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/* ldrand.c
*
* Pseudorandom number generator
*
*
*
* SYNOPSIS:
*
* double y;
* int ldrand();
*
* ldrand( &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.
*
* Versions invoked by the different arithmetic compile
* time options IBMPC, and MIEEE, produce the same sequences.
*
*/
/* Define MIEEE for 68k endianness, IBMPC for i386. */
/* #define MIEEE */
#define IBMPC 1
/* 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 {
long double d;
unsigned short s[8];
} unkans;
/* This function implements the three
* congruential generators.
*/
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;
}
/* ldrand.c
*
* Random double precision floating point number between 1 and 2.
*
* C callable:
* drand( &x );
*/
int ldrand( a )
long double *a;
{
unsigned short r;
/* This algorithm of Wichmann and Hill computes a floating point
* result:
*/
ranwh();
unkans.d = sx/30269.0L + sy/30307.0L + sz/30323.0L;
r = unkans.d;
unkans.d -= r;
unkans.d += 1.0L;
if( sizeof(long double) == 16 )
{
#ifdef MIEEE
ranwh();
r = sx * sy + sz;
unkans.s[7] = r;
ranwh();
r = sx * sy + sz;
unkans.s[6] = r;
ranwh();
r = sx * sy + sz;
unkans.s[5] = r;
ranwh();
r = sx * sy + sz;
unkans.s[4] = r;
ranwh();
r = sx * sy + sz;
unkans.s[3] = r;
#endif
#ifdef IBMPC
ranwh();
r = sx * sy + sz;
unkans.s[0] = r;
ranwh();
r = sx * sy + sz;
unkans.s[1] = r;
ranwh();
r = sx * sy + sz;
unkans.s[2] = r;
ranwh();
r = sx * sy + sz;
unkans.s[3] = r;
ranwh();
r = sx * sy + sz;
unkans.s[4] = r;
#endif
}
else
{
#ifdef MIEEE
ranwh();
r = sx * sy + sz;
unkans.s[5] = r;
ranwh();
r = sx * sy + sz;
unkans.s[4] = r;
#endif
#ifdef IBMPC
ranwh();
r = sx * sy + sz;
unkans.s[0] = r;
ranwh();
r = sx * sy + sz;
unkans.s[1] = r;
#endif
}
*a = unkans.d;
return 0;
}
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