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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;
- }
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