/*--------------------------------------------------------------------*/
/*     Copyright (C) 2004-2016  Serge Iovleff, Université Lille 1, Inria

    This program 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 2 of the
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    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 Lesser General Public License for more details.

    You should have received a copy of the GNU Lesser General Public
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    Contact : S..._Dot_I..._At_stkpp_Dot_org (see copyright for ...)
*/

/*
 * Project:  stkpp::
 * created on: 5 avr. 2015
 * Author:   iovleff, S..._Dot_I..._At_stkpp_Dot_org (see copyright for ...)
 **/

/** @file STK_Kernel_Exponential.h
 *  @brief In this file we define the class and methods for computing a Exponential Kernel.
 **/


#ifndef STK_KERNEL_EXPONENTIAL_H
#define STK_KERNEL_EXPONENTIAL_H

#include "STK_Kernel_IKernelBase.h"

namespace STK
{

namespace Kernel
{
/** @ingroup Kernel
 * The Exponential Kernel is a kernel of the form
 * \f[
 * k(x,y) = \exp\left(- \frac{\|x-y\|}{h} \right)
 * \f]
 * where @e h represents the bandwidth of the kernel.
 */
template<class Array>
class Exponential : public IKernelBase<Array>
{
  public:
    typedef IKernelBase<Array> Base;
    typedef typename Array::Row RowVector;
    using Base::p_data_;
    using Base::gram_;
    using Base::symmetrize;
    /** constructor with a constant pointer on the data set
     *  @param p_data a pointer on a data set that will be "kernelized"
     *  @param width the size of the windows to use in the kernel
     **/
    Exponential( Array const* p_data, Real const& width= 1.)
               : Base(p_data), width_(width) {}
    /** constructor with a constant pointer on the data set
     *  @param data a reference on a data set that will be "kernelized"
     *  @param width the size of the windows to use in the kernel
     **/
    Exponential( Array const& data, Real const& width= 1.)
               : Base(data), width_(width) {}
    /** destructor */
    virtual ~Exponential() {}
    /** @return the bandwidth of the kernel */
    Real const& width() const {return width_;}
    /** set the bandwidth of the kernel */
    void setWidth(Real const& width) {width_ = width;}

    /** compute the kernel value between two individuals
     *  @param ind1,ind2 two individuals to compare using the kernel metric */
    virtual Real kcomp(RowVector const& ind1, RowVector const& ind2) const;
    /** compute the kernel between an individual and himself
     *  @param ind the individual to evaluate using the kernel */
    virtual Real kdiag(RowVector const& ind) const;

  private:
    /** bandwidth of the kernel */
    Real width_;
};

template<class Array>
Real Exponential<Array>::kcomp(RowVector const& ind1, RowVector const& ind2) const
{ return std::exp(-(ind1 - ind2).norm()/width_);}

template<class Array>
Real Exponential<Array>::kdiag(RowVector const& ind) const
{ return 1.0;}

} // namespace Kernel

} // namespace STK

#endif /* STK_KERNEL_EXPONENTIAL_H */
