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//
// G4ExactHelixStepper
//
// Class description:
//
// Concrete class for particle motion in constant magnetic field.
// Helix a-la-Explicity Euler: x_1 = x_0 + helix(h)
// with helix(h) being a helix piece of length h.
// simplest approach for solving linear differential equations.
// Take the current derivative and add it to the current position.
//
// As the field is assumed constant, an error is not calculated.

// Author: J.Apostolakis, 28.01.2005.
//         Implementation adapted from ExplicitEuler by W.Wander 
// --------------------------------------------------------------------
#ifndef G4EXACTHELIXSTEPPER_HH
#define G4EXACTHELIXSTEPPER_HH

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

#include "G4MagIntegratorStepper.hh"
#include "G4MagHelicalStepper.hh"
#include "G4Mag_EqRhs.hh"

class G4ExactHelixStepper : public G4MagHelicalStepper
{
  public:  // with description

    G4ExactHelixStepper(G4Mag_EqRhs* EqRhs);
    ~G4ExactHelixStepper();
  
    G4ExactHelixStepper(const G4ExactHelixStepper&) = delete;
    G4ExactHelixStepper& operator=(const G4ExactHelixStepper&) = delete;

    void Stepper( const G4double y[],
                  const G4double dydx[],
                        G4double h,
                        G4double yout[],
                        G4double yerr[]  );
      // Step 'integration' for step size 'h'
      // Provides helix starting at y[0 to 6]
      // Outputs yout[] and ZERO estimated error yerr[]=0.
  
    void DumbStepper( const G4double y[],
                            G4ThreeVector   Bfld,
                            G4double  h,
                            G4double yout[] );
      // Performs a 'dump' Step without error calculation.
  
    G4double DistChord() const;
      // Estimate maximum distance of curved solution and chord ... 

    virtual G4int IntegratorOrder() const;

  private:

    G4ThreeVector fBfieldValue;
      //  Initial value of field at last step
};

#endif