stdlib/arc4random_uniform.c

   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.  */
  33 uint32_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 }
  70 libc_hidden_def (__arc4random_uniform)
  71 weak_alias (__arc4random_uniform, arc4random_uniform)