/* rng/taus.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 <stdlib.h>

#include <gsl/gsl_rng.h>

/* This is a maximally equidistributed combined Tausworthe

   generator. The sequence is,

   x_n = (s1_n ^ s2_n ^ s3_n) 

   s1_{n+1} = (((s1_n & 4294967294) <<12) ^ (((s1_n <<13) ^ s1_n) >>19))

   s2_{n+1} = (((s2_n & 4294967288) << 4) ^ (((s2_n << 2) ^ s2_n) >>25))

   s3_{n+1} = (((s3_n & 4294967280) <<17) ^ (((s3_n << 3) ^ s3_n) >>11))

   computed modulo 2^32. In the three formulas above '^' means

   exclusive-or (C-notation), not exponentiation. Note that the

   algorithm relies on the properties of 32-bit unsigned integers (it

   is formally defined on bit-vectors of length 32). I have added a

   bitmask to make it work on 64 bit machines.

   We initialize the generator with s1_1 .. s3_1 = s_n MOD m, where

   s_n = (69069 * s_{n-1}) mod 2^32, and s_0 = s is the user-supplied

   seed.

   The theoretical value of x_{10007} is 2733957125. The subscript

   10007 means (1) seed the generator with s=1 (2) do six warm-up

   iterations, (3) then do 10000 actual iterations.

   The period of this generator is about 2^88.

   From: P. L'Ecuyer, "Maximally Equidistributed Combined Tausworthe

   Generators", Mathematics of Computation, 65, 213 (1996), 203--213.

   This is available on the net from L'Ecuyer's home page,

   http://www.iro.umontreal.ca/~lecuyer/myftp/papers/tausme.ps

   ftp://ftp.iro.umontreal.ca/pub/simulation/lecuyer/papers/tausme.ps 

   Update: April 2002

   There is an erratum in the paper "Tables of Maximally

   Equidistributed Combined LFSR Generators", Mathematics of

   Computation, 68, 225 (1999), 261--269:

   http://www.iro.umontreal.ca/~lecuyer/myftp/papers/tausme2.ps

        ... the k_j most significant bits of z_j must be non-

        zero, for each j. (Note: this restriction also applies to the 

        computer code given in [4], but was mistakenly not mentioned in

        that paper.)

   This affects the seeding procedure by imposing the requirement

   s1 > 1, s2 > 7, s3 > 15.

   The generator taus2 has been added to satisfy this requirement.

   The original taus generator is unchanged.

   Update: November 2002

   There was a bug in the correction to the seeding procedure for s2.

   It affected the following seeds 254679140 1264751179 1519430319

   2274823218 2529502358 3284895257 3539574397 (s2 < 8).

*/

static inline unsigned long int taus_get (void *vstate);

static double taus_get_double (void *vstate);

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

typedef struct

  {

    unsigned long int s1, s2, s3;

  }

taus_state_t;

static inline unsigned long

taus_get (void *vstate)

{

  taus_state_t *state = (taus_state_t *) vstate;

#define MASK 0xffffffffUL

#define TAUSWORTHE(s,a,b,c,d) (((s &c) <<d) &MASK) ^ ((((s <<a) &MASK)^s) >>b)

  state->s1 = TAUSWORTHE (state->s1, 13, 19, 4294967294UL, 12);

  state->s2 = TAUSWORTHE (state->s2, 2, 25, 4294967288UL, 4);

  state->s3 = TAUSWORTHE (state->s3, 3, 11, 4294967280UL, 17);

  return (state->s1 ^ state->s2 ^ state->s3);

}

static double

taus_get_double (void *vstate)

{

  return taus_get (vstate) / 4294967296.0 ;

}

static void

taus_set (void *vstate, unsigned long int s)

{

  taus_state_t *state = (taus_state_t *) vstate;

  if (s == 0)

    s = 1;      /* default seed is 1 */

#define LCG(n) ((69069 * n) & 0xffffffffUL)

  state->s1 = LCG (s);

  state->s2 = LCG (state->s1);

  state->s3 = LCG (state->s2);

  /* "warm it up" */

  taus_get (state);

  taus_get (state);

  taus_get (state);

  taus_get (state);

  taus_get (state);

  taus_get (state);

  return;

}

static void

taus2_set (void *vstate, unsigned long int s)

{

  taus_state_t *state = (taus_state_t *) vstate;

  if (s == 0)

    s = 1;      /* default seed is 1 */

#define LCG(n) ((69069 * n) & 0xffffffffUL)

  state->s1 = LCG (s);

  if (state->s1 < 2) state->s1 += 2UL;

  state->s2 = LCG (state->s1);

  if (state->s2 < 8) state->s2 += 8UL;

  state->s3 = LCG (state->s2);

  if (state->s3 < 16) state->s3 += 16UL;

  /* "warm it up" */

  taus_get (state);

  taus_get (state);

  taus_get (state);

  taus_get (state);

  taus_get (state);

  taus_get (state);

  return;

}

static const gsl_rng_type taus_type =

{"taus",                        /* name */

 0xffffffffUL,                  /* RAND_MAX */

 0,                             /* RAND_MIN */

 sizeof (taus_state_t),

 &taus_set,

 &taus_get,

 &taus_get_double};

const gsl_rng_type *gsl_rng_taus = &taus_type;

static const gsl_rng_type taus2_type =

{"taus2",                       /* name */

 0xffffffffUL,                  /* RAND_MAX */

 0,                             /* RAND_MIN */

 sizeof (taus_state_t),

 &taus2_set,

 &taus_get,

 &taus_get_double};

const gsl_rng_type *gsl_rng_taus2 = &taus2_type;