| 1 | /* Random pseudo generator number which returns a single 32 bit value |
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| 2 | uniformly distributed but with an upper_bound. |
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| 3 | Copyright (C) 2022-2024 Free Software Foundation, Inc. |
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| 4 | This file is part of the GNU C Library. |
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| 5 | |
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| 6 | The GNU C Library is free software; you can redistribute it and/or |
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| 7 | modify it under the terms of the GNU Lesser General Public |
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| 8 | License as published by the Free Software Foundation; either |
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| 9 | version 2.1 of the License, or (at your option) any later version. |
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| 10 | |
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| 11 | The GNU C Library is distributed in the hope that it will be useful, |
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| 12 | but WITHOUT ANY WARRANTY; without even the implied warranty of |
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| 13 | MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU |
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| 14 | Lesser General Public License for more details. |
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| 15 | |
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| 16 | You should have received a copy of the GNU Lesser General Public |
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| 17 | License along with the GNU C Library; if not, see |
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| 18 | <https://www.gnu.org/licenses/>. */ |
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| 19 | |
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| 20 | #include <stdlib.h> |
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| 21 | #include <sys/param.h> |
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| 22 | |
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| 23 | /* Return a uniformly distributed random number less than N. The algorithm |
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| 24 | calculates a mask being the lowest power of two bounding the upper bound |
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| 25 | N, successively queries new random values, and rejects values outside of |
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| 26 | the request range. |
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| 27 | |
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| 28 | For reject values, it also tries if the remaining entropy could fit on |
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| 29 | the asked range after range adjustment. |
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| 30 | |
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| 31 | The algorithm avoids modulo and divide operations, which might be costly |
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| 32 | depending on the architecture. */ |
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| 33 | uint32_t |
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| 34 | __arc4random_uniform (uint32_t n) |
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| 35 | { |
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| 36 | if (n <= 1) |
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| 37 | /* There is no valid return value for a zero limit, and 0 is the |
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| 38 | only possible result for limit 1. */ |
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| 39 | return 0; |
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| 40 | |
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| 41 | /* Powers of two are easy. */ |
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| 42 | if (powerof2 (n)) |
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| 43 | return __arc4random () & (n - 1); |
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| 44 | |
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| 45 | /* mask is the smallest power of 2 minus 1 number larger than n. */ |
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| 46 | int z = __builtin_clz (n); |
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| 47 | uint32_t mask = ~UINT32_C(0) >> z; |
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| 48 | int bits = CHAR_BIT * sizeof (uint32_t) - z; |
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| 49 | |
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| 50 | while (1) |
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| 51 | { |
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| 52 | uint32_t value = __arc4random (); |
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| 53 | |
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| 54 | /* Return if the lower power of 2 minus 1 satisfy the condition. */ |
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| 55 | uint32_t r = value & mask; |
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| 56 | if (r < n) |
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| 57 | return r; |
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| 58 | |
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| 59 | /* Otherwise check if remaining bits of entropy provides fits in the |
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| 60 | bound. */ |
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| 61 | for (int bits_left = z; bits_left >= bits; bits_left -= bits) |
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| 62 | { |
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| 63 | value >>= bits; |
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| 64 | r = value & mask; |
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| 65 | if (r < n) |
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| 66 | return r; |
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| 67 | } |
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| 68 | } |
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| 69 | } |
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| 70 | libc_hidden_def (__arc4random_uniform) |
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| 71 | weak_alias (__arc4random_uniform, arc4random_uniform) |
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| 72 | |
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