1d25a1430SXin LI /* $OpenBSD: arc4random_uniform.c,v 1.3 2019/01/20 02:59:07 bcook Exp $ */
26b97c2e3SConrad Meyer
36b97c2e3SConrad Meyer /*
46b97c2e3SConrad Meyer * Copyright (c) 2008, Damien Miller <djm@openbsd.org>
56b97c2e3SConrad Meyer *
66b97c2e3SConrad Meyer * Permission to use, copy, modify, and distribute this software for any
76b97c2e3SConrad Meyer * purpose with or without fee is hereby granted, provided that the above
86b97c2e3SConrad Meyer * copyright notice and this permission notice appear in all copies.
96b97c2e3SConrad Meyer *
106b97c2e3SConrad Meyer * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
116b97c2e3SConrad Meyer * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
126b97c2e3SConrad Meyer * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
136b97c2e3SConrad Meyer * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
146b97c2e3SConrad Meyer * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
156b97c2e3SConrad Meyer * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
166b97c2e3SConrad Meyer * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
176b97c2e3SConrad Meyer */
186b97c2e3SConrad Meyer
19d25a1430SXin LI #include <stdint.h>
206b97c2e3SConrad Meyer #include <stdlib.h>
216b97c2e3SConrad Meyer
226b97c2e3SConrad Meyer /*
236b97c2e3SConrad Meyer * Calculate a uniformly distributed random number less than upper_bound
246b97c2e3SConrad Meyer * avoiding "modulo bias".
256b97c2e3SConrad Meyer *
266b97c2e3SConrad Meyer * Uniformity is achieved by generating new random numbers until the one
276b97c2e3SConrad Meyer * returned is outside the range [0, 2**32 % upper_bound). This
286b97c2e3SConrad Meyer * guarantees the selected random number will be inside
296b97c2e3SConrad Meyer * [2**32 % upper_bound, 2**32) which maps back to [0, upper_bound)
306b97c2e3SConrad Meyer * after reduction modulo upper_bound.
316b97c2e3SConrad Meyer */
326b97c2e3SConrad Meyer uint32_t
arc4random_uniform(uint32_t upper_bound)336b97c2e3SConrad Meyer arc4random_uniform(uint32_t upper_bound)
346b97c2e3SConrad Meyer {
356b97c2e3SConrad Meyer uint32_t r, min;
366b97c2e3SConrad Meyer
376b97c2e3SConrad Meyer if (upper_bound < 2)
386b97c2e3SConrad Meyer return 0;
396b97c2e3SConrad Meyer
406b97c2e3SConrad Meyer /* 2**32 % x == (2**32 - x) % x */
416b97c2e3SConrad Meyer min = -upper_bound % upper_bound;
426b97c2e3SConrad Meyer
436b97c2e3SConrad Meyer /*
446b97c2e3SConrad Meyer * This could theoretically loop forever but each retry has
456b97c2e3SConrad Meyer * p > 0.5 (worst case, usually far better) of selecting a
466b97c2e3SConrad Meyer * number inside the range we need, so it should rarely need
476b97c2e3SConrad Meyer * to re-roll.
486b97c2e3SConrad Meyer */
496b97c2e3SConrad Meyer for (;;) {
506b97c2e3SConrad Meyer r = arc4random();
516b97c2e3SConrad Meyer if (r >= min)
526b97c2e3SConrad Meyer break;
536b97c2e3SConrad Meyer }
546b97c2e3SConrad Meyer
556b97c2e3SConrad Meyer return r % upper_bound;
566b97c2e3SConrad Meyer }
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