/* | |

* Copyright (c) 2015 The WebRTC project authors. All Rights Reserved. | |

* | |

* Use of this source code is governed by a BSD-style license | |

* that can be found in the LICENSE file in the root of the source | |

* tree. An additional intellectual property rights grant can be found | |

* in the file PATENTS. All contributing project authors may | |

* be found in the AUTHORS file in the root of the source tree. | |

*/ | |

#include "webrtc/base/random.h" | |

#include <math.h> | |

#include "webrtc/base/checks.h" | |

namespace webrtc { | |

Random::Random(uint64_t seed) { | |

RTC_DCHECK(seed != 0x0ull); | |

state_ = seed; | |

} | |

uint32_t Random::Rand(uint32_t t) { | |

// Casting the output to 32 bits will give an almost uniform number. | |

// Pr[x=0] = (2^32-1) / (2^64-1) | |

// Pr[x=k] = 2^32 / (2^64-1) for k!=0 | |

// Uniform would be Pr[x=k] = 2^32 / 2^64 for all 32-bit integers k. | |

uint32_t x = NextOutput(); | |

// If x / 2^32 is uniform on [0,1), then x / 2^32 * (t+1) is uniform on | |

// the interval [0,t+1), so the integer part is uniform on [0,t]. | |

uint64_t result = x * (static_cast<uint64_t>(t) + 1); | |

result >>= 32; | |

return result; | |

} | |

uint32_t Random::Rand(uint32_t low, uint32_t high) { | |

RTC_DCHECK(low <= high); | |

return Rand(high - low) + low; | |

} | |

int32_t Random::Rand(int32_t low, int32_t high) { | |

RTC_DCHECK(low <= high); | |

// We rely on subtraction (and addition) to be the same for signed and | |

// unsigned numbers in two-complement representation. Thus, although | |

// high - low might be negative as an int, it is the correct difference | |

// when interpreted as an unsigned. | |

return Rand(high - low) + low; | |

} | |

template <> | |

float Random::Rand<float>() { | |

double result = NextOutput() - 1; | |

result = result / 0xFFFFFFFFFFFFFFFEull; | |

return static_cast<float>(result); | |

} | |

template <> | |

double Random::Rand<double>() { | |

double result = NextOutput() - 1; | |

result = result / 0xFFFFFFFFFFFFFFFEull; | |

return result; | |

} | |

template <> | |

bool Random::Rand<bool>() { | |

return Rand(0, 1) == 1; | |

} | |

double Random::Gaussian(double mean, double standard_deviation) { | |

// Creating a Normal distribution variable from two independent uniform | |

// variables based on the Box-Muller transform, which is defined on the | |

// interval (0, 1]. Note that we rely on NextOutput to generate integers | |

// in the range [1, 2^64-1]. Normally this behavior is a bit frustrating, | |

// but here it is exactly what we need. | |

const double kPi = 3.14159265358979323846; | |

double u1 = static_cast<double>(NextOutput()) / 0xFFFFFFFFFFFFFFFFull; | |

double u2 = static_cast<double>(NextOutput()) / 0xFFFFFFFFFFFFFFFFull; | |

return mean + standard_deviation * sqrt(-2 * log(u1)) * cos(2 * kPi * u2); | |

} | |

double Random::Exponential(double lambda) { | |

double uniform = Rand<double>(); | |

return -log(uniform) / lambda; | |

} | |

} // namespace webrtc |