source: trunk/minix/lib/other/lrand.c@ 20

/*  lrand(3)
 *
 *  Author: Terrence W. Holm          Nov. 1988
 *
 *
 *  A prime modulus multiplicative linear congruential
 *  generator (PMMLCG), or "Lehmer generator".
 *  Implementation directly derived from the article:
 *
 *      S. K. Park and K. W. Miller
 *      Random Number Generators: Good Ones are Hard to Find
 *      CACM vol 31, #10. Oct. 1988. pp 1192-1201.
 *
 *
 *  Using the following multiplier and modulus, we obtain a
 *  generator which:
 *
 *      1)  Has a full period: 1 to 2^31 - 2.
 *      2)  Is testably "random" (see the article).
 *      3)  Has a known implementation by E. L. Schrage.
 */

#include <lib.h>

_PROTOTYPE( long seed, (long lseed));
_PROTOTYPE( long lrand, (void));

#define  A        16807L        /* A "good" multiplier    */
#define  M   2147483647L        /* Modulus: 2^31 - 1      */
#define  Q       127773L        /* M / A                  */
#define  R         2836L        /* M % A                  */

PRIVATE long _lseed = 1L;

long seed(lseed)
long lseed;
{
  long previous_seed = _lseed;

  _lseed = lseed;

  return(previous_seed);
}


long lrand()
{
  _lseed = A * (_lseed % Q) - R * (_lseed / Q);

  if (_lseed < 0) _lseed += M;

  return(_lseed);
}