/*
 * Basic support for random number distributions.
 * Discrete event simulations require reproducible random
 * numbers.  Using drand48_r/srand48_r, this header library
 * implements uniform and an exponential distribution.
 * 
 * @author gback, Spring '24
 */
#include <stdlib.h>
#include <math.h>

#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wunused-function"

/* uniformly dist. pseudo random floating point nrs over [0.0, 1.0) */
struct rnd {
    struct drand48_data state;
};

static void
rnd_init(struct rnd *r, long seedval)
{
    srand48_r(seedval, &r->state);
}

static double
rnd_fdraw(struct rnd *r)
{
    double result;
    drand48_r(&r->state, &result);
    return result;
}

/* uniformly dist. pseudo random integers over [low, high] */
struct urnd {
    struct rnd base;
    int low;
    int high;
    double range;
};

static void
urnd_init(struct urnd *r, int low, int high, long seedval)
{
    rnd_init(&r->base, seedval);
    r->low = low;
    r->high = high;
    r->range = (double)(high-low+1);
}

/* draw from [low, high] */
static int
urnd_draw(struct urnd *r)
{
    return r->low+(int)(r->range*rnd_fdraw(&r->base));
}

/* exponentially dist. pseudo random nrs 
   (float or int) */
struct ernd {
    struct rnd base;
    double lambda;      /* mean */
};

static void
ernd_init(struct ernd *r, double lambda, long seedval)
{
    rnd_init(&r->base, seedval);
    r->lambda = lambda;
}

static double
ernd_fdraw(struct ernd *r) /* draw a double */
{
    return (double)(-log((double)(rnd_fdraw(&r->base)))*r->lambda);
}

static int
ernd_draw(struct ernd *r)   /* draw an int */
{
    return (int)round(ernd_fdraw(r));
}
#pragma GCC diagnostic pop