vonMisesKappaFunctor.hpp

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#ifndef VONMISESKAPPAFUNCTOR_H
#define VONMISESKAPPAFUNCTOR_H

#include <boost/math/special_functions/bessel.hpp>

namespace canopy
{

// Specific functor for the problem at hand (finding kappa)
struct vonMisesKappaFunctor
{
    float R; 

    vonMisesKappaFunctor(float Rin) {R = Rin;}

    int operator()(const Eigen::VectorXd &x, Eigen::VectorXd &fvec) const
    {
        using boost::math::cyl_bessel_i;
        // The function to solve for Kappa
        fvec(0,0) = cyl_bessel_i(1,x[0]) - R*cyl_bessel_i(0,x[0]);
        return 0;
    }

    int df(const Eigen::VectorXd &x, Eigen::MatrixXd &fjac) const
    {
        using boost::math::cyl_bessel_i;
        // Derivative of the above function using simple Bessel function identities
        fjac(0,0) = 0.5*(cyl_bessel_i(0,x[0]) + cyl_bessel_i(2,x[0])) - R*cyl_bessel_i(1,x[0]);
        return 0;
    }
};

}// end of namespace

#endif