49abb2a4f8
seed->first value correlation. It breaks rand_r()... Other possible methods like shuffling inside aray will breaks rand_r() too, because it assumes only one word state, i.e. nothing extra can be added after seed assignment in srand(). BTW, for old formulae seed->first value correlation is not so monotonically increased as with other Linear Congruential Generators of this type only becase arithmetic overflow happens. But overflow affects distribution and lower bits very badly, as many articles says, such type of overflow not improves PRNG. So, monotonically increased seed->first value correlation problem remains...
173 lines
4.4 KiB
C
173 lines
4.4 KiB
C
/*-
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* Copyright (c) 1990, 1993
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* The Regents of the University of California. All rights reserved.
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*
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* Redistribution and use in source and binary forms, with or without
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* modification, are permitted provided that the following conditions
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* are met:
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* 1. Redistributions of source code must retain the above copyright
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* notice, this list of conditions and the following disclaimer.
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* 2. Redistributions in binary form must reproduce the above copyright
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* notice, this list of conditions and the following disclaimer in the
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* documentation and/or other materials provided with the distribution.
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* 3. All advertising materials mentioning features or use of this software
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* must display the following acknowledgement:
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* This product includes software developed by the University of
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* California, Berkeley and its contributors.
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* 4. Neither the name of the University nor the names of its contributors
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* may be used to endorse or promote products derived from this software
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* without specific prior written permission.
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*
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* THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
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* ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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* IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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* ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
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* FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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* DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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* OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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* HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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* LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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* OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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* SUCH DAMAGE.
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*
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* Posix rand_r function added May 1999 by Wes Peters <wes@softweyr.com>.
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*/
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#if defined(LIBC_SCCS) && !defined(lint)
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static char sccsid[] = "@(#)rand.c 8.1 (Berkeley) 6/14/93";
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#endif /* LIBC_SCCS and not lint */
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#include <sys/cdefs.h>
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__FBSDID("$FreeBSD$");
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#include "namespace.h"
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#include <sys/time.h> /* for sranddev() */
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#include <sys/types.h>
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#include <fcntl.h> /* for sranddev() */
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#include <stdlib.h>
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#include <unistd.h> /* for sranddev() */
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#include "un-namespace.h"
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#ifdef TEST
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#include <stdio.h>
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#endif /* TEST */
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static int
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do_rand(unsigned long *ctx)
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{
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#ifdef USE_WEAK_SEEDING
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/*
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* Historic implementation compatibility.
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* The random sequences do not vary much with the seed,
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* even with overflowing.
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*/
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return ((*ctx = *ctx * 1103515245 + 12345) % ((u_long)RAND_MAX + 1));
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#else /* !USE_WEAK_SEEDING */
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/*
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* Compute x = (7^5 * x) mod (2^31 - 1)
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* wihout overflowing 31 bits:
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* (2^31 - 1) = 127773 * (7^5) + 2836
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* From "Random number generators: good ones are hard to find",
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* Park and Miller, Communications of the ACM, vol. 31, no. 10,
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* October 1988, p. 1195.
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*/
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long hi, lo, x;
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/* Can't be initialized with 0, so use another value. */
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if (*ctx == 0)
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*ctx = 123459876;
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hi = *ctx / 127773;
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lo = *ctx % 127773;
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x = 16807 * lo - 2836 * hi;
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if (x < 0)
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x += 0x7fffffff;
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return ((*ctx = x) % ((u_long)RAND_MAX + 1));
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#endif /* !USE_WEAK_SEEDING */
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}
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int
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rand_r(unsigned int *ctx)
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{
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u_long val = (u_long) *ctx;
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int r = do_rand(&val);
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*ctx = (unsigned int) val;
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return (r);
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}
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static u_long next = 1;
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int
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rand()
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{
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return (do_rand(&next));
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}
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void
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srand(seed)
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u_int seed;
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{
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next = seed;
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}
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/*
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* sranddev:
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*
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* Many programs choose the seed value in a totally predictable manner.
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* This often causes problems. We seed the generator using the much more
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* secure random(4) interface.
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*/
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void
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sranddev()
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{
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int fd, done;
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done = 0;
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fd = _open("/dev/random", O_RDONLY, 0);
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if (fd >= 0) {
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if (_read(fd, (void *) &next, sizeof(next)) == sizeof(next))
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done = 1;
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_close(fd);
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}
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if (!done) {
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struct timeval tv;
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unsigned long junk;
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gettimeofday(&tv, NULL);
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srand((getpid() << 16) ^ tv.tv_sec ^ tv.tv_usec ^ junk);
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}
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}
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#ifdef TEST
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main()
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{
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int i;
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unsigned myseed;
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printf("seeding rand with 0x19610910: \n");
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srand(0x19610910);
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printf("generating three pseudo-random numbers:\n");
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for (i = 0; i < 3; i++)
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{
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printf("next random number = %d\n", rand());
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}
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printf("generating the same sequence with rand_r:\n");
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myseed = 0x19610910;
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for (i = 0; i < 3; i++)
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{
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printf("next random number = %d\n", rand_r(&myseed));
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}
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return 0;
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}
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#endif /* TEST */
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