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/* lrand.c
*
* Pseudorandom number generator
*
*
*
* SYNOPSIS:
*
* long y, drand();
*
* drand( &y );
*
*
*
* DESCRIPTION:
*
* Yields a long integer random number.
*
* The three-generator congruential algorithm by Brian
* Wichmann and David Hill (BYTE magazine, March, 1987,
* pp 127-8) is used. The period, given by them, is
* 6953607871644.
*
*
*/
#include <math.h>
/* Three-generator random number algorithm
* of Brian Wichmann and David Hill
* BYTE magazine, March, 1987 pp 127-8
*
* The period, given by them, is (p-1)(q-1)(r-1)/4 = 6.95e12.
*/
static int sx = 1;
static int sy = 10000;
static int sz = 3000;
/* This function implements the three
* congruential generators.
*/
long lrand()
{
int r, s;
unsigned long ans;
/*
if( arg )
{
sx = 1;
sy = 10000;
sz = 3000;
}
*/
/* sx = sx * 171 mod 30269 */
r = sx/177;
s = sx - 177 * r;
sx = 171 * s - 2 * r;
if( sx < 0 )
sx += 30269;
/* sy = sy * 172 mod 30307 */
r = sy/176;
s = sy - 176 * r;
sy = 172 * s - 35 * r;
if( sy < 0 )
sy += 30307;
/* sz = 170 * sz mod 30323 */
r = sz/178;
s = sz - 178 * r;
sz = 170 * s - 63 * r;
if( sz < 0 )
sz += 30323;
ans = sx * sy * sz;
return(ans);
}
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