G4ImplicitEuler.cc

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//
// G4ImplicitEuler implementation
//
//  Implicit Euler:
//
//        x_1 = x_0 + h/2 * ( dx(t_0,x_0) + dx(t_0+h,x_0+h*dx(t_0,x_0) ) )
//
// Second order solver.
// Take the current derivative and add it to the current position.
// Take the output and its derivative. Add the mean of both derivatives
// to form the final output.
//
// Author: W. Wander <wwc@mit.edu>, 12.09.1997
// --------------------------------------------------------------------

#include "G4ImplicitEuler.hh"
#include "G4ThreeVector.hh"

//
// Constructor

G4ImplicitEuler::G4ImplicitEuler(G4EquationOfMotion* EqRhs, 
                                 G4int numberOfVariables)
  : G4MagErrorStepper(EqRhs, numberOfVariables)
{
  unsigned int noVariables = std::max(numberOfVariables,8); // For Time .. 7+1
  dydxTemp = new G4double[noVariables] ;
  yTemp    = new G4double[noVariables] ;
}


//
// Destructor
//
G4ImplicitEuler::~G4ImplicitEuler()
{
  delete [] dydxTemp;
  delete [] yTemp;
}

//
// DumbStepper
//
void
G4ImplicitEuler::DumbStepper( const G4double yIn[],
                              const G4double dydx[],
                                    G4double h,
                                    G4double yOut[] )
{
  const G4int numberOfVariables = GetNumberOfVariables();

  // 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 < numberOfVariables; ++i ) 
  {
    yTemp[i] = yIn[i] + h*dydx[i] ;          
  }
  
  RightHandSide(yTemp,dydxTemp);
  
  for( G4int i = 0; i < numberOfVariables; ++i ) 
  {
    yOut[i] = yIn[i] + 0.5 * h * ( dydx[i] + dydxTemp[i] );
  }

  return;
}