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// G4BorisDriver
//
// Class description:
//
//   G4BorisDriver is a driver class using the second order Boris 
// method to integrate the equation of motion.
// 
//
// Author: Divyansh Tiwari, Google Summer of Code 2022
// Supervision: John Apostolakis,Renee Fatemi, Soon Yung Jun 
// --------------------------------------------------------------------
#ifndef G4BORIS_DRIVER_HH
#define G4BORIS_DRIVER_HH

#include "G4VIntegrationDriver.hh"
#include "G4BorisScheme.hh"
#include "G4ChordFinderDelegate.hh"


class G4BorisDriver : public G4VIntegrationDriver,
                      public G4ChordFinderDelegate<G4BorisDriver>
{
  public:

    G4BorisDriver( G4double hminimum,
                   G4BorisScheme* Boris,
                   G4int numberOfComponents = 6,
                   G4bool verbosity = false);
   
    inline ~G4BorisDriver() override = default;

    inline G4BorisDriver(const G4BorisDriver&) = delete;
    inline G4BorisDriver& operator=(const G4BorisDriver&) = delete;

    // 1. Core methods that advance the integration

    G4bool AccurateAdvance( G4FieldTrack& track,
                            G4double stepLen,
                            G4double epsilon,
                            G4double beginStep = 0) override;
      // Advance integration accurately
      // - by relative accuracy better than 'epsilon'

    G4bool QuickAdvance(       G4FieldTrack& y_val,   // In/Out
                         const G4double dydx[],    
                               G4double       hstep,
                               G4double&   missDist,  // Out: estimated sagitta
                               G4double&   dyerr   ) override;
      // Attempt one integration step, and return estimated error 'dyerr'

    void OneGoodStep(G4double  yCurrentState[],  // In/Out: state ('y')
                     G4double& curveLength,      // In/Out: 'x'
                     G4double  htry,             // step to attempt
                     G4double  epsilon_rel,      // relative accuracy
                     G4double  restMass,         
                     G4double  charge,
                     G4double& hdid,             // Out: step achieved
                     G4double& hnext);           // Out: proposed next step
      // Method to implement Accurate Advance
   
    // 2. Methods needed to co-work with G4ChordFinder

    G4double AdvanceChordLimited(G4FieldTrack& track,
                                 G4double hstep,
                                 G4double eps,
                                 G4double chordDistance) override
    {
      return ChordFinderDelegate::
             AdvanceChordLimitedImpl(track, hstep, eps, chordDistance);
    }

    void OnStartTracking() override
    {
      ChordFinderDelegate::ResetStepEstimate();
    }

    void OnComputeStep(const G4FieldTrack*) override {}

    // 3. Does the method redo integrations when called to obtain values for
    //    internal, smaller intervals? (when needed to identify an intersection)

    G4bool DoesReIntegrate() const override { return true; }
      // It would be no if it just used interpolation to provide a result.

    // 4. Relevant for calculating a new step size to achieve required accuracy

    inline G4double ComputeNewStepSize(G4double  errMaxNorm, // normalised error
                                       G4double  hstepCurrent) override; // current step size

    G4double ShrinkStepSize2(G4double h, G4double error2) const;
    G4double GrowStepSize2(G4double h, G4double error2) const;
      // Calculate the next step size given the square of the relative error
   
    // 5. Auxiliary Methods ...

    void GetDerivatives( const G4FieldTrack& track,
                               G4double dydx[] ) const override;

    void GetDerivatives( const G4FieldTrack& track,
                               G4double dydx[],
                               G4double field[] ) const override;

    inline void SetVerboseLevel(G4int level) override;
    inline G4int GetVerboseLevel() const override;

    inline G4EquationOfMotion* GetEquationOfMotion() override;
    inline const G4EquationOfMotion* GetEquationOfMotion() const;
    void SetEquationOfMotion(G4EquationOfMotion* equation) override;

    void  StreamInfo( std::ostream& os ) const override;
     // Write out the parameters / state of the driver
   
    // 6. Not relevant for Boris and other non-RK methods

    inline const G4MagIntegratorStepper* GetStepper() const override;
    inline G4MagIntegratorStepper* GetStepper() override;

  private:

    inline G4int GetNumberOfVariables() const;

    inline void CheckStep(const G4ThreeVector& posIn,                              
                          const G4ThreeVector& posOut,
                                G4double hdid) const;
   
  private:

    // INVARIANTS -- remain unchanged during tracking / integration 
    // Parameters
    G4double fMinimumStep;
    G4bool   fVerbosity;

    // State -- The core stepping algorithm
    G4BorisScheme* boris;
    
    // STATE -- intermediate state (to avoid creation / churn )
    G4double yIn[G4FieldTrack::ncompSVEC],
             yMid[G4FieldTrack::ncompSVEC],
             yOut[G4FieldTrack::ncompSVEC],
             yError[G4FieldTrack::ncompSVEC];

    G4double yCurrent[G4FieldTrack::ncompSVEC];

    // - Unused 2022.11.03:   
    // G4double derivs[2][6][G4FieldTrack::ncompSVEC];
    // const G4int interval_sequence[2];
   
    // INVARIANTS -- Parameters for ensuring that one call has finite number of integration steps
    static constexpr G4int    fMaxNoSteps = 300; 
    static constexpr G4double fSmallestFraction= 1e-12; // To avoid FP underflow !  ( 1.e-6 for single prec)

    static constexpr G4int    fIntegratorOrder= 2; //  2nd order method -- needed for error control
    static constexpr G4double fSafetyFactor = 0.9; //

    static constexpr G4double fMaxSteppingIncrease= 10.0; //  Increase no more than 10x   
    static constexpr G4double fMaxSteppingDecrease= 0.1;  //  Reduce   no more than 10x
    static constexpr G4double fPowerShrink = -1.0 / fIntegratorOrder;
    static constexpr G4double fPowerGrow   = -1.0 / (1.0 + fIntegratorOrder);

    static const G4double fErrorConstraintShrink;
    static const G4double fErrorConstraintGrow;
   
    using ChordFinderDelegate = G4ChordFinderDelegate<G4BorisDriver>;
};

#include "G4BorisDriver.icc"

#endif