87 lines
2.7 KiB
C++
87 lines
2.7 KiB
C++
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/*
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* Copyright (c) 2015 The WebRTC project authors. All Rights Reserved.
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*
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* Use of this source code is governed by a BSD-style license
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* that can be found in the LICENSE file in the root of the source
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* tree. An additional intellectual property rights grant can be found
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* in the file PATENTS. All contributing project authors may
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* be found in the AUTHORS file in the root of the source tree.
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*/
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#include "webrtc/base/random.h"
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#include <math.h>
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#include "webrtc/base/checks.h"
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namespace webrtc {
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Random::Random(uint64_t seed) {
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RTC_DCHECK(seed != 0x0ull);
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state_ = seed;
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}
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uint32_t Random::Rand(uint32_t t) {
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// Casting the output to 32 bits will give an almost uniform number.
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// Pr[x=0] = (2^32-1) / (2^64-1)
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// Pr[x=k] = 2^32 / (2^64-1) for k!=0
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// Uniform would be Pr[x=k] = 2^32 / 2^64 for all 32-bit integers k.
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uint32_t x = NextOutput();
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// If x / 2^32 is uniform on [0,1), then x / 2^32 * (t+1) is uniform on
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// the interval [0,t+1), so the integer part is uniform on [0,t].
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uint64_t result = x * (static_cast<uint64_t>(t) + 1);
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result >>= 32;
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return result;
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}
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uint32_t Random::Rand(uint32_t low, uint32_t high) {
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RTC_DCHECK(low <= high);
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return Rand(high - low) + low;
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}
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int32_t Random::Rand(int32_t low, int32_t high) {
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RTC_DCHECK(low <= high);
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// We rely on subtraction (and addition) to be the same for signed and
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// unsigned numbers in two-complement representation. Thus, although
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// high - low might be negative as an int, it is the correct difference
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// when interpreted as an unsigned.
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return Rand(high - low) + low;
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}
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template <>
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float Random::Rand<float>() {
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double result = NextOutput() - 1;
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result = result / 0xFFFFFFFFFFFFFFFEull;
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return static_cast<float>(result);
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}
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template <>
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double Random::Rand<double>() {
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double result = NextOutput() - 1;
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result = result / 0xFFFFFFFFFFFFFFFEull;
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return result;
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}
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template <>
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bool Random::Rand<bool>() {
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return Rand(0, 1) == 1;
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}
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double Random::Gaussian(double mean, double standard_deviation) {
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// Creating a Normal distribution variable from two independent uniform
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// variables based on the Box-Muller transform, which is defined on the
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// interval (0, 1]. Note that we rely on NextOutput to generate integers
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// in the range [1, 2^64-1]. Normally this behavior is a bit frustrating,
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// but here it is exactly what we need.
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const double kPi = 3.14159265358979323846;
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double u1 = static_cast<double>(NextOutput()) / 0xFFFFFFFFFFFFFFFFull;
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double u2 = static_cast<double>(NextOutput()) / 0xFFFFFFFFFFFFFFFFull;
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return mean + standard_deviation * sqrt(-2 * log(u1)) * cos(2 * kPi * u2);
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}
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double Random::Exponential(double lambda) {
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double uniform = Rand<double>();
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return -log(uniform) / lambda;
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}
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} // namespace webrtc
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