G4MagErrorStepper.cc

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// $Id: G4MagErrorStepper.cc 66356 2012-12-18 09:02:32Z gcosmo $
//
// --------------------------------------------------------------------

#include "G4MagErrorStepper.hh"
#include "G4LineSection.hh"

G4MagErrorStepper::~G4MagErrorStepper()
{
   delete[] yMiddle;
   delete[] dydxMid;
   delete[] yInitial;
   delete[] yOneStep;
}

void
G4MagErrorStepper::Stepper( const G4double yInput[],
                    const G4double dydx[],
                          G4double hstep,
                          G4double yOutput[],
                          G4double yError []      )
{  
   const G4int nvar = this->GetNumberOfVariables() ;
   const G4int maxvar= GetNumberOfStateVariables();

   G4int i;
   // correction for Richardson Extrapolation.
   G4double  correction = 1. / ( (1 << IntegratorOrder()) -1 );
   
   //  Saving yInput because yInput and yOutput can be aliases for same array

   for(i=0;i<nvar;i++) yInitial[i]=yInput[i];
   yInitial[7]= yInput[7];    // Copy the time in case ... even if not really needed
   yMiddle[7] = yInput[7];  // Copy the time from initial value 
   yOneStep[7] = yInput[7]; // As it contributes to final value of yOutput ?
   // yOutput[7] = yInput[7];  // -> dumb stepper does it too for RK4
   for(i=nvar;i<maxvar;i++) yOutput[i]=yInput[i];
   // yError[7] = 0.0;         

   G4double halfStep = hstep * 0.5; 

   // Do two half steps

   DumbStepper  (yInitial,  dydx,   halfStep, yMiddle);
   RightHandSide(yMiddle, dydxMid);    
   DumbStepper  (yMiddle, dydxMid, halfStep, yOutput); 

   // Store midpoint, chord calculation

   fMidPoint = G4ThreeVector( yMiddle[0],  yMiddle[1],  yMiddle[2]); 

   // Do a full Step
   DumbStepper(yInitial, dydx, hstep, yOneStep);
   for(i=0;i<nvar;i++) {
      yError [i] = yOutput[i] - yOneStep[i] ;
      yOutput[i] += yError[i]*correction ;  // Provides accuracy increased
                                            // by 1 order via the 
                                            // Richardson Extrapolation  
   }

   fInitialPoint = G4ThreeVector( yInitial[0], yInitial[1], yInitial[2]); 
   fFinalPoint   = G4ThreeVector( yOutput[0],  yOutput[1],  yOutput[2]); 

   return ;
}



G4double
G4MagErrorStepper::DistChord() const 
{
  // Estimate the maximum distance from the curve to the chord
  //
  //  We estimate this using the distance of the midpoint to 
  //  chord (the line between 
  // 
  //  Method below is good only for angle deviations < 2 pi, 
  //   This restriction should not a problem for the Runge cutta methods, 
  //   which generally cannot integrate accurately for large angle deviations.
  G4double distLine, distChord; 

  if (fInitialPoint != fFinalPoint) {
     distLine= G4LineSection::Distline( fMidPoint, fInitialPoint, fFinalPoint );
     // This is a class method that gives distance of Mid 
     //  from the Chord between the Initial and Final points.

     distChord = distLine;
  }else{
     distChord = (fMidPoint-fInitialPoint).mag();
  }

  return distChord;
}