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// G4CashKarpRKF45
//
// Class description:
//
// The Cash-Karp Runge-Kutta-Fehlberg 4/5 method is an embedded fourth
// order method (giving fifth-order accuracy) for the solution of an ODE.
// Two different fourth order estimates are calculated; their difference
// gives an error estimate. [ref. Numerical Recipes in C, 2nd Edition]
// It is used to integrate the equations of the motion of a particle 
// in a magnetic field.

// Authors: J.Apostolakis, V.Grichine - 30.01.1997
// -------------------------------------------------------------------
#ifndef G4CASHKARP_RKF45_HH
#define G4CASHKARP_RKF45_HH

#include "G4MagIntegratorStepper.hh"

class G4CashKarpRKF45 : public G4MagIntegratorStepper
{

  public:  // with description

    G4CashKarpRKF45( G4EquationOfMotion* EqRhs,
                     G4int numberOfVariables = 6,
                     G4bool primary = true ) ;
   ~G4CashKarpRKF45() ;

    void Stepper( const G4double y[],
                  const G4double dydx[],
                        G4double h,
                        G4double yout[],
                        G4double yerr[] ) ;

  public:  // without description

    G4double  DistChord()   const; 
    G4int IntegratorOrder() const { return 4; }

  private:

    void StepWithEst( const G4double yIn[],
                      const G4double dydx[],
                            G4double Step,
                            G4double yOut[],
                            G4double& alpha2,
                            G4double& beta2,
                      const G4double B1[],
                            G4double B2[]    );  
      // No longer used. Obsolete.

    G4CashKarpRKF45(const G4CashKarpRKF45&);
    G4CashKarpRKF45& operator=(const G4CashKarpRKF45&);
      // Private copy constructor and assignment operator.

  private:

   G4double *ak2, *ak3, *ak4, *ak5, *ak6, *yTemp, *yIn; // *ak7
      // scratch space

    G4double fLastStepLength = 0.0;
    G4double *fLastInitialVector, *fLastFinalVector,
             *fLastDyDx, *fMidVector, *fMidError;
      // for DistChord calculations

    G4CashKarpRKF45* fAuxStepper = nullptr; 

};

#endif
