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#include <u.h>
#include <libn.h>
// ----------------------------------------------------------------------------
// Internal structure
uint64
rol64(uint64 x, int k)
{
return (x << k) | (x >> (64 - k));
}
typedef struct Rng {
uint64 s[4];
} Rng;
uint64
xoshiro256ss(Rng *state)
{
uint64 *s = state->s;
uint64 result = rol64(s[1] * 5, 7) * 9;
uint64 t = s[1] << 17;
s[2] ^= s[0];
s[3] ^= s[1];
s[1] ^= s[2];
s[0] ^= s[3];
s[2] ^= t;
s[3] = rol64(s[3], 45);
return result;
}
typedef struct Mix
{
uint64 s;
} Mix;
uint64
splitmix64(struct Mix *state) {
uint64 result = state->s;
state->s = result + 0x9E3779B97f4A7C15;
result = (result ^ (result >> 30)) * 0xBF58476D1CE4E5B9;
result = (result ^ (result >> 27)) * 0x94D049BB133111EB;
return result ^ (result >> 31);
}
static Rng RNG;
// ----------------------------------------------------------------------------
// Exported functions
/* Initializes the global RNG */
error
rng·init(uint64 seed)
{
Mix smstate = {seed};
for (int i=0; i < 4; i++)
RNG.s[i] = splitmix64(&smstate);
return 0;
}
/* Returns a random float64 between 0 and 1 */
double
rng·random(void)
{
uint64 r = xoshiro256ss(&RNG);
return (double)r / (double)UINT64_MAX;
}
double
rng·exponential(double lambda)
{
double f;
f = rng·random();
return -log(1 - f)/lambda;
}
double
rng·normal(void)
{
double f;
f = rng·random();
}
/* Returns true or false on success of trial */
bool
rng·bernoulli(double f)
{
return rng·random() < f;
}
/* Returns a random integer between 0 and max
* TODO: Modulo throws off uniformity
*/
uint64
rng·randi(int max)
{
uint64 r = xoshiro256ss(&RNG);
return r % max;
}
uint64
rng·poisson(double mean)
{
uint64 n;
double c;
if(mean<10.0) {
for(n=0, c=rng·exponential(1.0); c<mean; ++n, c+=rng·exponential(1.0))
;
return n;
}
return 0;
}
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