lib/other/lrand.c

   1 /*  lrand(3)
   2  *
   3  *  Author: Terrence W. Holm          Nov. 1988
   4  *
   5  *
   6  *  A prime modulus multiplicative linear congruential
   7  *  generator (PMMLCG), or "Lehmer generator".
   8  *  Implementation directly derived from the article:
   9  *
  10  *      S. K. Park and K. W. Miller
  11  *      Random Number Generators: Good Ones are Hard to Find
  12  *      CACM vol 31, #10. Oct. 1988. pp 1192-1201.
  13  *
  14  *
  15  *  Using the following multiplier and modulus, we obtain a
  16  *  generator which:
  17  *
  18  *      1)  Has a full period: 1 to 2^31 - 2.
  19  *      2)  Is testably "random" (see the article).
  20  *      3)  Has a known implementation by E. L. Schrage.
  21  */
  22
  23 #include <lib.h>
  24
  25 _PROTOTYPE( long seed, (long lseed));
  26 _PROTOTYPE( long lrand, (void));
  27
  28 #define  A        16807L        /* A "good" multiplier    */
  29 #define  M   2147483647L        /* Modulus: 2^31 - 1      */
  30 #define  Q       127773L        /* M / A                  */
  31 #define  R         2836L        /* M % A                  */
  32
  33 PRIVATE long _lseed = 1L;
  34
  35 long seed(lseed)
  36 long lseed;
  37 {
  38   long previous_seed = _lseed;
  39
  40   _lseed = lseed;
  41
  42   return(previous_seed);
  43 }
  44
  45
  46 long lrand()
  47 {
  48   _lseed = A * (_lseed % Q) - R * (_lseed / Q);
  49
  50   if (_lseed < 0) _lseed += M;
  51
  52   return(_lseed);
  53 }