random.c

/* random.c */

#include <time.h>
#include "random.h"

/*
	urand(), urand0() return uniformly distributed unsigned random ints
		all available bits are random, e.g. 32 bits on many platforms
	usrand(seed) initializes the generator
		a seed of 0 uses the current time as seed
	
	urand0() is the additive number generator "Program A" on p.27 of Knuth v.2
	urand() is urand0() randomized by shuffling, "Algorithm B" on p.32 of Knuth v.2
	urand0() is one of the fastest known generators that passes randomness tests
	urand() is somewhat slower, and is presumably better
*/

static unsigned rand_x[56], rand_y[256], rand_z;
static int rand_j, rand_k;

void usrand (unsigned seed)
{
	int j;

	rand_x[1] = 1;
	if (seed) rand_x[2] = seed;
	else rand_x[2] = time (NULL);
	for (j=3; j<56; ++j) rand_x[j] = rand_x[j-1] + rand_x[j-2];
	
	rand_j = 24;
	rand_k = 55;
	for (j=255; j>=0; --j) urand0 ();
	for (j=255; j>=0; --j) rand_y[j] = urand0 ();
	rand_z = urand0 ();
}

unsigned urand0 (void)
{
	if (--rand_j == 0) rand_j = 55;
	if (--rand_k == 0) rand_k = 55;
	return rand_x[rand_k] += rand_x[rand_j];
}

unsigned urand (void)
{
	int j;
	
	j =  rand_z >> 24;
	rand_z = rand_y[j];
	if (--rand_j == 0) rand_j = 55;
	if (--rand_k == 0) rand_k = 55;
	rand_y[j] = rand_x[rand_k] += rand_x[rand_j];
	return rand_z;
}

int randint(int upto)
//gives a random integer uniformly in [0..upto-1]
{
//	return((float)upto*(float)rand())/(RAND_MAX+1.0);
	return(((double)upto*(double)urand())/((double)MAX_URAND+1.0));
}

double randreal()
//gives a random integer uniformly in (0,1)
{
//	return (rand()/(RAND_MAX+1.0));
	return (urand()/(double)((double)MAX_URAND+1.0));	
}