用于对C兰特哪些常见的算法()? [英] What common algorithms are used for C's rand()?
问题描述
据我所知,C规范没有给出任何规格约的具体实施兰特()
。通常用在不同的主流平台有什么不同的算法?它们有什么不同?
I understand that the C specification does not give any specification about the specific implementation of rand()
. What different algorithms are commonly used on different major platforms? How do they differ?
推荐答案
请参阅这篇文章:的http:// en.wikipedia.org/wiki/List_of_random_number_generators
这是glibc的的兰特源$ C $ C()
:
/* Reentrant random function from POSIX.1c.
Copyright (C) 1996, 1999, 2009 Free Software Foundation, Inc.
This file is part of the GNU C Library.
Contributed by Ulrich Drepper <drepper@cygnus.com>, 1996.
The GNU C Library is free software; you can redistribute it and/or
modify it under the terms of the GNU Lesser General Public
License as published by the Free Software Foundation; either
version 2.1 of the License, or (at your option) any later version.
The GNU C Library 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
Lesser General Public License for more details.
You should have received a copy of the GNU Lesser General Public
License along with the GNU C Library; if not, write to the Free
Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA
02111-1307 USA. */
#include <stdlib.h>
/* This algorithm is mentioned in the ISO C standard, here extended
for 32 bits. */
int
rand_r (unsigned int *seed)
{
unsigned int next = *seed;
int result;
next *= 1103515245;
next += 12345;
result = (unsigned int) (next / 65536) % 2048;
next *= 1103515245;
next += 12345;
result <<= 10;
result ^= (unsigned int) (next / 65536) % 1024;
next *= 1103515245;
next += 12345;
result <<= 10;
result ^= (unsigned int) (next / 65536) % 1024;
*seed = next;
return result;
}
来源:的https://sourceware.org/git/?p=glibc.git;a=blob_plain;f=stdlib/rand_r.c;hb=HEAD
正如你所看到的,它只是乘以加法和移位。这些值都经过精心挑选,以确保你没有重复的输出为RAND_MAX迭代。
As you can see, it's simply multiply with an addition and a shift. The values are carefully chosen to make sure that you get no repeat of the output for RAND_MAX iterations.
请注意,这是一个古老的实现,它已取代了更复杂的算法:的https://sourceware.org/git/?p=glibc.git;a=blob_plain;f=stdlib/random_r.c;hb=HEAD
Note that this is an old implementation which has been replaced by a more complex algorithm: https://sourceware.org/git/?p=glibc.git;a=blob_plain;f=stdlib/random_r.c;hb=HEAD
如果,如果打破了链接,谷歌的glibc rand_r
If the link if broken, Google for "glibc rand_r"
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