1 /* 2 * Copyright (c) 2008, Damien Miller <djm@openbsd.org> 3 * 4 * Permission to use, copy, modify, and distribute this software for any 5 * purpose with or without fee is hereby granted, provided that the above 6 * copyright notice and this permission notice appear in all copies. 7 * 8 * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES 9 * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF 10 * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR 11 * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES 12 * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN 13 * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF 14 * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE. 15 */ 16 17 #include <stdint.h> 18 #include <stdlib.h> 19 20 uint32_t uniform_random(uint32_t); 21 unsigned long prng_uint32(void); 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 uniform_random(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 = (uint32_t)prng_uint32(); 52 if (r >= min) 53 break; 54 } 55 56 return r % upper_bound; 57 } 58