mirror of
https://sourceware.org/git/glibc.git
synced 2024-11-08 14:20:07 +00:00
72 lines
2.4 KiB
C
72 lines
2.4 KiB
C
/* Random pseudo generator number which returns a single 32 bit value
|
|
uniformly distributed but with an upper_bound.
|
|
Copyright (C) 2022-2023 Free Software Foundation, Inc.
|
|
This file is part of the GNU C Library.
|
|
|
|
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, see
|
|
<https://www.gnu.org/licenses/>. */
|
|
|
|
#include <stdlib.h>
|
|
#include <sys/param.h>
|
|
|
|
/* Return a uniformly distributed random number less than N. The algorithm
|
|
calculates a mask being the lowest power of two bounding the upper bound
|
|
N, successively queries new random values, and rejects values outside of
|
|
the request range.
|
|
|
|
For reject values, it also tries if the remaining entropy could fit on
|
|
the asked range after range adjustment.
|
|
|
|
The algorithm avoids modulo and divide operations, which might be costly
|
|
depending on the architecture. */
|
|
uint32_t
|
|
__arc4random_uniform (uint32_t n)
|
|
{
|
|
if (n <= 1)
|
|
/* There is no valid return value for a zero limit, and 0 is the
|
|
only possible result for limit 1. */
|
|
return 0;
|
|
|
|
/* Powers of two are easy. */
|
|
if (powerof2 (n))
|
|
return __arc4random () & (n - 1);
|
|
|
|
/* mask is the smallest power of 2 minus 1 number larger than n. */
|
|
int z = __builtin_clz (n);
|
|
uint32_t mask = ~UINT32_C(0) >> z;
|
|
int bits = CHAR_BIT * sizeof (uint32_t) - z;
|
|
|
|
while (1)
|
|
{
|
|
uint32_t value = __arc4random ();
|
|
|
|
/* Return if the lower power of 2 minus 1 satisfy the condition. */
|
|
uint32_t r = value & mask;
|
|
if (r < n)
|
|
return r;
|
|
|
|
/* Otherwise check if remaining bits of entropy provides fits in the
|
|
bound. */
|
|
for (int bits_left = z; bits_left >= bits; bits_left -= bits)
|
|
{
|
|
value >>= bits;
|
|
r = value & mask;
|
|
if (r < n)
|
|
return r;
|
|
}
|
|
}
|
|
}
|
|
libc_hidden_def (__arc4random_uniform)
|
|
weak_alias (__arc4random_uniform, arc4random_uniform)
|