Minix1.5/lib/other/lrand.c

#include <lib.h>
/*  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.
 */


#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);
}