1 /* 2 * Copyright (c) 2015 The WebRTC project authors. All Rights Reserved. 3 * 4 * Use of this source code is governed by a BSD-style license 5 * that can be found in the LICENSE file in the root of the source 6 * tree. An additional intellectual property rights grant can be found 7 * in the file PATENTS. All contributing project authors may 8 * be found in the AUTHORS file in the root of the source tree. 9 */ 10 #include "webrtc/base/random.h" 11 12 #include <math.h> 13 14 #include "webrtc/base/checks.h" 15 16 namespace webrtc { 17 18 Random::Random(uint64_t seed) { 19 RTC_DCHECK(seed != 0x0ull); 20 state_ = seed; 21 } 22 23 uint32_t Random::Rand(uint32_t t) { 24 // Casting the output to 32 bits will give an almost uniform number. 25 // Pr[x=0] = (2^32-1) / (2^64-1) 26 // Pr[x=k] = 2^32 / (2^64-1) for k!=0 27 // Uniform would be Pr[x=k] = 2^32 / 2^64 for all 32-bit integers k. 28 uint32_t x = NextOutput(); 29 // If x / 2^32 is uniform on [0,1), then x / 2^32 * (t+1) is uniform on 30 // the interval [0,t+1), so the integer part is uniform on [0,t]. 31 uint64_t result = x * (static_cast<uint64_t>(t) + 1); 32 result >>= 32; 33 return result; 34 } 35 36 uint32_t Random::Rand(uint32_t low, uint32_t high) { 37 RTC_DCHECK(low <= high); 38 return Rand(high - low) + low; 39 } 40 41 int32_t Random::Rand(int32_t low, int32_t high) { 42 RTC_DCHECK(low <= high); 43 // We rely on subtraction (and addition) to be the same for signed and 44 // unsigned numbers in two-complement representation. Thus, although 45 // high - low might be negative as an int, it is the correct difference 46 // when interpreted as an unsigned. 47 return Rand(high - low) + low; 48 } 49 50 template <> 51 float Random::Rand<float>() { 52 double result = NextOutput() - 1; 53 result = result / 0xFFFFFFFFFFFFFFFEull; 54 return static_cast<float>(result); 55 } 56 57 template <> 58 double Random::Rand<double>() { 59 double result = NextOutput() - 1; 60 result = result / 0xFFFFFFFFFFFFFFFEull; 61 return result; 62 } 63 64 template <> 65 bool Random::Rand<bool>() { 66 return Rand(0, 1) == 1; 67 } 68 69 double Random::Gaussian(double mean, double standard_deviation) { 70 // Creating a Normal distribution variable from two independent uniform 71 // variables based on the Box-Muller transform, which is defined on the 72 // interval (0, 1]. Note that we rely on NextOutput to generate integers 73 // in the range [1, 2^64-1]. Normally this behavior is a bit frustrating, 74 // but here it is exactly what we need. 75 const double kPi = 3.14159265358979323846; 76 double u1 = static_cast<double>(NextOutput()) / 0xFFFFFFFFFFFFFFFFull; 77 double u2 = static_cast<double>(NextOutput()) / 0xFFFFFFFFFFFFFFFFull; 78 return mean + standard_deviation * sqrt(-2 * log(u1)) * cos(2 * kPi * u2); 79 } 80 81 double Random::Exponential(double lambda) { 82 double uniform = Rand<double>(); 83 return -log(uniform) / lambda; 84 } 85 86 } // namespace webrtc 87
