1 /** 2 * 3 * Copyright (c) 2005-2021 by Pierre-Henri WUILLEMIN(_at_LIP6) & Christophe GONZALES(_at_AMU) 4 * info_at_agrum_dot_org 5 * 6 * This library is free software: you can redistribute it and/or modify 7 * it under the terms of the GNU Lesser General Public License as published by 8 * the Free Software Foundation, either version 3 of the License, or 9 * (at your option) any later version. 10 * 11 * This library is distributed in the hope that it will be useful, 12 * but WITHOUT ANY WARRANTY; without even the implied warranty of 13 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the 14 * GNU Lesser General Public License for more details. 15 * 16 * You should have received a copy of the GNU Lesser General Public License 17 * along with this library. If not, see <http://www.gnu.org/licenses/>. 18 * 19 */ 20 21 22 /** 23 * @file 24 * @brief The class for computing Log2 (Gamma(x)). 25 * 26 * @author Christophe GONZALES(_at_AMU) and Pierre-Henri WUILLEMIN(_at_LIP6) 27 */ 28 29 namespace gum { 30 gammaLog2(double x)31 ALWAYS_INLINE double GammaLog2::gammaLog2(double x) const { 32 if (x <= 0) GUM_ERROR(OutOfBounds, "log2(gamma()) should be called with a positive argument") 33 34 // if x is small, use precomputed values 35 if (x < 50) { 36 if (x >= 0.01) { 37 if (_requires_precision_) { 38 const Idx index = int(x * 100); 39 return _small_values_[index] 40 + (_small_values_[index + 1] - _small_values_[index]) * double(x * 100 - index); 41 } else { 42 const Idx index = int(x * 100 + 0.5); 43 return _small_values_[index]; 44 } 45 } else { 46 // for very small values of x, Gamma(x) is approximately equal to 47 // 1/x. Hence gammaLog2(x) is approximately equal to log2(1/x) 48 return std::log2(1.0 / x); 49 } 50 } 51 52 // returns the approximation by the stirling formula 53 return (_log_sqrt_2pi_ + (x - 0.5f) * log(x) - x + log(1.0 + 1.0 / (12 * x))) * _inv_log2_; 54 } 55 operator()56 ALWAYS_INLINE double GammaLog2::operator()(double x) const { return gammaLog2(x); } 57 setPrecision(bool prec)58 INLINE void GammaLog2::setPrecision(bool prec) { _requires_precision_ = prec; } 59 60 } /* namespace gum */ 61