// Copyright (C) 2021  EDF
// All Rights Reserved
// This code is published under the GNU Lesser General Public License (GNU LGPL)
#ifndef LOCALLINEARDISCRLASTDIMREGRESSION_H
#define LOCALLINEARDISCRLASTDIMREGRESSION_H
#include <vector>
#include <memory>
#include <array>
#include <Eigen/Dense>
#include "StOpt/regression/LocalDiscrLastDimRegression.h"
#include "StOpt/core/grids/InterpolatorSpectral.h"

/** \file   LocalLinearDiscrLastDimRegression.h
 *  \brief  Compute conditional expectations by using local regressions.
 *          See article "Monte-Carlo valorisation of American options: facts and new algorithms to improve existing methods"
 *          by Bouchard, Warin in "Numerical methods in finance", Springer,2012
 *          Here the approximation in the last dimension has discrete values gathered in constant approximation in the lasty dimension.
 *  \author Xavier Warin
 */
namespace StOpt
{
/**
 * \defgroup LocalLinearDiscrLastDim Piecewise linear regression
 * \brief It implements local linear functions for performing local regression
 * using P-1 finite elements.
 *@{
 */
/// \class LocalLinearDiscrLastDimRegression LocalLinearDiscrLastDimRegression.h
/// To be used in Monte Carlo methods. LinearDiscrLastDim regression on each cell which is constructed such that each cell has
/// roughly the same number of particles, except in last dimension with discrete values.
class LocalLinearDiscrLastDimRegression : public LocalDiscrLastDimRegression
{
protected :
    Eigen::ArrayXXd m_matReg    ;            ///< Regression matrix (rank  \f$ (n_d+1)^2 \f$ by the number of meshes ). Also store lower part of factorized matrix.
    Eigen::ArrayXXd m_diagReg    ;          ///< Diagonal part of the factorized matrix during Choleski factorization



public :

    /// \brief Default constructor
    LocalLinearDiscrLastDimRegression() {}

    /// \brief Constructor
    /// \param  p_nbMesh       discretization in each direction
    /// \param  p_bRotationAndRecale do we use SVD
    LocalLinearDiscrLastDimRegression(const Eigen::ArrayXi   &p_nbMesh, bool  p_bRotationAndRecale = false);

    /// \brief Constructor for object constructed at each time step
    /// \param  p_bZeroDate    first date is 0?
    /// \param  p_particles    particles used for the meshes.
    ///                        First dimension  : dimension of the problem,
    ///                        second dimension : the  number of particles
    /// \param  p_nbMesh       discretization in each direction
    /// \param  p_bRotationAndRecale do we use SVD
    LocalLinearDiscrLastDimRegression(const bool &p_bZeroDate,
                                      const Eigen::ArrayXXd   &p_particles,
                                      const Eigen::ArrayXi    &p_nbMesh,
                                      bool  p_bRotationAndRecale = false);

    /// \brief Second constructor , only to be used in simulation
    /// \param  p_bRotationAndRecale do we use SVD
    LocalLinearDiscrLastDimRegression(const bool &p_bZeroDate,
                                      const   Eigen::ArrayXi &p_nbMesh,
                                      const   Eigen::Array< std::array< double, 2>, Eigen::Dynamic, Eigen::Dynamic >   &p_mesh,
                                      const std::vector< std::shared_ptr< Eigen::ArrayXd > > &p_mesh1D, const   Eigen::ArrayXd &p_meanX,
                                      const   Eigen::ArrayXd   &p_etypX, const   Eigen::MatrixXd   &p_svdMatrix,
                                      const bool   &p_bRotationAndRecale) ;

    /// \brief Copy constructor
    /// \param p_objet object to copy
    LocalLinearDiscrLastDimRegression(const LocalLinearDiscrLastDimRegression   &p_objet);


    /// \brief update the particles used in regression  and construct the matrices
    /// \param  p_bZeroDate    first date is 0?
    /// \param  p_particles    particles used for the meshes.
    ///                        First dimension  : dimension of the problem,
    ///                        second dimension : the  number of particles
    void updateSimulations(const bool &p_bZeroDate, const Eigen::ArrayXXd &p_particles);

    /// \brief Get some local accessors
    ///@{
    inline const Eigen::ArrayXXd &getMatReg() const
    {
        return m_matReg;
    }
    inline const Eigen::ArrayXXd &getDiagReg() const
    {
        return m_diagReg;
    }
    ///@}
    /// \brief conditional expectation basis function coefficient calculation
    /// \param  p_fToRegress  function to regress associated to each simulation used in optimization
    /// \return regression coordinates on the basis  (size : number of meshes multiplied by the dimension plus one)
    /// @{
    Eigen::ArrayXd getCoordBasisFunction(const Eigen::ArrayXd &p_fToRegress) const;
    ///@}
    /// \brief conditional expectation basis function coefficient calculation for multiple functions to regress
    /// \param  p_fToRegress  function to regress associated to each simulation used in optimization (size : number of functions to regress \times the number of Monte Carlo simulations)
    /// \return regression coordinates on the basis  (size :  number of function to regress  \times number of meshes multiplied by the dimension plus one)
    /// @{
    Eigen::ArrayXXd getCoordBasisFunctionMultiple(const Eigen::ArrayXXd &p_fToRegress) const ;
    ///@}

    /// \brief conditional expectation calculation
    /// \param  p_fToRegress  simulations  to regress used in optimization
    /// \return regressed value function
    /// @{
    Eigen::ArrayXd getAllSimulations(const Eigen::ArrayXd &p_fToRegress) const ;
    Eigen::ArrayXXd getAllSimulationsMultiple(const Eigen::ArrayXXd &p_fToRegress) const;
    ///@}


    /// \brief Use basis functions to reconstruct the solution
    /// \param p_basisCoefficients basis coefficients
    ///@{
    Eigen::ArrayXd reconstruction(const Eigen::ArrayXd   &p_basisCoefficients) const;
    Eigen::ArrayXXd reconstructionMultiple(const Eigen::ArrayXXd   &p_basisCoefficients) const;
    /// @}
    /// \brief use basis function to reconstruct a given simulation
    /// \param p_isim               simulation number
    /// \param p_basisCoefficients  basis coefficients to reconstruct a given conditional expectation
    double reconstructionASim(const int &p_isim, const Eigen::ArrayXd   &p_basisCoefficients) const ;

    /// \brief conditional expectation reconstruction
    /// \param  p_coordinates        coordinates to interpolate (uncertainty sample)
    /// \param  p_coordBasisFunction regression coordinates on the basis  (size: number of meshes multiplied by the dimension plus one)
    /// \return regressed value function reconstructed for each simulation
    double getValue(const Eigen::ArrayXd   &p_coordinates,
                    const Eigen::ArrayXd   &p_coordBasisFunction) const;


    /// \brief permits to reconstruct a function with basis functions coefficients values given on a grid
    /// \param  p_coordinates          coordinates  (uncertainty sample)
    /// \param  p_ptOfStock            grid point
    /// \param  p_interpFuncBasis      spectral interpolator to interpolate the basis functions  coefficients used in regression on the grid (given for each basis function)
    double getAValue(const Eigen::ArrayXd &p_coordinates,  const Eigen::ArrayXd &p_ptOfStock,
                     const std::vector< std::shared_ptr<InterpolatorSpectral> > &p_interpFuncBasis) const;


    /// \brief get the number of basis functions
    inline int getNumberOfFunction() const
    {
        if (m_bZeroDate)
            return 1;
        else
            return m_nbMesh.prod() * (m_nbMesh.size() + 1);
    }


    /// \brief Clone the regressor
    std::shared_ptr<BaseRegression> clone() const
    {
        return std::static_pointer_cast<BaseRegression>(std::make_shared<LocalLinearDiscrLastDimRegression>(*this));
    }

    /// \brief conditional expectation basis function coefficient calculation for a special cell
    /// \param  p_iCell     cell number
    /// \param  p_fToRegress  function to regress associated to each simulation used in optimization and corresponding to the cell
    /// \return regression coordinates on the basis  (size : the dimension of the problem plus one)
    /// @{
    Eigen::ArrayXd getCoordBasisFunctionOneCell(const int &p_iCell, const Eigen::ArrayXd &p_fToRegress) const ;
    Eigen::ArrayXXd getCoordBasisFunctionMultipleOneCell(const int &p_iCell, const Eigen::ArrayXXd &p_fToRegress) const ;
    ///@}

    /// \brief Given a particle and the coordinates of the mesh it belongs to, get back the conditional expectation
    /// \param p_oneParticle  The particle generated
    /// \param p_cell      the cell it belongs to
    /// \param p_foncBasisCoef  function basis on the current cell (the row is the number of reconstruction to achieve, the columns the number of function basis)
    /// \return send back the array of conditional expectations
    Eigen::ArrayXd getValuesOneCell(const Eigen::ArrayXd &p_oneParticle, const int &p_cell, const Eigen::ArrayXXd   &p_foncBasisCoef) const;
};
/**@}*/
}
#endif /*LOCALLINEARDISCRLASTDIMREGRESSION_H*/
