/* rng/ranf.c

 * 

 * Copyright (C) 1996, 1997, 1998, 1999, 2000, 2007 James Theiler, Brian Gough

 * 

 * This program is free software; you can redistribute it and/or modify

 * it under the terms of the GNU General Public License as published by

 * the Free Software Foundation; either version 3 of the License, or (at

 * your option) any later version.

 * 

 * This program is distributed in the hope that it will be useful, but

 * WITHOUT ANY WARRANTY; without even the implied warranty of

 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU

 * General Public License for more details.

 * 

 * You should have received a copy of the GNU General Public License

 * along with this program; if not, write to the Free Software

 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.

 */

#include <config.h>

#include <math.h>

#include <stdlib.h>

#include <gsl/gsl_rng.h>

#include <gsl/gsl_sys.h>

/* This is the CRAY RANF generator. The generator returns the

   upper 32 bits from each term of the sequence,

   x_{n+1} = (a x_n) mod m 

   using 48-bit unsigned arithmetic, with a = 0x2875A2E7B175 and m =

   2^48. The seed specifies the lower 32 bits of the initial value,

   x_1, with the lowest bit set (to prevent the seed taking an even

   value), and the upper 16 bits set to 0.

   There is a subtlety in the implementation of the seed. The initial

   state is put one step back by multiplying by the modular inverse of

   a mod m. This is done for compatibility with the original CRAY

   implementation.

   Note, you can only seed the generator with integers up to 2^32,

   while the CRAY uses wide integers which can cover all 2^48 states

   of the generator.

   The theoretical value of x_{10001} is 141091827447341.

   The period of this generator is 2^{46}. */

static inline void ranf_advance (void *vstate);

static unsigned long int ranf_get (void *vstate);

static double ranf_get_double (void *vstate);

static void ranf_set (void *state, unsigned long int s);

static const unsigned short int a0 = 0xB175 ;

static const unsigned short int a1 = 0xA2E7 ;

static const unsigned short int a2 = 0x2875 ;

typedef struct

  {

    unsigned short int x0, x1, x2;

  }

ranf_state_t;

static inline void

ranf_advance (void *vstate)

{

  ranf_state_t *state = (ranf_state_t *) vstate;

  const unsigned long int x0 = (unsigned long int) state->x0 ;

  const unsigned long int x1 = (unsigned long int) state->x1 ;

  const unsigned long int x2 = (unsigned long int) state->x2 ;

  unsigned long int r ;

  r = a0 * x0 ;

  state->x0 = (r & 0xFFFF) ;

  r >>= 16 ;

  r += a0 * x1 + a1 * x0 ;

  state->x1 = (r & 0xFFFF) ;

  r >>= 16 ;

  r += a0 * x2 + a1 * x1 + a2 * x0 ;

  state->x2 = (r & 0xFFFF) ;

}

static unsigned long int 

ranf_get (void *vstate)

{

  unsigned long int x1, x2;

  ranf_state_t *state = (ranf_state_t *) vstate;

  ranf_advance (state) ;  

  x1 = (unsigned long int) state->x1;

  x2 = (unsigned long int) state->x2;

  return (x2 << 16) + x1;

}

static double

ranf_get_double (void * vstate)

{

  ranf_state_t *state = (ranf_state_t *) vstate;

  ranf_advance (state) ; 

  return (ldexp((double) state->x2, -16)

          + ldexp((double) state->x1, -32) 

          + ldexp((double) state->x0, -48)) ;

}

static void

ranf_set (void *vstate, unsigned long int s)

{

  ranf_state_t *state = (ranf_state_t *) vstate;

  unsigned short int x0, x1, x2 ;

  unsigned long int r ;

  unsigned long int b0 = 0xD6DD ;

  unsigned long int b1 = 0xB894 ;

  unsigned long int b2 = 0x5CEE ;

  if (s == 0)  /* default seed */

    {

      x0 = 0x9CD1 ;

      x1 = 0x53FC ;

      x2 = 0x9482 ;

    }

  else 

    {

      x0 = (s | 1) & 0xFFFF ;

      x1 = s >> 16 & 0xFFFF ;

      x2 = 0 ;

    }

  r = b0 * x0 ;

  state->x0 = (r & 0xFFFF) ;

  r >>= 16 ;

  r += b0 * x1 + b1 * x0 ;

  state->x1 = (r & 0xFFFF) ;

  r >>= 16 ;

  r += b0 * x2 + b1 * x1 + b2 * x0 ;

  state->x2 = (r & 0xFFFF) ;

  return;

}

static const gsl_rng_type ranf_type =

{"ranf",                        /* name */

 0xffffffffUL,                  /* RAND_MAX */

 0,                             /* RAND_MIN */

 sizeof (ranf_state_t),

 &ranf_set,

 &ranf_get,

 &ranf_get_double

};

const gsl_rng_type *gsl_rng_ranf = &ranf_type;