webrtc/test/random.cc

/*
 *  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/test/random.h"

#include <math.h>

#include "webrtc/base/checks.h"

namespace webrtc {

namespace test {

Random::Random(uint32_t seed) : a_(0x531FDB97 ^ seed), b_(0x6420ECA8 + seed) {
}

uint32_t Random::Rand(uint32_t t) {
  // If b / 2^32 is uniform on [0,1), then b / 2^32 * (t+1) is uniform on
  // the interval [0,t+1), so the integer part is uniform on [0,t].
  uint64_t result = b_ * (static_cast<uint64_t>(t) + 1);
  result >>= 32;
  a_ ^= b_;
  b_ += a_;
  return result;
}

uint32_t Random::Rand(uint32_t low, uint32_t high) {
  RTC_DCHECK(low <= high);
  return Rand(high - low) + low;
}

template <>
float Random::Rand<float>() {
  const double kScale = 1.0f / (static_cast<uint64_t>(1) << 32);
  double result = kScale * b_;
  a_ ^= b_;
  b_ += a_;
  return static_cast<float>(result);
}

template <>
bool Random::Rand<bool>() {
  return Rand(0, 1) == 1;
}

int Random::Gaussian(int mean, int 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], hence the mask+add below.
  const double kPi = 3.14159265358979323846;
  const double kScale = 1.0 / 0x80000000ul;
  double u1 = kScale * ((a_ & 0x7ffffffful) + 1);
  double u2 = kScale * ((b_ & 0x7ffffffful) + 1);
  a_ ^= b_;
  b_ += a_;
  return static_cast<int>(
      mean + standard_deviation * sqrt(-2 * log(u1)) * cos(2 * kPi * u2));
}

int Random::Exponential(float lambda) {
  float uniform = Rand<float>();
  return static_cast<int>(-log(uniform) / lambda);
}
}  // namespace test
}  // namespace webrtc