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// G4VIntegrationDriver
//
// Class description:
//
// Abstract base class for 'driver' classes which are responsible for
// undertaking integration of an state given an equation of motion and
// within acceptable error bound(s).
//
// Different integration methods are meant to be provided via this
// common interface, and can span the original type (explicit Runge Kutta
// methods), enhanced RK methods and alternatives such as the
// Bulirsch-Stoer and multi-step methods.
//
// The drivers' key mission is to insure that the error is below set values. 

// Author: Dmitry Sorokin (CERN, Google Summer of Code 2017), 20.10.2017
// Supervision: John Apostolakis (CERN)
// --------------------------------------------------------------------
#ifndef G4VINTEGRATION_DRIVER_HH
#define G4VINTEGRATION_DRIVER_HH

#include "G4Types.hh"
#include "G4FieldTrack.hh"
#include "G4EquationOfMotion.hh"

class G4MagIntegratorStepper;

class G4VIntegrationDriver
{
  public:

    /**
     * Default Destructor.
     */
    virtual ~G4VIntegrationDriver() = default;

    /**
     * Computes the step to take, based on chord limits.
     *  @param[in,out] track The current track in field.
     *  @param[in] hstep Proposed step length.
     *  @param[in] eps Requested accuracy, y_err/hstep.
     *  @param[in] chordDistance Maximum sagitta distance.
     *  @returns The length of step taken.
     */
    virtual G4double AdvanceChordLimited(G4FieldTrack& track,
                                         G4double hstep,
                                         G4double eps,
                                         G4double chordDistance) = 0;

    /**
     * Advances integration accurately by relative accuracy better than 'eps'.
     * On output the track is replaced by the value at the end of interval.
     *  @param[in,out] track The current track in field.
     *  @param[in] hstep Proposed step length.
     *  @param[in] eps Requested accuracy, y_err/hstep.
     *  @param[in] hinitial Initial minimum integration step.
     *  @returns true if integration succeeds.
     */
    virtual G4bool AccurateAdvance(G4FieldTrack& track,
                                   G4double hstep,
                                   G4double eps, // Requested y_err/hstep
                                   G4double hinitial = 0 ) = 0;

    /**
     * Setter and getter for the equation of motion.
     */
    virtual void SetEquationOfMotion(G4EquationOfMotion* equation) = 0;
    virtual G4EquationOfMotion* GetEquationOfMotion() = 0;

    /**
     * Method for compatibility -- relevant only for G4MagIntegratorDriver.
     */
    virtual void RenewStepperAndAdjust(G4MagIntegratorStepper* pItsStepper);
   
    /**
     * Setter and getter for verbosity.
     */
    virtual void SetVerboseLevel(G4int level) = 0;
    virtual G4int GetVerboseLevel() const = 0;

    /**
     * Dispatch interface method for computing step.
     */
    virtual void OnComputeStep(const G4FieldTrack* /*track*/ = nullptr) = 0;

    /**
     * Dispatch interface method for initialisation/reset of driver.
     */
    virtual void OnStartTracking() = 0;

    /**
     * Whether the driver implements re-integration:
     * Does this driver *Recalculate* when AccurateAdvance() is called ?
     */
    virtual G4bool DoesReIntegrate() const = 0;

    /**
     * Writes out to stream the parameters/state of the driver.
     */
    virtual void StreamInfo( std::ostream& os ) const = 0;

    /**
     * Streaming operator.
     */
    friend std::ostream& operator<<( std::ostream& os, const G4VIntegrationDriver& id);

    // ------------------------------------------------------------------------

    //[[deprecated("will be removed")]]
    virtual G4bool QuickAdvance(G4FieldTrack& /*track*/,   // INOUT
                                const G4double /*dydx*/[],
                                G4double /*hstep*/,
                                G4double& /*dchord_step*/,
                                G4double& /*dyerr*/) { return false; }

    //[[deprecated("will be removed")]]
    virtual void GetDerivatives(const G4FieldTrack& track,
                                G4double dydx[]) const = 0;

    //[[deprecated("will be removed")]]
    virtual void GetDerivatives(const G4FieldTrack& track,
                                G4double dydx[],
                                G4double field[]) const = 0;

    //[[deprecated("use GetEquationOfMotion() instead of GetStepper()->GetEquationOfMotion()")]]
    virtual const G4MagIntegratorStepper* GetStepper() const = 0;
    virtual G4MagIntegratorStepper* GetStepper() = 0;

    //[[deprecated("will be removed")]]
    virtual G4double ComputeNewStepSize(G4double errMaxNorm, // normalised error
                                        G4double hstepCurrent) = 0;
      // Taking the last step's normalised error, calculate
      // a step size for the next step.
      // - Can limit the next step's size within a factor of the current one.
   
  protected:

    static constexpr G4double max_stepping_increase = 5;
    static constexpr G4double max_stepping_decrease = 0.1;
};

#endif