_t x) {
    return (double)(x >> 12) * two_powm52 + two_powm53;
}
 
double i64_to_open01(const int64_t x) {
    // Modulo 2^64
    const uint64_t ux = (uint64_t)x;
    return u64_to_open01(ux);
}
 
uint64_t splitmix64_mixer(const uint64_t x) {
    uint64_t z = x + 0x9e3779b97f4a7c15ULL;
    z = (z ^ (z >> 30)) * 0xbf58476d1ce4e5b9ULL;
    z = (z ^ (z >> 27)) * 0x94d049bb133111ebULL;
    return z ^ (z >> 31);
}
 
uint64_t splitmix64_mixer_2pow63(const uint64_t x){
    // force top bit to 0 (positive number)
    return splitmix64_mixer(x) & 0x7FFFFFFFFFFFFFFFULL;
}
 
uint64_t hash_combine_ids(const uint64_t *ids, const uint32_t n) {
    assert(n > 0);
    uint64_t s = ids[0];
    for(uint32_t i=1; i<n; i++) {
        s = s ^ (ids[i] + GOLDEN_RATIO + (s << 6) + (s >> 2));
    }
    // force top bit to 0 (positive number)
    return s & 0x7FFFFFFFFFFFFFFFULL;
}