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
16namespace webrtc {
17
18Random::Random(uint64_t seed) {
19  RTC_DCHECK(seed != 0x0ull);
20  state_ = seed;
21}
22
23uint32_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
36uint32_t Random::Rand(uint32_t low, uint32_t high) {
37  RTC_DCHECK(low <= high);
38  return Rand(high - low) + low;
39}
40
41int32_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
50template <>
51float Random::Rand<float>() {
52  double result = NextOutput() - 1;
53  result = result / 0xFFFFFFFFFFFFFFFEull;
54  return static_cast<float>(result);
55}
56
57template <>
58double Random::Rand<double>() {
59  double result = NextOutput() - 1;
60  result = result / 0xFFFFFFFFFFFFFFFEull;
61  return result;
62}
63
64template <>
65bool Random::Rand<bool>() {
66  return Rand(0, 1) == 1;
67}
68
69double 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
81double Random::Exponential(double lambda) {
82  double uniform = Rand<double>();
83  return -log(uniform) / lambda;
84}
85
86}  // namespace webrtc
87