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// $Id: G4ErrorMag_UsualEqRhs.cc,v 1.1 2007/05/16 12:54:02 gcosmo Exp $
// GEANT4 tag $Name: geant4-09-02-cand-01 $
//
//
// --------------------------------------------------------------------
//      GEANT 4 class implementation file 
// --------------------------------------------------------------------

#include "G4ErrorMag_UsualEqRhs.hh"
#include "G4ErrorPropagatorData.hh"
 
//---------------------------------------------------------------------

G4ErrorMag_UsualEqRhs::G4ErrorMag_UsualEqRhs( G4MagneticField* MagField )
  : G4Mag_UsualEqRhs( MagField )
{
}

G4ErrorMag_UsualEqRhs::~G4ErrorMag_UsualEqRhs()
{
}

//---------------------------------------------------------------------
void
G4ErrorMag_UsualEqRhs::EvaluateRhsGivenB( const G4double y[],
                                          const G4double B[3],
                                                G4double dydx[] ) const
{

  G4Mag_UsualEqRhs::EvaluateRhsGivenB(y, B, dydx );
  
  if(G4ErrorPropagatorData::GetErrorPropagatorData()->GetMode()
     == G4ErrorMode_PropBackwards)
  {
    G4double momentum_mag_square = sqr(y[3]) + sqr(y[4]) + sqr(y[5]);
    G4double inv_momentum_magnitude = 1.0 / std::sqrt( momentum_mag_square );
    
    G4double cof = FCof()*inv_momentum_magnitude;

    dydx[3] = cof*(y[4]*(-B[2]) - y[5]*(-B[1])) ;
    dydx[4] = cof*(y[5]*(-B[0]) - y[3]*(-B[2])) ;
    dydx[5] = cof*(y[3]*(-B[1]) - y[4]*(-B[0])) ;    
  }
  return;
}
