gammaLog2_inl.h

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/***************************************************************************
 *   Copyright (C) 2005 by Christophe GONZALES and Pierre-Henri WUILLEMIN  *
 *   {prenom.nom}_at_lip6.fr                                               *
 *                                                                         *
 *   This program is free software; you can redistribute it and/or modify  *
 *   it under the terms of the GNU General Public License as published by  *
 *   the Free Software Foundation; either version 2 of the License, or     *
 *   (at your option) any later version.                                   *
 *                                                                         *
 *   This program is distributed in the hope that it will be useful,       *
 *   but WITHOUT ANY WARRANTY; without even the implied warranty of        *
 *   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the         *
 *   GNU General Public License for more details.                          *
 *                                                                         *
 *   You should have received a copy of the GNU General Public License     *
 *   along with this program; if not, write to the                         *
 *   Free Software Foundation, Inc.,                                       *
 *   59 Temple Place - Suite 330, Boston, MA  02111-1307, USA.             *
 ***************************************************************************/
namespace gum {

  ALWAYS_INLINE double GammaLog2::gammaLog2(double x) const {
    if (x <= 0)
      GUM_ERROR(OutOfBounds,
                "log2(gamma()) should be called with a positive argument");

    // if x is small, use precomputed values
    if (x < 50) {
      if (__requires_precision) {
        Idx index = int(x * 100);
        return __small_values[index]
               + (__small_values[index + 1] - __small_values[index])
                    * double(x * 100 - index);
      } else {
        Idx index = int(x * 100 + 0.5);
        return __small_values[index];
      }
    }

    // returns the approximation by the stirling formula
    return (__log_sqrt_2pi + (x - 0.5f) * log(x) - x + log(1.0 + 1.0 / (12 * x)))
           * __1log2;
  }

  ALWAYS_INLINE double GammaLog2::operator()(double x) const {
    return gammaLog2(x);
  }

  INLINE void GammaLog2::setPrecision(bool prec) { __requires_precision = prec; }

} /* namespace gum */