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It uses the bitmask with rejection [1], which calculates a mask being the lowest power of two bounding the request upper bound, successively queries new random values, and rejects values outside the requested range. Performance-wise, there is no much gain in trying to conserve bits since arc4random is wrapper on getrandom syscall. It should be cheaper to just query a uint32_t value. The algorithm also avoids modulo and divide operations, which might be costly depending of the architecture. [1] https://www.pcg-random.org/posts/bounded-rands.html Reviewed-by: Yann Droneaud <ydroneaud@opteya.com>
72 lines
2.4 KiB
C
72 lines
2.4 KiB
C
/* Random pseudo generator number which returns a single 32 bit value
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uniformly distributed but with an upper_bound.
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Copyright (C) 2022 Free Software Foundation, Inc.
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This file is part of the GNU C Library.
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The GNU C Library is free software; you can redistribute it and/or
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modify it under the terms of the GNU Lesser General Public
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License as published by the Free Software Foundation; either
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version 2.1 of the License, or (at your option) any later version.
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The GNU C Library is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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Lesser General Public License for more details.
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You should have received a copy of the GNU Lesser General Public
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License along with the GNU C Library; if not, see
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<https://www.gnu.org/licenses/>. */
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#include <stdlib.h>
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#include <sys/param.h>
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/* Return a uniformly distributed random number less than N. The algorithm
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calculates a mask being the lowest power of two bounding the upper bound
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N, successively queries new random values, and rejects values outside of
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the request range.
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For reject values, it also tries if the remaining entropy could fit on
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the asked range after range adjustment.
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The algorithm avoids modulo and divide operations, which might be costly
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depending on the architecture. */
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uint32_t
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__arc4random_uniform (uint32_t n)
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{
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if (n <= 1)
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/* There is no valid return value for a zero limit, and 0 is the
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only possible result for limit 1. */
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return 0;
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/* Powers of two are easy. */
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if (powerof2 (n))
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return __arc4random () & (n - 1);
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/* mask is the smallest power of 2 minus 1 number larger than n. */
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int z = __builtin_clz (n);
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uint32_t mask = ~UINT32_C(0) >> z;
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int bits = CHAR_BIT * sizeof (uint32_t) - z;
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while (1)
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{
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uint32_t value = __arc4random ();
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/* Return if the lower power of 2 minus 1 satisfy the condition. */
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uint32_t r = value & mask;
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if (r < n)
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return r;
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/* Otherwise check if remaining bits of entropy provides fits in the
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bound. */
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for (int bits_left = z; bits_left >= bits; bits_left -= bits)
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{
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value >>= bits;
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r = value & mask;
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if (r < n)
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return r;
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}
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}
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}
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libc_hidden_def (__arc4random_uniform)
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weak_alias (__arc4random_uniform, arc4random_uniform)
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