/* ************************************************************************
   *   File: random.c                                      Part of CircleMUD *
   *  Usage: pseudo-random number generator                                  *
   ************************************************************************ */

/*
 * I am bothered by the non-portablility of 'rand' and 'random' -- rand
 * is ANSI C, but on some systems such as Suns, rand has seriously tragic
 * spectral properties (the low bit alternates between 0 and 1!).  random
 * is better but isn't supported by all systems.  So, in my quest for Ultimate
 * CircleMUD Portability, I decided to include this code for a simple but
 * relatively effective random number generator.  It's not the best RNG code
 * around, but I like it because it's very short and simple, and for our
 * purposes it's "random enough".
 *               --Jeremy Elson  2/23/95
 *
 * Now that we're using GNU's autoconf, I've coded Circle to always use
 * random(), and automatically link in this object file if random() isn't
 * supported on the target system.  -JE 2/3/96
 *
 * Well, despite autoconf we're back to using this random all the
 * time.  Oh well, there's no harm in changing my mind on this one
 * from release to release...  -JE 10/28/97
 */

/***************************************************************************/

/*

 * This program is public domain and was written by William S. England
 * (Oct 1988).  It is based on an article by:
 *
 * Stephen K. Park and Keith W. Miller. RANDOM NUMBER GENERATORS:
 * GOOD ONES ARE HARD TO FIND. Communications of the ACM,
 * New York, NY.,October 1988 p.1192

 The following is a portable c program for generating random numbers.
 The modulus and multipilier have been extensively tested and should
 not be changed except by someone who is a professional Lehmer generator
 writer.  THIS GENERATOR REPRESENTS THE MINIMUM STANDARD AGAINST WHICH
 OTHER GENERATORS SHOULD BE JUDGED. ("Quote from the referenced article's
 authors. WSE" )
 */

#define	m  (unsigned long)2147483647
#define	q  (unsigned long)127773

#define	a (unsigned int)16807
#define	r (unsigned int)2836

/*
   ** F(z)      = (az)%m
   **   = az-m(az/m)
   **
   ** F(z)  = G(z)+mT(z)
   ** G(z)  = a(z%q)- r(z/q)
   ** T(z)  = (z/q) - (az/m)
   **
   ** F(z)  = a(z%q)- rz/q+ m((z/q) - a(z/m))
   **    = a(z%q)- rz/q+ m(z/q) - az
 */

static unsigned long seed;

void 
circle_srandom (unsigned long initial_seed)
{
  seed = initial_seed;
}


unsigned long 
circle_random (void)
{
  register int lo, hi, test;

  hi = seed / q;
  lo = seed % q;

  test = a * lo - r * hi;

  if (test > 0)
    seed = test;
  else
    seed = test + m;

  return seed;
}