projects / apple / network_cmds.git / blame
| Commit | Line | Data |
|---|---|---|
| 89c4ed63 A |
1 | /* $OpenBSD: arc4random_uniform.c,v 1.1 2014/07/12 13:24:54 deraadt Exp $ */ |
| 2 | ||
| 3 | /* | |
| 4 | * Copyright (c) 2008, Damien Miller <djm@openbsd.org> | |
| 5 | * | |
| 6 | * Permission to use, copy, modify, and distribute this software for any | |
| 7 | * purpose with or without fee is hereby granted, provided that the above | |
| 8 | * copyright notice and this permission notice appear in all copies. | |
| 9 | * | |
| 10 | * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES | |
| 11 | * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF | |
| 12 | * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR | |
| 13 | * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES | |
| 14 | * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN | |
| 15 | * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF | |
| 16 | * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE. | |
| 17 | */ | |
| 18 | ||
| 19 | #include "config.h" | |
| 20 | #include <sys/types.h> | |
| 21 | #include <stdlib.h> | |
| 22 | ||
| 23 | /* | |
| 24 | * Calculate a uniformly distributed random number less than upper_bound | |
| 25 | * avoiding "modulo bias". | |
| 26 | * | |
| 27 | * Uniformity is achieved by generating new random numbers until the one | |
| 28 | * returned is outside the range [0, 2**32 % upper_bound). This | |
| 29 | * guarantees the selected random number will be inside | |
| 30 | * [2**32 % upper_bound, 2**32) which maps back to [0, upper_bound) | |
| 31 | * after reduction modulo upper_bound. | |
| 32 | */ | |
| 33 | uint32_t | |
| 34 | arc4random_uniform(uint32_t upper_bound) | |
| 35 | { | |
| 36 | uint32_t r, min; | |
| 37 | ||
| 38 | if (upper_bound < 2) | |
| 39 | return 0; | |
| 40 | ||
| 41 | /* 2**32 % x == (2**32 - x) % x */ | |
| 42 | min = -upper_bound % upper_bound; | |
| 43 | ||
| 44 | /* | |
| 45 | * This could theoretically loop forever but each retry has | |
| 46 | * p > 0.5 (worst case, usually far better) of selecting a | |
| 47 | * number inside the range we need, so it should rarely need | |
| 48 | * to re-roll. | |
| 49 | */ | |
| 50 | for (;;) { | |
| 51 | r = arc4random(); | |
| 52 | if (r >= min) | |
| 53 | break; | |
| 54 | } | |
| 55 | ||
| 56 | return r % upper_bound; | |
| 57 | } |