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/* drand.c
*
* Pseudorandom number generator
*
*
*
* SYNOPSIS:
*
* double y, drand();
*
* drand( &y );
*
*
*
* DESCRIPTION:
*
* Yields a random number 1.0 <= y < 2.0.
*
* 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.
*
* Versions invoked by the different arithmetic compile
* time options DEC, IBMPC, and MIEEE, produce
* approximately the same sequences, differing only in the
* least significant bits of the numbers. The UNK option
* implements the algorithm as recommended in the BYTE
* article. It may be used on all computers. However,
* the low order bits of a double precision number may
* not be adequately random, and may vary due to arithmetic
* implementation details on different computers.
*
* The other compile options generate an additional random
* integer that overwrites the low order bits of the double
* precision number. This reduces the period by a factor of
* two but tends to overcome the problems mentioned.
*
*/
#include "mconf.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;
static union {
double d;
unsigned short s[4];
} unkans;
/* This function implements the three
* congruential generators.
*/
static int ranwh()
{
int r, s;
/* 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;
/* The results are in static sx, sy, sz. */
return 0;
}
/* drand.c
*
* Random double precision floating point number between 1 and 2.
*
* C callable:
* drand( &x );
*/
int drand( a )
double *a;
{
unsigned short r;
#ifdef DEC
unsigned short s, t;
#endif
/* This algorithm of Wichmann and Hill computes a floating point
* result:
*/
ranwh();
unkans.d = sx/30269.0 + sy/30307.0 + sz/30323.0;
r = unkans.d;
unkans.d -= r;
unkans.d += 1.0;
/* if UNK option, do nothing further.
* Otherwise, make a random 16 bit integer
* to overwrite the least significant word
* of unkans.
*/
#ifdef UNK
/* do nothing */
#else
ranwh();
r = sx * sy + sz;
#endif
#ifdef DEC
/* To make the numbers as similar as possible
* in all arithmetics, the random integer has
* to be inserted 3 bits higher up in a DEC number.
* An alternative would be put it 3 bits lower down
* in all the other number types.
*/
s = unkans.s[2];
t = s & 07; /* save these bits to put in at the bottom */
s &= 0177770;
s |= (r >> 13) & 07;
unkans.s[2] = s;
t |= r << 3;
unkans.s[3] = t;
#endif
#ifdef IBMPC
unkans.s[0] = r;
#endif
#ifdef MIEEE
unkans.s[3] = r;
#endif
*a = unkans.d;
return 0;
}
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