#include <stdio.h>
/* Generating pseudo random numbers by a modulo technique:
   basic idea: new_seed = (a * seed + r) % m 
   This program lets you experiment with different values of a, r and m
   in order to investigate quality of the pseudo-random sequence 
   Usage:
   % pseudorandomexpt          --- to accept all defaults
   % pseudorandomexpt M        --- to accept all defaults except m
   % pseudorandomexpt M A      --- to accept all defaults except m,a
   % pseudorandomexpt M A R    --- to accept all defaults except m,a,r
   % pseudorandomexpt M A R S  --- to accept no defaults 
   M A R S are new values for m a r s

   Try it with the defaults.  See the problem?  Period is ?
   Try it with pseudorandomexpt 101 19 51.  Is it better?  Is it good?
*/

long m = 100; /* is this good? */
long a = 19; /* is this good? */
long r = 51; /* how about this? */

long next(s) 
  long s;
{ 
  return (a * s + r) % m;
}

main(argc, argv)
     int argc;
     char *argv[];
{
  long s;
  int i;
  char continu;
  s = 25;
  if (argc > 1) m = (long) atoi(argv[1]);
  if (argc > 2) a = (long) atoi(argv[2]);
  if (argc > 3) r = (long) atoi(argv[3]);
  if (argc > 4) s = (long) atoi(argv[4]);
  continu = 'g';
  while (continu != 'q') {
    for (i=0; i<20; i++) {
      printf("%d ", s);
      s  = next(s);
    }
    printf("\n");
    printf("Hit q to quit, ENTER to continue > ");
    continu = getchar();
  }
  getchar(); /* eat the CR/LF */
}