#include <config.h>
#include <stdlib.h>
#include <gsl_errno.h>
#include <gsl_rng.h>

/* This is an implementation of the algorithm used in Numerical
   Recipe's ran0 generator. It is the same as MINSTD with an XOR mask
   of 123459876 on the seed.

   The period of this generator is 2^31.  

   Note, if you choose a seed of 123459876 it would give a degenerate
   series 0,0,0,0, ...  I've made that into an error. */

inline unsigned long int ran0_get (void *vstate);
double ran0_get_double (void *vstate);
void ran0_set (void *state, unsigned long int s);

static const long int m = 2147483647, a = 16807, q = 127773, r = 2836;
static const unsigned long int mask = 123459876;

typedef struct
  {
    unsigned long int x;
  }
ran0_state_t;

inline unsigned long int
ran0_get (void *vstate)
{
  ran0_state_t *state = (ran0_state_t *) vstate;

  const unsigned long int x = state->x;

  const long int h = x / q;
  const long int t = a * (x - h * q) - h * r;

  if (t < 0)
    {
      state->x = t + m;
    }
  else
    {
      state->x = t;
    }

  return state->x;
}

double
ran0_get_double (void *vstate)
{
  return ran0_get (vstate) / 2147483647.0 ;
}

void
ran0_set (void *vstate, unsigned long int s)
{
  ran0_state_t *state = (ran0_state_t *) vstate;

  if (s == mask)
    {
      GSL_ERROR_RETURN_NOTHING ("ran0 should not use seed == mask", 
				GSL_EINVAL);
    }

  state->x = s ^ mask;

  return;
}

static const gsl_rng_type ran0_type =
{"ran0",			/* name */
 2147483646,			/* RAND_MAX */
 1,				/* RAND_MIN */
 sizeof (ran0_state_t),
 &ran0_set,
 &ran0_get,
 &ran0_get_double};

const gsl_rng_type *gsl_rng_ran0 = &ran0_type;