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/* rng/rand48.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_sys.h>
#include <gsl/gsl_rng.h>
/* This is the Unix rand48() generator. The generator returns the
upper 32 bits from each term of the sequence,
x_{n+1} = (a x_n + c) mod m
using 48-bit unsigned arithmetic, with a = 0x5DEECE66D , c = 0xB
and m = 2^48. The seed specifies the upper 32 bits of the initial
value, x_1, with the lower 16 bits set to 0x330E.
The theoretical value of x_{10001} is 244131582646046.
The period of this generator is ? FIXME (probably around 2^48). */
static inline void rand48_advance (void *vstate);
static unsigned long int rand48_get (void *vstate);
static double rand48_get_double (void *vstate);
static void rand48_set (void *state, unsigned long int s);
static const unsigned short int a0 = 0xE66D ;
static const unsigned short int a1 = 0xDEEC ;
static const unsigned short int a2 = 0x0005 ;
static const unsigned short int c0 = 0x000B ;
typedef struct
{
unsigned short int x0, x1, x2;
}
rand48_state_t;
static inline void
rand48_advance (void *vstate)
{
rand48_state_t *state = (rand48_state_t *) vstate;
/* work with unsigned long ints throughout to get correct integer
promotions of any unsigned short ints */
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 a ;
a = a0 * x0 + c0 ;
state->x0 = (a & 0xFFFF) ;
a >>= 16 ;
/* although the next line may overflow we only need the top 16 bits
in the following stage, so it does not matter */
a += a0 * x1 + a1 * x0 ;
state->x1 = (a & 0xFFFF) ;
a >>= 16 ;
a += a0 * x2 + a1 * x1 + a2 * x0 ;
state->x2 = (a & 0xFFFF) ;
}
static unsigned long int
rand48_get (void *vstate)
{
unsigned long int x1, x2;
rand48_state_t *state = (rand48_state_t *) vstate;
rand48_advance (state) ;
x2 = (unsigned long int) state->x2;
x1 = (unsigned long int) state->x1;
return (x2 << 16) + x1;
}
static double
rand48_get_double (void * vstate)
{
rand48_state_t *state = (rand48_state_t *) vstate;
rand48_advance (state) ;
return (ldexp((double) state->x2, -16)
+ ldexp((double) state->x1, -32)
+ ldexp((double) state->x0, -48)) ;
}
static void
rand48_set (void *vstate, unsigned long int s)
{
rand48_state_t *state = (rand48_state_t *) vstate;
if (s == 0) /* default seed */
{
state->x0 = 0x330E ;
state->x1 = 0xABCD ;
state->x2 = 0x1234 ;
}
else
{
state->x0 = 0x330E ;
state->x1 = s & 0xFFFF ;
state->x2 = (s >> 16) & 0xFFFF ;
}
return;
}
static const gsl_rng_type rand48_type =
{"rand48", /* name */
0xffffffffUL, /* RAND_MAX */
0, /* RAND_MIN */
sizeof (rand48_state_t),
&rand48_set,
&rand48_get,
&rand48_get_double
};
const gsl_rng_type *gsl_rng_rand48 = &rand48_type;
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