/libfido2/fuzz/uniform_random.c
Line | Count | Source (jump to first uncovered line) |
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 | 18.0M | { |
36 | 18.0M | uint32_t r, min; |
37 | | |
38 | 18.0M | if (upper_bound < 2) |
39 | 0 | return 0; |
40 | | |
41 | | /* 2**32 % x == (2**32 - x) % x */ |
42 | 18.0M | 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 | 18.0M | for (;;) { |
51 | 18.0M | r = (uint32_t)prng_uint32(); |
52 | 18.0M | if (r >= min) |
53 | 18.0M | break; |
54 | 18.0M | } |
55 | | |
56 | 18.0M | return r % upper_bound; |
57 | 18.0M | } |