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//
//
//
//
// class G4InterpolationDriver
//
// Class description:
//
// Driver class which uses Runge-Kutta stepper with interpolation property
// to integrate track with error control 

// History:
// - Created. D.Sorokin
// --------------------------------------------------------------------

#ifndef G4INTERPOLATION_DRIVER_HH
#define G4INTERPOLATION_DRIVER_HH

#include "G4RKIntegrationDriver.hh"
#include "G4FieldUtils.hh"

#include "globals.hh"

#include <vector>
#include <memory>


template <class T>
class G4InterpolationDriver : public G4RKIntegrationDriver<T> {
public:
    G4InterpolationDriver(G4double hminimum,
                          T* stepper,
                          G4int numberOfComponents = 6,
                          G4int statisticsVerbosity = 1);

    G4InterpolationDriver(const G4InterpolationDriver &) = delete;
    const G4InterpolationDriver& operator =(const G4InterpolationDriver &) = delete;

    virtual G4double AdvanceChordLimited(G4FieldTrack& track,
                                         G4double hstep,
                                         G4double eps,
                                         G4double chordDistance) override;

    virtual void OnStartTracking() override
    {
        fChordStepEstimate = DBL_MAX;
        fhnext = DBL_MAX;
        fTotalStepsForTrack = 0;
    }

    // Integrates ODE from current s (s=s0) to s=s0+h with accuracy eps.
    // On output track is replaced by value at end of interval.
    // The concept is similar to the odeint routine from NRC p.721-722.
    virtual G4bool AccurateAdvance(G4FieldTrack& track,
                                   G4double hstep,
                                   G4double eps,                     // Requested y_err/hstep
                                   G4double hinitial = 0) override;  // Suggested 1st interval

    virtual void OnComputeStep() override 
    { 
        fKeepLastStepper = false;
        fFirstStep = true;
        fLastStepper = fSteppers.end();
    }

    virtual void SetVerboseLevel(G4int level) override { fVerboseLevel = level; }
    virtual G4int GetVerboseLevel() const override { return fVerboseLevel; }

private:
    struct InterpStepper {
        std::unique_ptr<T> stepper;
        G4double begin;
        G4double end;
        G4double inverseLength;
    };

    using StepperIterator = typename std::vector<InterpStepper>::iterator;
    using ConstStepperIterator = typename std::vector<InterpStepper>::const_iterator;

    // This takes one Step that is of size htry, or as large 
    // as possible while satisfying the accuracy criterion of:
    //     yerr < eps * |y_end-y_start|
    // return hdid
    G4double OneGoodStep(StepperIterator it,
                         field_utils::State& y,
                         field_utils::State& dydx,
                         G4double& hstep,
                         G4double eps,
                         G4double curveLength);

    void Interpolate(G4double curveLength, field_utils::State& y) const;

    void InterpolateImpl(G4double curveLength, 
                         ConstStepperIterator it, 
                         field_utils::State& y) const;

    G4double DistChord(const field_utils::State& yBegin, 
                       G4double curveLengthBegin, 
                       const field_utils::State& yEnd, 
                       G4double curveLengthEnd) const;

    G4double FindNextChord(const field_utils::State& yBegin,
                           G4double curveLengthBegin,
                           field_utils::State& yEnd,
                           G4double curveLengthEnd,
                           G4double dChord,
                           G4double maxChordDistance);

    G4double CalcChordStep(G4double stepTrialOld, 
                           G4double dChordStep,
                           G4double fDeltaChord);

    void PrintState() const;

    void CheckState() const;

    std::vector<InterpStepper> fSteppers;
    typename std::vector<InterpStepper>::iterator fLastStepper;
    G4bool fKeepLastStepper = false;

    G4double fhnext = DBL_MAX;

    // Minimum Step allowed in a Step (in absolute units)
    G4double fMinimumStep;

    G4double fChordStepEstimate = DBL_MAX;
    const G4double fFractionNextEstimate = 0.98;
    const G4double fSmallestCurveFraction = 0.01;

    G4int fVerboseLevel;

    field_utils::State fdydx;
    G4bool fFirstStep = true;

    const G4int fMaxTrials = 100; 
    G4int fTotalStepsForTrack = 0;

    using Base = G4RKIntegrationDriver<T>;
};

#include "G4InterpolationDriver.icc"

#endif