/data/refman/pasoursint/CMSSW_4_1_8_patch9/src/TrackPropagation/RungeKutta/interface/RK4OneStepTempl.h

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#ifndef RK4OneStepTempl_H
#define RK4OneStepTempl_H

#include "TrackPropagation/RungeKutta/interface/RKSmallVector.h"
#include "TrackPropagation/RungeKutta/interface/RKDerivative.h"

template <typename T, int N>
class RK4OneStepTempl {
 public:

    typedef T                                   Scalar;
    typedef RKSmallVector<T,N>                  Vector;

  
    Vector operator()( Scalar startPar, const Vector& startState,
                       const RKDerivative<T,N>& deriv, Scalar step) const {

        // cout << "RK4OneStepTempl: starting from " << startPar << startState << endl;

        Vector k1 = step * deriv( startPar, startState);
        Vector k2 = step * deriv( startPar+step/2, startState+k1/2);
        Vector k3 = step * deriv( startPar+step/2, startState+k2/2);
        Vector k4 = step * deriv( startPar+step, startState+k3);

        Vector result = startState + k1/6 + k2/3 + k3/3 + k4/6;

        // cout << "RK4OneStepTempl: result for step " << step << " is " << result << endl;

        return result;
    }
};

#endif