1 // Copyright 2017 The Abseil Authors.
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
7 // https://www.apache.org/licenses/LICENSE-2.0
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.
15 // -----------------------------------------------------------------------------
16 // File: uniform_int_distribution.h
17 // -----------------------------------------------------------------------------
19 // This header defines a class for representing a uniform integer distribution
20 // over the closed (inclusive) interval [a,b]. You use this distribution in
21 // combination with an Abseil random bit generator to produce random values
22 // according to the rules of the distribution.
24 // `absl::uniform_int_distribution` is a drop-in replacement for the C++11
25 // `std::uniform_int_distribution` [rand.dist.uni.int] but is considerably
26 // faster than the libstdc++ implementation.
28 #ifndef ABSL_RANDOM_UNIFORM_INT_DISTRIBUTION_H_
29 #define ABSL_RANDOM_UNIFORM_INT_DISTRIBUTION_H_
34 #include <type_traits>
36 #include "absl/base/optimization.h"
37 #include "absl/random/internal/distribution_impl.h"
38 #include "absl/random/internal/fast_uniform_bits.h"
39 #include "absl/random/internal/iostream_state_saver.h"
40 #include "absl/random/internal/traits.h"
44 // absl::uniform_int_distribution<T>
46 // This distribution produces random integer values uniformly distributed in the
47 // closed (inclusive) interval [a, b].
53 // // Use the distribution to produce a value between 1 and 6, inclusive.
54 // int die_roll = absl::uniform_int_distribution<int>(1, 6)(gen);
56 template <typename IntType = int>
57 class uniform_int_distribution {
60 typename random_internal::make_unsigned_bits<IntType>::type;
63 using result_type = IntType;
67 using distribution_type = uniform_int_distribution;
71 result_type hi = (std::numeric_limits<result_type>::max)())
73 range_(static_cast<unsigned_type>(hi) -
74 static_cast<unsigned_type>(lo)) {
75 // [rand.dist.uni.int] precondition 2
79 result_type a() const { return lo_; }
80 result_type b() const {
81 return static_cast<result_type>(static_cast<unsigned_type>(lo_) + range_);
84 friend bool operator==(const param_type& a, const param_type& b) {
85 return a.lo_ == b.lo_ && a.range_ == b.range_;
88 friend bool operator!=(const param_type& a, const param_type& b) {
93 friend class uniform_int_distribution;
94 unsigned_type range() const { return range_; }
99 static_assert(std::is_integral<result_type>::value,
100 "Class-template absl::uniform_int_distribution<> must be "
101 "parameterized using an integral type.");
104 uniform_int_distribution() : uniform_int_distribution(0) {}
106 explicit uniform_int_distribution(
108 result_type hi = (std::numeric_limits<result_type>::max)())
111 explicit uniform_int_distribution(const param_type& param) : param_(param) {}
113 // uniform_int_distribution<T>::reset()
115 // Resets the uniform int distribution. Note that this function has no effect
116 // because the distribution already produces independent values.
119 template <typename URBG>
120 result_type operator()(URBG& gen) { // NOLINT(runtime/references)
121 return (*this)(gen, param());
124 template <typename URBG>
125 result_type operator()(
126 URBG& gen, const param_type& param) { // NOLINT(runtime/references)
127 return param.a() + Generate(gen, param.range());
130 result_type a() const { return param_.a(); }
131 result_type b() const { return param_.b(); }
133 param_type param() const { return param_; }
134 void param(const param_type& params) { param_ = params; }
136 result_type(min)() const { return a(); }
137 result_type(max)() const { return b(); }
139 friend bool operator==(const uniform_int_distribution& a,
140 const uniform_int_distribution& b) {
141 return a.param_ == b.param_;
143 friend bool operator!=(const uniform_int_distribution& a,
144 const uniform_int_distribution& b) {
149 // Generates a value in the *closed* interval [0, R]
150 template <typename URBG>
151 unsigned_type Generate(URBG& g, // NOLINT(runtime/references)
156 // -----------------------------------------------------------------------------
157 // Implementation details follow
158 // -----------------------------------------------------------------------------
159 template <typename CharT, typename Traits, typename IntType>
160 std::basic_ostream<CharT, Traits>& operator<<(
161 std::basic_ostream<CharT, Traits>& os,
162 const uniform_int_distribution<IntType>& x) {
164 typename random_internal::stream_format_type<IntType>::type;
165 auto saver = random_internal::make_ostream_state_saver(os);
166 os << static_cast<stream_type>(x.a()) << os.fill()
167 << static_cast<stream_type>(x.b());
171 template <typename CharT, typename Traits, typename IntType>
172 std::basic_istream<CharT, Traits>& operator>>(
173 std::basic_istream<CharT, Traits>& is,
174 uniform_int_distribution<IntType>& x) {
175 using param_type = typename uniform_int_distribution<IntType>::param_type;
176 using result_type = typename uniform_int_distribution<IntType>::result_type;
178 typename random_internal::stream_format_type<IntType>::type;
183 auto saver = random_internal::make_istream_state_saver(is);
187 param_type(static_cast<result_type>(a), static_cast<result_type>(b)));
192 template <typename IntType>
193 template <typename URBG>
194 typename random_internal::make_unsigned_bits<IntType>::type
195 uniform_int_distribution<IntType>::Generate(
196 URBG& g, // NOLINT(runtime/references)
197 typename random_internal::make_unsigned_bits<IntType>::type R) {
198 random_internal::FastUniformBits<unsigned_type> fast_bits;
199 unsigned_type bits = fast_bits(g);
200 const unsigned_type Lim = R + 1;
201 if ((R & Lim) == 0) {
202 // If the interval's length is a power of two range, just take the low bits.
206 // Generates a uniform variate on [0, Lim) using fixed-point multiplication.
207 // The above fast-path guarantees that Lim is representable in unsigned_type.
209 // Algorithm adapted from
210 // http://lemire.me/blog/2016/06/30/fast-random-shuffling/, with added
213 // The algorithm creates a uniform variate `bits` in the interval [0, 2^N),
214 // and treats it as the fractional part of a fixed-point real value in [0, 1),
215 // multiplied by 2^N. For example, 0.25 would be represented as 2^(N - 2),
216 // because 2^N * 0.25 == 2^(N - 2).
218 // Next, `bits` and `Lim` are multiplied with a wide-multiply to bring the
219 // value into the range [0, Lim). The integral part (the high word of the
220 // multiplication result) is then very nearly the desired result. However,
221 // this is not quite accurate; viewing the multiplication result as one
222 // double-width integer, the resulting values for the sample are mapped as
225 // If the result lies in this interval: Return this value:
229 // [K * 2^N, (K + 1) * 2^N) K
231 // [(Lim - 1) * 2^N, Lim * 2^N) Lim - 1
233 // While all of these intervals have the same size, the result of `bits * Lim`
234 // must be a multiple of `Lim`, and not all of these intervals contain the
235 // same number of multiples of `Lim`. In particular, some contain
236 // `F = floor(2^N / Lim)` and some contain `F + 1 = ceil(2^N / Lim)`. This
237 // difference produces a small nonuniformity, which is corrected by applying
238 // rejection sampling to one of the values in the "larger intervals" (i.e.,
239 // the intervals containing `F + 1` multiples of `Lim`.
241 // An interval contains `F + 1` multiples of `Lim` if and only if its smallest
242 // value modulo 2^N is less than `2^N % Lim`. The unique value satisfying
243 // this property is used as the one for rejection. That is, a value of
244 // `bits * Lim` is rejected if `(bit * Lim) % 2^N < (2^N % Lim)`.
246 using helper = random_internal::wide_multiply<unsigned_type>;
247 auto product = helper::multiply(bits, Lim);
249 // Two optimizations here:
250 // * Rejection occurs with some probability less than 1/2, and for reasonable
251 // ranges considerably less (in particular, less than 1/(F+1)), so
252 // ABSL_PREDICT_FALSE is apt.
253 // * `Lim` is an overestimate of `threshold`, and doesn't require a divide.
254 if (ABSL_PREDICT_FALSE(helper::lo(product) < Lim)) {
255 // This quantity is exactly equal to `2^N % Lim`, but does not require high
256 // precision calculations: `2^N % Lim` is congruent to `(2^N - Lim) % Lim`.
257 // Ideally this could be expressed simply as `-X` rather than `2^N - X`, but
258 // for types smaller than int, this calculation is incorrect due to integer
260 const unsigned_type threshold =
261 ((std::numeric_limits<unsigned_type>::max)() - Lim + 1) % Lim;
262 while (helper::lo(product) < threshold) {
264 product = helper::multiply(bits, Lim);
268 return helper::hi(product);
273 #endif // ABSL_RANDOM_UNIFORM_INT_DISTRIBUTION_H_