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0001 //------------------------------- -*- C++ -*- -------------------------------// 0002 // Copyright Celeritas contributors: see top-level COPYRIGHT file for details 0003 // SPDX-License-Identifier: (Apache-2.0 OR MIT) 0004 //---------------------------------------------------------------------------// 0005 //! \file corecel/random/distribution/UniformRealDistribution.hh 0006 //---------------------------------------------------------------------------// 0007 #pragma once 0008 0009 #include <cmath> 0010 #include <type_traits> 0011 0012 #include "corecel/Assert.hh" 0013 #include "corecel/Macros.hh" 0014 #include "corecel/Types.hh" 0015 0016 #include "GenerateCanonical.hh" 0017 0018 namespace celeritas 0019 { 0020 //---------------------------------------------------------------------------// 0021 /*! 0022 * Sample from a uniform distribution. 0023 * 0024 * This distribution is defined between two arbitrary real numbers \em a and 0025 * \em b , and has a flat PDF between the two values. It \em is allowable for 0026 * the two numbers to have reversed order. 0027 * The normalized PDF is: 0028 * \f[ 0029 f(x; a, b) = \frac{1}{b - a} \quad \mathrm{for} \ a \le x < b 0030 \f] 0031 * which integrated into a CDF and inverted gives a sample: 0032 * \f[ 0033 x = (b - a) \xi + a 0034 \f] 0035 */ 0036 template<class RealType = ::celeritas::real_type> 0037 class UniformRealDistribution 0038 { 0039 static_assert(std::is_floating_point_v<RealType>); 0040 0041 public: 0042 //!@{ 0043 //! \name Type aliases 0044 using real_type = RealType; 0045 using result_type = real_type; 0046 //!@} 0047 0048 public: 0049 // Construct on [0, 1) 0050 CELER_CONSTEXPR_FUNCTION UniformRealDistribution(); 0051 0052 // Construct on an arbitrary interval 0053 CELER_CONSTEXPR_FUNCTION UniformRealDistribution(real_type a, real_type b); 0054 0055 // Sample a random number according to the distribution 0056 template<class Generator> 0057 inline CELER_FUNCTION result_type operator()(Generator& rng) const; 0058 0059 //// ACCESSORS //// 0060 0061 //! Get the lower bound of the distribution 0062 CELER_CONSTEXPR_FUNCTION real_type a() const { return a_; } 0063 0064 //! Get the upper bound of the distribution 0065 CELER_CONSTEXPR_FUNCTION real_type b() const { return delta_ + a_; } 0066 0067 private: 0068 RealType a_; 0069 RealType delta_; 0070 }; 0071 0072 //---------------------------------------------------------------------------// 0073 // INLINE DEFINITIONS 0074 //---------------------------------------------------------------------------// 0075 /*! 0076 * Construct on the interval [0, 1). 0077 * 0078 * This constructor is generally unused because it's simpler and more efficient 0079 * to directly call ``generate_canonical``. We leave it for compatibility with 0080 * the standard. 0081 */ 0082 template<class RealType> 0083 CELER_CONSTEXPR_FUNCTION 0084 UniformRealDistribution<RealType>::UniformRealDistribution() 0085 : UniformRealDistribution(0, 1) 0086 { 0087 } 0088 0089 //---------------------------------------------------------------------------// 0090 /*! 0091 * Construct on the interval [a, b). 0092 * 0093 * Note that it's allowable for these two to be out of order to support other 0094 * generators (inverse square, power) where they may be inverted and out of 0095 * order. 0096 */ 0097 template<class RealType> 0098 CELER_CONSTEXPR_FUNCTION 0099 UniformRealDistribution<RealType>::UniformRealDistribution(real_type a, 0100 real_type b) 0101 : a_(a), delta_(b - a) 0102 { 0103 } 0104 0105 //---------------------------------------------------------------------------// 0106 /*! 0107 * Sample a random number according to the distribution. 0108 */ 0109 template<class RealType> 0110 template<class Generator> 0111 CELER_FUNCTION auto 0112 UniformRealDistribution<RealType>::operator()(Generator& rng) const -> result_type 0113 { 0114 return std::fma(delta_, generate_canonical<RealType>(rng), a_); 0115 } 0116 0117 //---------------------------------------------------------------------------// 0118 } // namespace celeritas
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