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//
// G4RKG3_Stepper
//
// Class description:
//
// Runga-Kutta integrator stepper from Geant3.

// Created: J.Apostolakis, V.Grichine - 30.01.1997
// -------------------------------------------------------------------
#ifndef G4RKG3_STEPPER_HH
#define G4RKG3_STEPPER_HH

#include "G4Types.hh"
#include "G4MagIntegratorStepper.hh"
#include "G4ThreeVector.hh"

class G4Mag_EqRhs;

class G4RKG3_Stepper : public G4MagIntegratorStepper
{
  public:  // with description

    G4RKG3_Stepper(G4Mag_EqRhs* EqRhs);
      // Integrate over 6 variables only:  position & velocity.
      // Not implemented yet !

    ~G4RKG3_Stepper();

    void Stepper( const G4double yIn[],
                  const G4double dydx[],
                        G4double h,
                        G4double yOut[],
                        G4double yErr[] );
      // The method which must be provided, even if less efficient.

    G4double  DistChord() const ;
 
    void StepNoErr( const G4double tIn[8],
                    const G4double dydx[6],
                          G4double Step,
                          G4double tOut[8],
                          G4double B[3] );
      // Integrator RK Stepper from G3 with only two field evaluation per 
      // Step. It is used in propagation initial Step by small substeps
      // after solution error and delta geometry considerations. 
      // B[3] is magnetic field which is passed from substep to substep.

    void StepWithEst( const G4double  tIn[8],
                      const G4double dydx[6],
                            G4double Step,
                            G4double tOut[8],
                            G4double& alpha2,
                            G4double& beta2,
                      const G4double B1[3],
                            G4double B2[3] );
      // Integrator for RK from G3 with evaluation of error in solution and
      // delta geometry based on naive similarity with the case of uniform
      // magnetic field.
      // B1[3] is input  and is the first magnetic field values
      // B2[3] is output and is the final magnetic field values.

  public:  // without description

    inline G4int IntegratorOrder() const { return 4; }

  private:

    G4ThreeVector fyInitial,
                  fyMidPoint,
                  fyFinal;
    G4ThreeVector fpInitial;
    G4ThreeVector BfldIn;
    G4double      hStep = 0.0;
};

#endif
