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Diffstat (limited to 'stdlib/arc4random_uniform.c')
| -rw-r--r-- | stdlib/arc4random_uniform.c | 140 |
1 files changed, 140 insertions, 0 deletions
diff --git a/stdlib/arc4random_uniform.c b/stdlib/arc4random_uniform.c new file mode 100644 index 0000000000..1326dfa593 --- /dev/null +++ b/stdlib/arc4random_uniform.c @@ -0,0 +1,140 @@ +/* Random pseudo generator number which returns a single 32 bit value + uniformly distributed but with an upper_bound. + Copyright (C) 2022 Free Software Foundation, Inc. + This file is part of the GNU C Library. + + The GNU C Library is free software; you can redistribute it and/or + modify it under the terms of the GNU Lesser General Public + License as published by the Free Software Foundation; either + version 2.1 of the License, or (at your option) any later version. + + The GNU C Library is distributed in the hope that it will be useful, + but WITHOUT ANY WARRANTY; without even the implied warranty of + MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU + Lesser General Public License for more details. + + You should have received a copy of the GNU Lesser General Public + License along with the GNU C Library; if not, see + <https://www.gnu.org/licenses/>. */ + +#include <endian.h> +#include <libc-lock.h> +#include <stdlib.h> +#include <sys/param.h> + +/* Return the number of bytes which cover values up to the limit. */ +__attribute__ ((const)) +static uint32_t +byte_count (uint32_t n) +{ + if (n < (1U << 8)) + return 1; + else if (n < (1U << 16)) + return 2; + else if (n < (1U << 24)) + return 3; + else + return 4; +} + +/* Fill the lower bits of the result with randomness, according to the + number of bytes requested. */ +static void +random_bytes (uint32_t *result, uint32_t byte_count) +{ + *result = 0; + unsigned char *ptr = (unsigned char *) result; + if (__BYTE_ORDER == __BIG_ENDIAN) + ptr += 4 - byte_count; + __arc4random_buf (ptr, byte_count); +} + +uint32_t +__arc4random_uniform (uint32_t n) +{ + if (n <= 1) + /* There is no valid return value for a zero limit, and 0 is the + only possible result for limit 1. */ + return 0; + + /* The bits variable serves as a source for bits. Prefetch the + minimum number of bytes needed. */ + uint32_t count = byte_count (n); + uint32_t bits_length = count * CHAR_BIT; + uint32_t bits; + random_bytes (&bits, count); + + /* Powers of two are easy. */ + if (powerof2 (n)) + return bits & (n - 1); + + /* The general case. This algorithm follows Jérémie Lumbroso, + Optimal Discrete Uniform Generation from Coin Flips, and + Applications (2013), who credits Donald E. Knuth and Andrew + C. Yao, The complexity of nonuniform random number generation + (1976), for solving the general case. + + The implementation below unrolls the initialization stage of the + loop, where v is less than n. */ + + /* Use 64-bit variables even though the intermediate results are + never larger than 33 bits. This ensures the code is easier to + compile on 64-bit architectures. */ + uint64_t v; + uint64_t c; + + /* Initialize v and c. v is the smallest power of 2 which is larger + than n.*/ + { + uint32_t log2p1 = 32 - __builtin_clz (n); + v = 1ULL << log2p1; + c = bits & (v - 1); + bits >>= log2p1; + bits_length -= log2p1; + } + + /* At the start of the loop, c is uniformly distributed within the + half-open interval [0, v), and v < 2n < 2**33. */ + while (true) + { + if (v >= n) + { + /* If the candidate is less than n, accept it. */ + if (c < n) + /* c is uniformly distributed on [0, n). */ + return c; + else + { + /* c is uniformly distributed on [n, v). */ + v -= n; + c -= n; + /* The distribution was shifted, so c is uniformly + distributed on [0, v) again. */ + } + } + /* v < n here. */ + + /* Replenish the bit source if necessary. */ + if (bits_length == 0) + { + /* Overwrite the least significant byte. */ + random_bytes (&bits, 1); + bits_length = CHAR_BIT; + } + + /* Double the range. No overflow because v < n < 2**32. */ + v *= 2; + /* v < 2n here. */ + + /* Extract a bit and append it to c. c remains less than v and + thus 2**33. */ + c = (c << 1) | (bits & 1); + bits >>= 1; + --bits_length; + + /* At this point, c is uniformly distributed on [0, v) again, + and v < 2n < 2**33. */ + } +} +libc_hidden_def (__arc4random_uniform) +weak_alias (__arc4random_uniform, arc4random_uniform) |