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Diffstat (limited to 'webrtc/base/random.cc')
-rw-r--r-- | webrtc/base/random.cc | 86 |
1 files changed, 86 insertions, 0 deletions
diff --git a/webrtc/base/random.cc b/webrtc/base/random.cc new file mode 100644 index 0000000000..14a9faf5b3 --- /dev/null +++ b/webrtc/base/random.cc @@ -0,0 +1,86 @@ +/* + * 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 |