android / platform / external / dhcpcd-6.8.2 / refs/tags/android-cts-7.1_r5 / . / compat / arc4random_uniform.c

/* | |

* Copyright (c) 2008, Damien Miller <djm@openbsd.org> | |

* | |

* Permission to use, copy, modify, and distribute this software for any | |

* purpose with or without fee is hereby granted, provided that the above | |

* copyright notice and this permission notice appear in all copies. | |

* | |

* THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES | |

* WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF | |

* MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR | |

* ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES | |

* WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN | |

* ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF | |

* OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE. | |

*/ | |

#include <stdint.h> | |

#include <stdlib.h> | |

/* We need to include config.h so we pickup either the system arc4random | |

* or our compat one. */ | |

#include "../config.h" | |

/* | |

* Calculate a uniformly distributed random number less than upper_bound | |

* avoiding "modulo bias". | |

* | |

* Uniformity is achieved by generating new random numbers until the one | |

* returned is outside the range [0, 2**32 % upper_bound). This | |

* guarantees the selected random number will be inside | |

* [2**32 % upper_bound, 2**32) which maps back to [0, upper_bound) | |

* after reduction modulo upper_bound. | |

*/ | |

uint32_t | |

arc4random_uniform(uint32_t upper_bound) | |

{ | |

uint32_t r, min; | |

if (upper_bound < 2) | |

return 0; | |

/* 2**32 % x == (2**32 - x) % x */ | |

min = -upper_bound % upper_bound; | |

/* | |

* This could theoretically loop forever but each retry has | |

* p > 0.5 (worst case, usually far better) of selecting a | |

* number inside the range we need, so it should rarely need | |

* to re-roll. | |

*/ | |

do | |

r = arc4random(); | |

while (r < min); | |

return r % upper_bound; | |

} |