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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<decltype(Rng::max())>::value
117 && std::is_unsigned<decltype(Rng::min())>::value,
118 "Rng::max and Rng::min must be unsigned");
119 RealType r = RealType(Rng::max() - Rng::min()) + RealType(1);
120 RealType s = g() - Rng::min();
124 for (uint64_t i = 1; i < k; ++i)
126 s += RealType(g() - Rng::min()) * base;
131 // This implementation is specified by the C++ standard, but unfortunately it
132 // has a bug where 1.0 can be generated occasionally due to the limited
133 // precision of floating point, while 0.0 is only generated half as often as
134 // it should. We "solve" both these issues by swapping 1.0 for 0.0 when it happens.
137 // https://llvm.org/bugs/show_bug.cgi?id=18767
138 // https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63176
140 // Note that we prefer not to use the gcc 'fix' of looping until the result
141 // is smaller than 1.0, since that breaks the strict specification of the
142 // number of times the rng will be called.
144 // This can only happen when we ask for the same number of bits that fit
145 // in RealType, so by checking for that we avoid the extra code in all other
146 // cases. If you are worried about it: Use RealType=double with 32 bits.
148 if (realBits == digits && result == 1.0)
156 /*! \brief Uniform real distribution
158 * The C++ standard library does provide this distribution, but even
159 * though they all sample from the correct distribution different standard
160 * library implementations appear to return different sequences of numbers
161 * for the same random number generator. To make it easier to use GROMACS
162 * unit tests that depend on random numbers we have our own implementation.
164 * \tparam RealType Floating-point type, real by default in GROMACS.
166 template<class RealType = real>
167 class UniformRealDistribution
170 /*! \brief Type of values returned */
171 typedef RealType result_type;
173 /*! \brief Uniform real distribution parameters */
176 /*! \brief Lower end of range (inclusive) */
178 /*! \brief Upper end of range (exclusive) */
182 /*! \brief Reference back to the distribution class */
183 typedef UniformRealDistribution distribution_type;
185 /*! \brief Construct parameter block
187 * \param a Lower end of range (inclusive)
188 * \param b Upper end of range (exclusive)
190 explicit param_type(result_type a = 0.0, result_type b = 1.0) : a_(a), b_(b)
192 GMX_RELEASE_ASSERT(a < b, "The uniform real distribution requires a<b");
195 /*! \brief Return first parameter */
196 result_type a() const { return a_; }
197 /*! \brief Return second parameter */
198 result_type b() const { return b_; }
200 /*! \brief True if two parameter sets will return the same uniform real distribution.
202 * \param x Instance to compare with.
204 bool operator==(const param_type& x) const { return a_ == x.a_ && b_ == x.b_; }
206 /*! \brief True if two parameter sets will return different uniform real distributions
208 * \param x Instance to compare with.
210 bool operator!=(const param_type& x) const { return !operator==(x); }
214 /*! \brief Construct new distribution with given floating-point parameters.
216 * \param a Lower end of range (inclusive)
217 * \param b Upper end of range (exclusive)
219 explicit UniformRealDistribution(result_type a = 0.0, result_type b = 1.0) :
220 param_(param_type(a, b))
224 /*! \brief Construct new distribution from parameter class
226 * \param param Parameter class as defined inside gmx::UniformRealDistribution.
228 explicit UniformRealDistribution(const param_type& param) : param_(param) {}
230 /*! \brief Flush all internal saved values */
233 /*! \brief Return values from uniform real distribution with internal parameters
235 * \tparam Rng Random engine class
237 * \param g Random engine
240 result_type operator()(Rng& g)
242 return (*this)(g, param_);
245 /*! \brief Return value from uniform real distribution with given parameters
247 * \tparam Rng Random engine class
249 * \param g Random engine
250 * \param param Parameters to use
253 result_type operator()(Rng& g, const param_type& param)
255 result_type r = generateCanonical<RealType, std::numeric_limits<RealType>::digits>(g);
256 return (param.b() - param.a()) * r + param.a();
259 /*! \brief Return the lower range uniform real distribution */
260 result_type a() const { return param_.a(); }
262 /*! \brief Return the upper range of the uniform real distribution */
263 result_type b() const { return param_.b(); }
265 /*! \brief Return the full parameter class of the uniform real distribution */
266 param_type param() const { return param_; }
268 /*! \brief Smallest value that can be returned from uniform real distribution */
269 result_type min() const { return a(); }
271 /*! \brief Largest value that can be returned from uniform real distribution */
272 result_type max() const { return b(); }
274 /*! \brief True if two uniform real distributions will produce the same values.
276 * \param x Instance to compare with.
278 bool operator==(const UniformRealDistribution& x) const { return param_ == x.param_; }
280 /*! \brief True if two uniform real distributions will produce different values.
282 * \param x Instance to compare with.
284 bool operator!=(const UniformRealDistribution& x) const { return !operator==(x); }
287 /*! \brief Internal value for parameters, can be overridden at generation time. */
290 GMX_DISALLOW_COPY_AND_ASSIGN(UniformRealDistribution);
295 #endif // GMX_RANDOM_UNIFORMREALDISTRIBUTION_H