#ifndef NEOHOOKEANMATERIAL_H
#define NEOHOOKEANMATERIAL_H

#include "MaterialModel.h"

/*
* Neo-Hookean nonlinear material model, with energy density
* W = beta/2.0 (tr[M] - 2 - log det M) + alpha/2.0 (log det M / 2)^2
* where M = gbar^-1 g, and g and gbar are the current and rest metrics of the 
* shell volume (which vary in the thickness direction as defined by the surface 
* fundamental forms).
*/

template <class SFF>
class NeoHookeanMaterial : public MaterialModel<SFF>
{
public:
    NeoHookeanMaterial(double lameAlpha, double lameBeta) : lameAlpha_(lameAlpha), lameBeta_(lameBeta) {}

    /*
    * Lame parameters of the material (as in the energy density written above)
    */
    double lameAlpha_, lameBeta_;

    virtual double stretchingEnergy(
        const MeshConnectivity &mesh,
        const Eigen::MatrixXd &curPos,
        double thickness,
        const Eigen::Matrix2d &abar,
        int face,
        Eigen::Matrix<double, 1, 9> *derivative, // F(face, i)
        Eigen::Matrix<double, 9, 9> *hessian) const;

    virtual double bendingEnergy(
        const MeshConnectivity &mesh,
        const Eigen::MatrixXd &curPos,
        const Eigen::VectorXd &extraDOFs,
        double thickness,
        const Eigen::Matrix2d &abar, const Eigen::Matrix2d &bbar,
        int face,                        
        Eigen::Matrix<double, 1, 18 + 3*SFF::numExtraDOFs> *derivative, // F(face, i), then the three vertices opposite F(face,i), then the extra DOFs on oppositeEdge(face,i)
        Eigen::Matrix<double, 18 + 3*SFF::numExtraDOFs, 18 + 3*SFF::numExtraDOFs> *hessian) const;


};


#endif