/* source/rnd.c: random number generator

   Copyright (c) 1989-92 James E. Wilson

   This software may be copied and distributed for educational, research, and
   not for profit purposes provided that this copyright and statement are
   included in all such copies. */

#include "config.h"
#include "constant.h"
#include "types.h"

/* Define this to compile as a standalone test */
/* #define TEST_RNG */

/* This alg uses a prime modulus multiplicative congruential generator
   (PMMLCG), also known as a Lehmer Grammer, which satisfies the following
   properties

   (i)	 modulus: m - a large prime integer
   (ii)	 multiplier: a - an integer in the range 2, 3, ..., m - 1
   (iii) z[n+1] = f(z[n]), for n = 1, 2, ...
   (iv)	 f(z) = az mod m
   (v)	 u[n] = z[n] / m, for n = 1, 2, ...

   The sequence of z's must be initialized by choosing an initial seed
   z[1] from the range 1, 2, ..., m - 1.  The sequence of z's is a pseudo-
   random sequence drawn without replacement from the set 1, 2, ..., m - 1.
   The u's form a psuedo-random sequence of real numbers between (but not
   including) 0 and 1.

   Schrage's method is used to compute the sequence of z's.
   Let m = aq + r, where q = m div a, and r = m mod a.
   Then f(z) = az mod m = az - m * (az div m) =
	     = gamma(z) + m * delta(z)
   Where gamma(z) = a(z mod q) - r(z div q)
   and	 delta(z) = (z div q) - (az div m)

   If r < q, then for all z in 1, 2, ..., m - 1:
   (1) delta(z) is either 0 or 1
   (2) both a(z mod q) and r(z div q) are in 0, 1, ..., m - 1
   (3) absolute value of gamma(z) <= m - 1
   (4) delta(z) = 1 iff gamma(z) < 0

   Hence each value of z can be computed exactly without overflow as long
   as m can be represented as an integer.
 */

/* a good random number generator, correct on any machine with 32 bit
   integers, this algorithm is from:

Stephen K. Park and Keith W. Miller, "Random Number Generators:
	Good ones are hard to find", Communications of the ACM, October 1988,
	vol 31, number 10, pp. 1192-1201.

   If this algorithm is implemented correctly, then if z[1] = 1, then
   z[10001] will equal 1043618065

   Has a full period of 2^31 - 1.
   Returns integers in the range 1 to 2^31-1.
 */

#define RNG_M 2147483647L  /* m = 2^31 - 1 */
#define RNG_A 16807L
#define RNG_Q 127773L	   /* m div a */
#define RNG_R 2836L	   /* m mod a */

/* 32 bit seed */
static int32u rnd_seed;

int32u get_rnd_seed ()
{
  return rnd_seed;
}

void set_rnd_seed (seedval)
int32u seedval;
{
  /* set seed to value between 1 and m-1 */

  rnd_seed = (seedval % (RNG_M - 1)) + 1;
}

/* returns a pseudo-random number from set 1, 2, ..., RNG_M - 1 */
int32 rnd ()
{
  register long low, high, test;

  high = rnd_seed / RNG_Q;
  low = rnd_seed % RNG_Q;
  test = RNG_A * low - RNG_R * high;
  if (test > 0)
    rnd_seed = test;
  else
    rnd_seed = test + RNG_M;
  return rnd_seed;
}

#ifdef TEST_RNG

main ()
{
  long i, random;

  set_rnd_seed (0L);

  for (i = 1; i < 10000; i++)
    (void) rnd ();

  random = rnd ();
  printf ("z[10001] = %ld, should be 1043618065\n", random);
  if (random == 1043618065L)
    printf ("success!!!\n");
}

#endif
