1/* Random pseudo generator number which returns a single 32 bit value
2 uniformly distributed but with an upper_bound.
3 Copyright (C) 2022-2023 Free Software Foundation, Inc.
4 This file is part of the GNU C Library.
5
6 The GNU C Library is free software; you can redistribute it and/or
7 modify it under the terms of the GNU Lesser General Public
8 License as published by the Free Software Foundation; either
9 version 2.1 of the License, or (at your option) any later version.
10
11 The GNU C Library is distributed in the hope that it will be useful,
12 but WITHOUT ANY WARRANTY; without even the implied warranty of
13 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
14 Lesser General Public License for more details.
15
16 You should have received a copy of the GNU Lesser General Public
17 License along with the GNU C Library; if not, see
18 <https://www.gnu.org/licenses/>. */
19
20#include <stdlib.h>
21#include <sys/param.h>
22
23/* Return a uniformly distributed random number less than N. The algorithm
24 calculates a mask being the lowest power of two bounding the upper bound
25 N, successively queries new random values, and rejects values outside of
26 the request range.
27
28 For reject values, it also tries if the remaining entropy could fit on
29 the asked range after range adjustment.
30
31 The algorithm avoids modulo and divide operations, which might be costly
32 depending on the architecture. */
33uint32_t
34__arc4random_uniform (uint32_t n)
35{
36 if (n <= 1)
37 /* There is no valid return value for a zero limit, and 0 is the
38 only possible result for limit 1. */
39 return 0;
40
41 /* Powers of two are easy. */
42 if (powerof2 (n))
43 return __arc4random () & (n - 1);
44
45 /* mask is the smallest power of 2 minus 1 number larger than n. */
46 int z = __builtin_clz (n);
47 uint32_t mask = ~UINT32_C(0) >> z;
48 int bits = CHAR_BIT * sizeof (uint32_t) - z;
49
50 while (1)
51 {
52 uint32_t value = __arc4random ();
53
54 /* Return if the lower power of 2 minus 1 satisfy the condition. */
55 uint32_t r = value & mask;
56 if (r < n)
57 return r;
58
59 /* Otherwise check if remaining bits of entropy provides fits in the
60 bound. */
61 for (int bits_left = z; bits_left >= bits; bits_left -= bits)
62 {
63 value >>= bits;
64 r = value & mask;
65 if (r < n)
66 return r;
67 }
68 }
69}
70libc_hidden_def (__arc4random_uniform)
71weak_alias (__arc4random_uniform, arc4random_uniform)
72