mwc1616.c

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/* mwc1616.c - fast random number generator
 *
 * 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, 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., 59 Temple Place - Suite 330, Boston, MA
 * 02111-1307, USA.
 */

/*
 * MWC1616
 * return value made by concatenating two 16-bit
 * multiply-with-carry generators.
 * The two generators have the form
 *    x(n)=a*x(n-1)+carry mod 2^16      and
 *    y(n)=b*y(n-1)+carry mod 2^16,
 * in this case a and b are choosen as 18000 and 30903
 * 
 * Some other good choices for a and b
 * 18000 18030 18273 18513 18879 19074 19098 19164 19215 19584
 * 19599 19950 20088 20508 20544 20664 20814 20970 21153 21243
 * 21423 21723 21954 22125 22188 22293 22860 22938 22965 22974
 * 23109 23124 23163 23208 23508 23520 23553 23658 23865 24114
 * 24219 24660 24699 24864 24948 25023 25308 25443 26004 26088
 * 26154 26550 26679 26838 27183 27258 27753 27795 27810 27834
 * 27960 28320 28380 28689 28710 28794 28854 28959 28980 29013
 * 29379 29889 30135 30345 30459 30714 30903 30963 31059 31083
 * 
 * The `carry'  c works as follows:  If a and x are 16-bit  and
 * c at most 14 bits, then forming a*x+c produces an at-most 31-
 * bit result.  That result mod 2^16 (the rightmost 16 bits) is
 * the new x and the topmost 16 bits the new carry c.  The sequ-
 * ence of resulting x's has period the order of 2^16 in the
 * group of residues relatively prime to m=a*2^16-1, which will
 * be a*2^15-1 for the multipliers suggested here.
 * You will be prompted to choose a and b and two seeds.  Output
 * is a 32-bit integer, the pair x,y side by side.
 * This multiply-with-carry generator is best done in assembler,
 * where it takes about 200 nanosecs with a Pentium 120. A Fort-
 * ran version takes about 300 ns.  It seems to pass all tests
 * and is highly recommended for speed and simplicity.
 */

#include "mwc1616.h"

static unsigned long mwc1616_x = 1;
static unsigned long mwc1616_y = 2;

void seed_rand_mwc1616(unsigned long seed) {
    mwc1616_x = seed | 1;   /* Can't seed with 0 */
    mwc1616_y = seed | 2;
}

unsigned long rand_mwc1616(void) {
    mwc1616_x = 18000 * (mwc1616_x & 0xffff) + (mwc1616_x >> 16);
    mwc1616_y = 30903 * (mwc1616_y & 0xffff) + (mwc1616_y >> 16);
    return (mwc1616_x<<16)+(mwc1616_y&0xffff);
}