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/*
* 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 "rtc_base/random.h"
#include <math.h>
#include <cstdint>
#include <numbers>
#include "rtc_base/checks.h"
#include "rtc_base/numerics/safe_conversions.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);
const int64_t low_i64{low};
return dchecked_cast<int32_t>(Rand(dchecked_cast<uint32_t>(high - low_i64)) +
low_i64);
}
template <>
float Random::Rand<float>() {
double result = NextOutput() - 1;
result = result / static_cast<double>(0xFFFFFFFFFFFFFFFFull);
return static_cast<float>(result);
}
template <>
double Random::Rand<double>() {
double result = NextOutput() - 1;
result = result / static_cast<double>(0xFFFFFFFFFFFFFFFFull);
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 = std::numbers::pi;
double u1 = static_cast<double>(NextOutput()) /
static_cast<double>(0xFFFFFFFFFFFFFFFFull);
double u2 = static_cast<double>(NextOutput()) /
static_cast<double>(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
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