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/* rng/randu.c
*
* Copyright (C) 1996, 1997, 1998, 1999, 2000 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 2 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 reincarnation of the infamously bad RANDU generator.
The sequence is,
x_{n+1} = (a x_n) mod m
with a = 65539 and m = 2^31 = 2147483648. The seed specifies
the initial value, x_1.
The theoretical value of x_{10001} is 1623524161.
The period of this generator is 2^29.
Note: Knuth describes this generator as "really horrible".
From: Park and Miller, "Random Number Generators: Good ones are
hard to find" Communications of the ACM, October 1988, Volume 31,
No 10, pages 1192-1201. */
static inline unsigned long int randu_get (void *vstate);
static double randu_get_double (void *vstate);
static void randu_set (void *state, unsigned long int s);
static const long int a = 65539;
/* static const unsigned long int m = 2147483648UL; */
typedef struct
{
unsigned long int x;
}
randu_state_t;
static inline unsigned long int
randu_get (void *vstate)
{
randu_state_t *state = (randu_state_t *) vstate;
/* The following line relies on unsigned 32-bit arithmetic */
state->x = (a * state->x) & 0x7fffffffUL;
return state->x;
}
static double
randu_get_double (void *vstate)
{
return randu_get (vstate) / 2147483648.0 ;
}
static void
randu_set (void *vstate, unsigned long int s)
{
randu_state_t *state = (randu_state_t *) vstate;
if (s == 0)
s = 1; /* default seed is 1 */
state->x = s;
return;
}
static const gsl_rng_type randu_type =
{"randu", /* name */
0x7fffffffUL, /* RAND_MAX */
1, /* RAND_MIN */
sizeof (randu_state_t),
&randu_set,
&randu_get,
&randu_get_double};
const gsl_rng_type *gsl_rng_randu = &randu_type;
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