1 // Copyright 2017 The Abseil Authors.
2 //
3 // Licensed under the Apache License, Version 2.0 (the "License");
4 // you may not use this file except in compliance with the License.
5 // You may obtain a copy of the License at
6 //
7 // https://www.apache.org/licenses/LICENSE-2.0
8 //
9 // Unless required by applicable law or agreed to in writing, software
10 // distributed under the License is distributed on an "AS IS" BASIS,
11 // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
12 // See the License for the specific language governing permissions and
13 // limitations under the License.
14
15 #ifndef ABSL_RANDOM_EXPONENTIAL_DISTRIBUTION_H_
16 #define ABSL_RANDOM_EXPONENTIAL_DISTRIBUTION_H_
17
18 #include <cassert>
19 #include <cmath>
20 #include <istream>
21 #include <limits>
22 #include <type_traits>
23
24 #include "absl/meta/type_traits.h"
25 #include "absl/random/internal/fast_uniform_bits.h"
26 #include "absl/random/internal/generate_real.h"
27 #include "absl/random/internal/iostream_state_saver.h"
28
29 namespace absl {
30 ABSL_NAMESPACE_BEGIN
31
32 // absl::exponential_distribution:
33 // Generates a number conforming to an exponential distribution and is
34 // equivalent to the standard [rand.dist.pois.exp] distribution.
35 template <typename RealType = double>
36 class exponential_distribution {
37 public:
38 using result_type = RealType;
39
40 class param_type {
41 public:
42 using distribution_type = exponential_distribution;
43
lambda_(lambda)44 explicit param_type(result_type lambda = 1) : lambda_(lambda) {
45 assert(lambda > 0);
46 neg_inv_lambda_ = -result_type(1) / lambda_;
47 }
48
lambda()49 result_type lambda() const { return lambda_; }
50
51 friend bool operator==(const param_type& a, const param_type& b) {
52 return a.lambda_ == b.lambda_;
53 }
54
55 friend bool operator!=(const param_type& a, const param_type& b) {
56 return !(a == b);
57 }
58
59 private:
60 friend class exponential_distribution;
61
62 result_type lambda_;
63 result_type neg_inv_lambda_;
64
65 static_assert(
66 std::is_floating_point<RealType>::value,
67 "Class-template absl::exponential_distribution<> must be parameterized "
68 "using a floating-point type.");
69 };
70
exponential_distribution()71 exponential_distribution() : exponential_distribution(1) {}
72
exponential_distribution(result_type lambda)73 explicit exponential_distribution(result_type lambda) : param_(lambda) {}
74
exponential_distribution(const param_type & p)75 explicit exponential_distribution(const param_type& p) : param_(p) {}
76
reset()77 void reset() {}
78
79 // Generating functions
80 template <typename URBG>
operator()81 result_type operator()(URBG& g) { // NOLINT(runtime/references)
82 return (*this)(g, param_);
83 }
84
85 template <typename URBG>
86 result_type operator()(URBG& g, // NOLINT(runtime/references)
87 const param_type& p);
88
param()89 param_type param() const { return param_; }
param(const param_type & p)90 void param(const param_type& p) { param_ = p; }
91
result_type(min)92 result_type(min)() const { return 0; }
result_type(max)93 result_type(max)() const {
94 return std::numeric_limits<result_type>::infinity();
95 }
96
lambda()97 result_type lambda() const { return param_.lambda(); }
98
99 friend bool operator==(const exponential_distribution& a,
100 const exponential_distribution& b) {
101 return a.param_ == b.param_;
102 }
103 friend bool operator!=(const exponential_distribution& a,
104 const exponential_distribution& b) {
105 return a.param_ != b.param_;
106 }
107
108 private:
109 param_type param_;
110 random_internal::FastUniformBits<uint64_t> fast_u64_;
111 };
112
113 // --------------------------------------------------------------------------
114 // Implementation details follow
115 // --------------------------------------------------------------------------
116
117 template <typename RealType>
118 template <typename URBG>
119 typename exponential_distribution<RealType>::result_type
operator()120 exponential_distribution<RealType>::operator()(
121 URBG& g, // NOLINT(runtime/references)
122 const param_type& p) {
123 using random_internal::GenerateNegativeTag;
124 using random_internal::GenerateRealFromBits;
125 using real_type =
126 absl::conditional_t<std::is_same<RealType, float>::value, float, double>;
127
128 const result_type u = GenerateRealFromBits<real_type, GenerateNegativeTag,
129 false>(fast_u64_(g)); // U(-1, 0)
130
131 // log1p(-x) is mathematically equivalent to log(1 - x) but has more
132 // accuracy for x near zero.
133 return p.neg_inv_lambda_ * std::log1p(u);
134 }
135
136 template <typename CharT, typename Traits, typename RealType>
137 std::basic_ostream<CharT, Traits>& operator<<(
138 std::basic_ostream<CharT, Traits>& os, // NOLINT(runtime/references)
139 const exponential_distribution<RealType>& x) {
140 auto saver = random_internal::make_ostream_state_saver(os);
141 os.precision(random_internal::stream_precision_helper<RealType>::kPrecision);
142 os << x.lambda();
143 return os;
144 }
145
146 template <typename CharT, typename Traits, typename RealType>
147 std::basic_istream<CharT, Traits>& operator>>(
148 std::basic_istream<CharT, Traits>& is, // NOLINT(runtime/references)
149 exponential_distribution<RealType>& x) { // NOLINT(runtime/references)
150 using result_type = typename exponential_distribution<RealType>::result_type;
151 using param_type = typename exponential_distribution<RealType>::param_type;
152 result_type lambda;
153
154 auto saver = random_internal::make_istream_state_saver(is);
155 lambda = random_internal::read_floating_point<result_type>(is);
156 if (!is.fail()) {
157 x.param(param_type(lambda));
158 }
159 return is;
160 }
161
162 ABSL_NAMESPACE_END
163 } // namespace absl
164
165 #endif // ABSL_RANDOM_EXPONENTIAL_DISTRIBUTION_H_
166