sirandom.c

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#include "sirandom.h"

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


int siSeed = 1;

int siRandNext(int r)
{
  r = A * (r % Q) - R * (r / Q);

  if ( r < 0 )
    r += M;

  return( r );
}

int siRand()
{
  siSeed=siRandNext(siSeed);
  return siSeed;
}
int siRandPlus1(int r)
{
  return r+1;
}