SimulatedBinaryCrossover.h

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/*!
 * 
 *
 * \brief       Simulated binary crossover operator.
 * 
 * 
 *
 * \author      T.Voss
 * \date        2010
 *
 *
 * \par Copyright 1995-2017 Shark Development Team
 * 
 * <BR><HR>
 * This file is part of Shark.
 * <http://shark-ml.org/>
 * 
 * Shark is free software: you can redistribute it and/or modify
 * it under the terms of the GNU Lesser General Public License as published 
 * by the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 * 
 * Shark 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 Lesser General Public License for more details.
 * 
 * You should have received a copy of the GNU Lesser General Public License
 * along with Shark.  If not, see <http://www.gnu.org/licenses/>.
 *
 */
#ifndef SHARK_ALGORITHMS_DIRECT_SEARCH_OPERATORS_CROSSOVER_SBX_H
#define SHARK_ALGORITHMS_DIRECT_SEARCH_OPERATORS_CROSSOVER_SBX_H

#include <shark/Core/Random.h>
#include <shark/ObjectiveFunctions/BoxConstraintHandler.h>

namespace shark {

    /// \brief Simulated binary crossover operator.
    template<typename PointType>
    struct SimulatedBinaryCrossover {

        SimulatedBinaryCrossover() 
        : m_nc( 20.0 )
        , m_prob( 0.5 ) {}

        /// \brief Initializes the operator for the supplied box-constraints
        void init( RealVector const& lower, RealVector const& upper ) {
            SIZE_CHECK(lower.size() == upper.size());
            m_prob = 1./lower.size();
            m_lower = lower;
            m_upper = upper;
        }

        /// \brief Mates the supplied individuals.
        /// 
        /// \param [in,out] i1 Individual to be mated.
        /// \param [in,out] i2 Individual to be mated.
        template<class randomType, typename IndividualType>
        void operator()(randomType& rng, IndividualType & i1, IndividualType & i2 )const{   
            RealVector& point1 = i1.searchPoint();
            RealVector& point2 = i2.searchPoint();

            for( unsigned int i = 0; i < point1.size(); i++ ) {

                if( !random::coinToss(rng, m_prob ) )
                    continue;
                
                double y1 = 0;
                double y2 = 0;
                if( point2[i] < point1[i] ) {
                    y1 = point2[i];
                    y2 = point1[i];
                } else {
                    y1 = point1[i];
                    y2 = point2[i];
                }
                
                double betaQ1 = 0.0;
                double betaQ2 = 0.0;
                if( std::abs(y2 - y1) < 1E-7 )continue;//equal
                
                // Find beta value2
                double beta1 = 1 + 2 * (y1 - m_lower( i )) / (y2 - y1);
                double beta2 = 1 + 2 * (m_upper( i ) - y2) / (y2 - y1);
                double expp = m_nc + 1.;
                // Find alpha
                double alpha1 = 2. - std::pow(beta1 , -expp);
                double alpha2 = 2. - std::pow(beta2 , -expp);

                double u = random::uni(rng, 0., 1. );
                alpha1 *=u;
                alpha2 *=u;
                if( u > 1. / alpha1 ) {
                    alpha1 = 1. / (2. - alpha1);
                }
                if( u > 1. / alpha2 ) {
                    alpha2 = 1. / (2. - alpha2);
                }
                betaQ1 = std::pow( alpha1, 1.0/expp );
                betaQ2 = std::pow( alpha2, 1.0/expp );

                //recombine points
                point1[i] = 0.5 * ((y1 + y2) - betaQ1 * (y2 - y1));
                point2[i] = 0.5 * ((y1 + y2) + betaQ2 * (y2 - y1));
                // randomly swap loci
                if( random::coinToss(rng,0.5) ) std::swap(point1[i], point2[i]);


                //  -> from Deb's implementation, not contained in any paper
                point1[i] = std::max( point1[i], m_lower( i ) );
                point1[i] = std::min( point1[i], m_upper( i ) );
                point2[i] = std::max( point2[i], m_lower( i ) );
                point2[i] = std::min( point2[i], m_upper( i ) );
            }

        }

        /// \brief Serializes this instance to the supplied archive.
        /// \tparam Archive The type of the archive the instance shall be serialized to.
        /// \param [in,out] archive The archive to serialize to.
        /// \param [in] version Version information (optional and not used here).
        template<typename Archive>
        void serialize( Archive & archive, const unsigned int version ) {
            archive & BOOST_SERIALIZATION_NVP( m_nc );
            archive & BOOST_SERIALIZATION_NVP( m_prob );
            archive & BOOST_SERIALIZATION_NVP( m_upper );
            archive & BOOST_SERIALIZATION_NVP( m_lower );
        }

        double m_nc; ///< Parameter nc.
        double m_prob; ///< Crossover probability.

        RealVector m_upper; ///< Upper bound (box constraint).
        RealVector m_lower; ///< Lower bound (box constraint).
    };
}

#endif