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// G4TSimpleRunge
//
// Class description:
//
// Templated version of G4SimpleRunge.
// Adapted from G4G4TSimpleRunge class.

// Author: Josh Xie (CERN, Google Summer of Code 2014), June 2014
// Supervisors:  Sandro Wenzel, John Apostolakis (CERN)
// --------------------------------------------------------------------
#ifndef G4TSimpleRunge_HH
#define G4TSimpleRunge_HH

#include <cassert>
#include "G4TMagErrorStepper.hh"
#include "G4ThreeVector.hh"

/**
 * @brief G4TSimpleRunge is a templated version of G4SimpleRunge.
 */

template <class T_Equation, int N>
class G4TSimpleRunge
  : public G4TMagErrorStepper<G4TSimpleRunge<T_Equation, N>, T_Equation, N>
{
  public:

    static constexpr double IntegratorCorrection = 1. / ((1 << 2) - 1);

    G4TSimpleRunge(T_Equation* EqRhs, G4int numberOfVariables = 6)
      : G4TMagErrorStepper<G4TSimpleRunge<T_Equation, N>, T_Equation, N>(
          EqRhs, numberOfVariables)
      , fNumberOfVariables(numberOfVariables)
      , fEquation_Rhs(EqRhs)
      
    {
      // default GetNumberOfStateVariables() == 12
      assert(this->GetNumberOfStateVariables() <= 12);
    }

    ~G4TSimpleRunge() = default;

    inline void RightHandSide(G4double y[],
                              G4double dydx[])
    {
      fEquation_Rhs->T_Equation::RightHandSide(y, dydx);
    }

    inline void DumbStepper(const G4double yIn[],
                            const G4double dydx[],
                            G4double h, G4double yOut[]) // override final
    {
      // Initialise time to t0, needed when it is not updated by the integration.
      yTemp[7] = yOut[7] = yIn[7];  //  Better to set it to NaN;  // TODO

      for(G4int i = 0; i < N; ++i)
      {
        yTemp[i] = yIn[i] + 0.5 * h * dydx[i];
      }

      this->RightHandSide(yTemp, dydxTemp);

      for(G4int i = 0; i < N; ++i)
      {
        yOut[i] = yIn[i] + h * (dydxTemp[i]);
      }
    }

    inline G4int IntegratorOrder() const { return 2; }

  private:

    G4int fNumberOfVariables;
    G4double dydxTemp[N > 12 ? N : 12];
    G4double yTemp[N > 12 ? N : 12];

    T_Equation* fEquation_Rhs;
    // scratch space
};

#endif