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sys/libkern/arc4random_uniform.c
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| /* $OpenBSD: arc4random_uniform.c,v 1.3 2019/01/20 02:59:07 bcook Exp $ */ | |||||
| /* | |||||
| * Copyright (c) 2008, Damien Miller <djm@openbsd.org> | |||||
| * | |||||
| * Permission to use, copy, modify, and distribute this software for any | |||||
| * purpose with or without fee is hereby granted, provided that the above | |||||
| * copyright notice and this permission notice appear in all copies. | |||||
| * | |||||
| * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES | |||||
| * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF | |||||
| * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR | |||||
| * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES | |||||
| * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN | |||||
| * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF | |||||
| * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE. | |||||
| * | |||||
| * $FreeBSD$ | |||||
| */ | |||||
| #include <sys/types.h> | |||||
| #include <sys/libkern.h> | |||||
| /* | |||||
| * Calculate a uniformly distributed random number less than upper_bound | |||||
| * avoiding "modulo bias". | |||||
| * | |||||
| * Uniformity is achieved by generating new random numbers until the one | |||||
| * returned is outside the range [0, 2**32 % upper_bound). This | |||||
| * guarantees the selected random number will be inside | |||||
| * [2**32 % upper_bound, 2**32) which maps back to [0, upper_bound) | |||||
| * after reduction modulo upper_bound. | |||||
| */ | |||||
| uint32_t | |||||
| arc4random_uniform(uint32_t upper_bound) | |||||
| { | |||||
| uint32_t r, min; | |||||
| if (upper_bound < 2) | |||||
| return 0; | |||||
| /* 2**32 % x == (2**32 - x) % x */ | |||||
| min = -upper_bound % upper_bound; | |||||
| /* | |||||
| * This could theoretically loop forever but each retry has | |||||
| * p > 0.5 (worst case, usually far better) of selecting a | |||||
| * number inside the range we need, so it should rarely need | |||||
| * to re-roll. | |||||
| */ | |||||
| for (;;) { | |||||
| r = arc4random(); | |||||
| if (r >= min) | |||||
| break; | |||||
| } | |||||
| return r % upper_bound; | |||||
| } | |||||