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37 * \brief The uniform real distribution
39 * Portable version of the uniform real that generates the same sequence
40 * on all platforms. Since stdlibc++ and libc++ provide different sequences
41 * we prefer this one so unit tests produce the same values on all platforms.
43 * \author Erik Lindahl <erik.lindahl@gmail.com>
45 * \ingroup module_random
48 #ifndef GMX_RANDOM_UNIFORMREALDISTRIBUTION_H
49 #define GMX_RANDOM_UNIFORMREALDISTRIBUTION_H
55 #include <type_traits>
57 #include "gromacs/math/functions.h"
58 #include "gromacs/utility/basedefinitions.h"
59 #include "gromacs/utility/classhelpers.h"
60 #include "gromacs/utility/gmxassert.h"
61 #include "gromacs/utility/real.h"
64 * The portable version of the uniform real distribution (to make sure we get
65 * the same values on all platforms) has been modified from the LLVM libcxx
66 * headers, distributed under the MIT license:
68 * Copyright (c) The LLVM compiler infrastructure
70 * Permission is hereby granted, free of charge, to any person obtaining a copy
71 * of this software and associated documentation files (the "Software"), to deal
72 * in the Software without restriction, including without limitation the rights
73 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
74 * copies of the Software, and to permit persons to whom the Software is
75 * furnished to do so, subject to the following conditions:
77 * The above copyright notice and this permission notice shall be included in
78 * all copies or substantial portions of the Software.
80 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
81 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
82 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
83 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
84 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
85 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
92 /*! \brief Generate a floating-point value with specified number of random bits
94 * \tparam RealType Floating-point type to generate
95 * \tparam Bits Number of random bits to generate
96 * \tparam Rng Random number generator class
98 * \param g Random number generator to use
100 * This implementation avoids the bug in libc++ and stdlibc++ (which is due
101 * to the C++ standard being unclear) where 1.0 can be returned occasionally.
104 template<class RealType = real, unsigned int Bits, class Rng>
105 RealType generateCanonical(Rng& g)
107 // No point in using more bits than fit in RealType
108 const uint64_t digits = std::numeric_limits<RealType>::digits;
109 const uint64_t realBits = std::min(digits, static_cast<uint64_t>(Bits));
110 const uint64_t log2R = std::numeric_limits<typename Rng::result_type>::digits;
111 uint64_t k = realBits / log2R + (realBits % log2R != 0) + (realBits == 0);
112 // Note that Rng::max and Rng::min are typically an integer type.
113 // Only unsigned integer types can express the range using the
114 // same type. Converting to RealType before computing the range
115 // would work but we have no need for that.
116 static_assert(std::is_unsigned_v<decltype(Rng::max())> && std::is_unsigned_v<decltype(Rng::min())>,
117 "Rng::max and Rng::min must be unsigned");
118 RealType r = RealType(Rng::max() - Rng::min()) + RealType(1);
119 RealType s = g() - Rng::min();
123 for (uint64_t i = 1; i < k; ++i)
125 s += RealType(g() - Rng::min()) * base;
130 // This implementation is specified by the C++ standard, but unfortunately it
131 // has a bug where 1.0 can be generated occasionally due to the limited
132 // precision of floating point, while 0.0 is only generated half as often as
133 // it should. We "solve" both these issues by swapping 1.0 for 0.0 when it happens.
136 // https://llvm.org/bugs/show_bug.cgi?id=18767
137 // https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63176
139 // Note that we prefer not to use the gcc 'fix' of looping until the result
140 // is smaller than 1.0, since that breaks the strict specification of the
141 // number of times the rng will be called.
143 // This can only happen when we ask for the same number of bits that fit
144 // in RealType, so by checking for that we avoid the extra code in all other
145 // cases. If you are worried about it: Use RealType=double with 32 bits.
147 if (realBits == digits && result == 1.0)
155 /*! \brief Uniform real distribution
157 * The C++ standard library does provide this distribution, but even
158 * though they all sample from the correct distribution different standard
159 * library implementations appear to return different sequences of numbers
160 * for the same random number generator. To make it easier to use GROMACS
161 * unit tests that depend on random numbers we have our own implementation.
163 * \tparam RealType Floating-point type, real by default in GROMACS.
165 template<class RealType = real>
166 class UniformRealDistribution
169 /*! \brief Type of values returned */
170 typedef RealType result_type;
172 /*! \brief Uniform real distribution parameters */
175 /*! \brief Lower end of range (inclusive) */
177 /*! \brief Upper end of range (exclusive) */
181 /*! \brief Reference back to the distribution class */
182 typedef UniformRealDistribution distribution_type;
184 /*! \brief Construct parameter block
186 * \param a Lower end of range (inclusive)
187 * \param b Upper end of range (exclusive)
189 explicit param_type(result_type a = 0.0, result_type b = 1.0) : a_(a), b_(b)
191 GMX_RELEASE_ASSERT(a < b, "The uniform real distribution requires a<b");
194 /*! \brief Return first parameter */
195 result_type a() const { return a_; }
196 /*! \brief Return second parameter */
197 result_type b() const { return b_; }
199 /*! \brief True if two parameter sets will return the same uniform real distribution.
201 * \param x Instance to compare with.
203 bool operator==(const param_type& x) const { return a_ == x.a_ && b_ == x.b_; }
205 /*! \brief True if two parameter sets will return different uniform real distributions
207 * \param x Instance to compare with.
209 bool operator!=(const param_type& x) const { return !operator==(x); }
213 /*! \brief Construct new distribution with given floating-point parameters.
215 * \param a Lower end of range (inclusive)
216 * \param b Upper end of range (exclusive)
218 explicit UniformRealDistribution(result_type a = 0.0, result_type b = 1.0) :
219 param_(param_type(a, b))
223 /*! \brief Construct new distribution from parameter class
225 * \param param Parameter class as defined inside gmx::UniformRealDistribution.
227 explicit UniformRealDistribution(const param_type& param) : param_(param) {}
229 /*! \brief Flush all internal saved values */
232 /*! \brief Return values from uniform real distribution with internal parameters
234 * \tparam Rng Random engine class
236 * \param g Random engine
239 result_type operator()(Rng& g)
241 return (*this)(g, param_);
244 /*! \brief Return value from uniform real distribution with given parameters
246 * \tparam Rng Random engine class
248 * \param g Random engine
249 * \param param Parameters to use
252 result_type operator()(Rng& g, const param_type& param)
254 result_type r = generateCanonical<RealType, std::numeric_limits<RealType>::digits>(g);
255 return (param.b() - param.a()) * r + param.a();
258 /*! \brief Return the lower range uniform real distribution */
259 result_type a() const { return param_.a(); }
261 /*! \brief Return the upper range of the uniform real distribution */
262 result_type b() const { return param_.b(); }
264 /*! \brief Return the full parameter class of the uniform real distribution */
265 param_type param() const { return param_; }
267 /*! \brief Smallest value that can be returned from uniform real distribution */
268 result_type min() const { return a(); }
270 /*! \brief Largest value that can be returned from uniform real distribution */
271 result_type max() const { return b(); }
273 /*! \brief True if two uniform real distributions will produce the same values.
275 * \param x Instance to compare with.
277 bool operator==(const UniformRealDistribution& x) const { return param_ == x.param_; }
279 /*! \brief True if two uniform real distributions will produce different values.
281 * \param x Instance to compare with.
283 bool operator!=(const UniformRealDistribution& x) const { return !operator==(x); }
286 /*! \brief Internal value for parameters, can be overridden at generation time. */
289 GMX_DISALLOW_COPY_AND_ASSIGN(UniformRealDistribution);
294 #endif // GMX_RANDOM_UNIFORMREALDISTRIBUTION_H