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File indexing completed on 2025-09-14 08:50:58
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/InverseSquareDistribution.hh 0006 //---------------------------------------------------------------------------// 0007 #pragma once 0008 0009 #include "corecel/Assert.hh" 0010 #include "corecel/Macros.hh" 0011 #include "corecel/Types.hh" 0012 0013 #include "UniformRealDistribution.hh" 0014 0015 namespace celeritas 0016 { 0017 //---------------------------------------------------------------------------// 0018 /*! 0019 * Sample \f$ 1/x^2 \f$ over a given domain. 0020 * 0021 * This distribution is defined on a positive range \f$ [a, b) \f$ and has the 0022 * normalized PDF: 0023 * \f[ 0024 f(x; a, b) = \frac{a b}{x^2 (b - a)} \quad \mathrm{for} \ a \le x < b 0025 \f] 0026 * which integrated into a CDF and inverted gives a sample: 0027 * \f[ 0028 x = \frac{a b}{a + \xi (b - a)} 0029 \f] 0030 */ 0031 template<class RealType = ::celeritas::real_type> 0032 class InverseSquareDistribution 0033 { 0034 public: 0035 //!@{ 0036 //! \name Type aliases 0037 using real_type = RealType; 0038 using result_type = real_type; 0039 //!@} 0040 0041 public: 0042 // Construct on an interval 0043 inline CELER_FUNCTION InverseSquareDistribution(real_type a, real_type b); 0044 0045 // Sample a random number according to the distribution 0046 template<class Generator> 0047 inline CELER_FUNCTION result_type operator()(Generator& rng) const; 0048 0049 private: 0050 RealType product_; 0051 UniformRealDistribution<RealType> sample_denom_; 0052 }; 0053 0054 //---------------------------------------------------------------------------// 0055 // INLINE DEFINITIONS 0056 //---------------------------------------------------------------------------// 0057 /*! 0058 * Construct on the interval [a, b). 0059 * 0060 * As with UniformRealDistribution, it is allowable for the two bounds to be 0061 * out of order. 0062 */ 0063 template<class RealType> 0064 CELER_FUNCTION 0065 InverseSquareDistribution<RealType>::InverseSquareDistribution(real_type a, 0066 real_type b) 0067 : product_{a * b}, sample_denom_{a, b} 0068 { 0069 CELER_EXPECT(a > 0); 0070 CELER_EXPECT(b >= a); 0071 } 0072 0073 //---------------------------------------------------------------------------// 0074 /*! 0075 * Sample a random number according to the distribution. 0076 */ 0077 template<class RealType> 0078 template<class Generator> 0079 CELER_FUNCTION auto 0080 InverseSquareDistribution<RealType>::operator()(Generator& rng) const -> result_type 0081 { 0082 return product_ / sample_denom_(rng); 0083 } 0084 0085 //---------------------------------------------------------------------------// 0086 } // namespace celeritas
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