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//
// G4ClassicalRK4 implementation
//
// Authors: J.Apostolakis, V.Grichine (CERN), 30.01.1997
// -------------------------------------------------------------------

#include "G4ClassicalRK4.hh"
#include "G4ThreeVector.hh"

//////////////////////////////////////////////////////////////////
//
// Constructor sets the number of variables (default = 6)
//
G4ClassicalRK4::
G4ClassicalRK4(G4EquationOfMotion* EqRhs, G4int numberOfVariables)
  : G4MagErrorStepper(EqRhs, numberOfVariables)
{
   unsigned int noVariables= std::max(numberOfVariables,8); // For Time .. 7+1
 
   dydxm = new G4double[noVariables];
   dydxt = new G4double[noVariables]; 
   yt    = new G4double[noVariables]; 
}

////////////////////////////////////////////////////////////////
//
// Destructor
//
G4ClassicalRK4::~G4ClassicalRK4()
{
  delete [] dydxm;
  delete [] dydxt;
  delete [] yt;
}

//////////////////////////////////////////////////////////////////////
//
// Given values for the variables y[0,..,n-1] and their derivatives
// dydx[0,...,n-1] known at x, use the classical 4th Runge-Kutta
// method to advance the solution over an interval h and return the
// incremented variables as yout[0,...,n-1], which not be a distinct
// array from y. The user supplies the routine RightHandSide(x,y,dydx),
// which returns derivatives dydx at x. The source is routine rk4 from
// NRC p. 712-713 .
//
void
G4ClassicalRK4::DumbStepper( const G4double yIn[],
                             const G4double dydx[],
                                   G4double h,
                                   G4double yOut[])
{
  const G4int nvar = GetNumberOfVariables();   //  fNumberOfVariables(); 
  G4int i;
  G4double hh = h*0.5, h6 = h/6.0;

  // Initialise time to t0, needed when it is not updated by the integration.
  //        [ Note: Only for time dependent fields (usually electric) 
  //                  is it neccessary to integrate the time.] 
  yt[7]   = yIn[7]; 
  yOut[7] = yIn[7];

  for(i=0; i<nvar; ++i)
  {
    yt[i] = yIn[i] + hh*dydx[i] ;             // 1st Step K1=h*dydx
  }
  RightHandSide(yt,dydxt) ;                   // 2nd Step K2=h*dydxt

  for(i=0; i<nvar; ++i)
  { 
    yt[i] = yIn[i] + hh*dydxt[i] ;
  }
  RightHandSide(yt,dydxm) ;                   // 3rd Step K3=h*dydxm

  for(i=0; i<nvar; ++i)
  {
    yt[i] = yIn[i] + h*dydxm[i] ;
    dydxm[i] += dydxt[i] ;                    // now dydxm=(K2+K3)/h
  }
  RightHandSide(yt,dydxt) ;                   // 4th Step K4=h*dydxt
 
  for(i=0; i<nvar; ++i)    // Final RK4 output
  {
    yOut[i] = yIn[i]+h6*(dydx[i]+dydxt[i]+2.0*dydxm[i]); //+K1/6+K4/6+(K2+K3)/3
  }
  if ( nvar == 12 )  { NormalisePolarizationVector ( yOut ); }
  
}  // end of DumbStepper ....................................................
