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// G4NystromRK4
//
// Class description:
//
// Integrates the equations of the motion of a particle in a magnetic field
// using 4th Runge-Kutta-Nystrom method with errors estimation 
// (ATL-SOFT-PUB-2009-01)
// Current form can be used only for 'pure' magnetic field.
// Notes: 1) field must be time-independent.
//        2) time is not integrated

// Created: I.Gavrilenko, 15.05.2009 (as G4AtlasRK4)
// Adaptations: J.Apostolakis, November 2009
// -------------------------------------------------------------------

#ifndef G4NYSTROMRK4_HH
#define G4NYSTROMRK4_HH

#include "G4MagIntegratorStepper.hh"
#include "G4Mag_EqRhs.hh"
#include "G4CachedMagneticField.hh"
#include "G4ThreeVector.hh"

#include <memory>

class G4NystromRK4 : public G4MagIntegratorStepper
{
  public: 

    G4NystromRK4(G4Mag_EqRhs* EquationMotion, 
                 G4double distanceConstField = 0.0); 
      // Can be used only for Magnetic Fields - and for 6 variables (x,p)
    ~G4NystromRK4() {}
   
    virtual void Stepper(const G4double y[],
                         const G4double dydx[],
                         G4double hstep,
                         G4double yOut[],
                         G4double yError[]) override;
      // Single call for integration result and error
      // Provides error via analytical method

    void SetDistanceForConstantField(G4double length); 
    G4double GetDistanceForConstantField() const; 
   
    virtual G4int IntegratorOrder() const override { return 4; }
    virtual G4double DistChord() const override; 
  
  private:

    inline void GetFieldValue(const G4double point[4], G4double field[3]);
    inline G4double GetFCof();

    G4CachedMagneticField* GetField();
    const G4CachedMagneticField* GetField() const;

    G4double fMomentum = 0.0;
    G4double fMomentum2 = 0.0;
    G4double fInverseMomentum = 0.0;
    G4double fCoefficient = 0.0;
    G4ThreeVector fInitialPoint;
    G4ThreeVector fMidPoint;
    G4ThreeVector fEndPoint;

    std::unique_ptr<G4CachedMagneticField> fCachedField;
};

#include "G4NystromRK4.icc"

#endif