/* rng/rand48.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 <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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