source: trunk/source/geometry/magneticfield/src/G4ExactHelixStepper.cc @ 1202

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// $Id: G4ExactHelixStepper.cc,v 1.9 2008/10/29 14:34:35 gcosmo Exp $ 
// GEANT4 tag $Name: geant4-09-02-cand-01 $
//
//  Helix a-la-Explicity Euler: x_1 = x_0 + helix(h)
//   with helix(h) being a helix piece of length h
//   simplest approach for solving linear differential equations.
//  Take the current derivative and add it to the current position.
//
//  As the field is assumed constant, an error is not calculated.
// 
//  Author: J. Apostolakis, 28 Jan 2005
//     Implementation adapted from ExplicitEuler of W.Wander 
// -------------------------------------------------------------------

#include "G4ExactHelixStepper.hh"
#include "G4ThreeVector.hh"
#include "G4LineSection.hh"


G4ExactHelixStepper::G4ExactHelixStepper(G4Mag_EqRhs *EqRhs)
  : G4MagHelicalStepper(EqRhs),
    fBfieldValue(DBL_MAX, DBL_MAX, DBL_MAX), yInitialEHS(DBL_MAX), yFinalEHS(-DBL_MAX)
    
{
   const G4int nvar = 6 ;
   G4int i; 
   for(i=0;i<nvar;i++)  {
     fYInSav[i]= DBL_MAX;
   }
    fPtrMagEqOfMot=EqRhs;
}

G4ExactHelixStepper::~G4ExactHelixStepper() {} 

void
G4ExactHelixStepper::Stepper( const G4double yInput[],
                              const G4double*,
                                    G4double hstep,
                                    G4double yOut[],
                                    G4double yErr[]      )
{  
   const G4int nvar = 6 ;

   G4int i;
   // G4double      yTemp[7], yIn[7] ;
   G4ThreeVector Bfld_value;

   for(i=0;i<nvar;i++)  {
      // yIn[i]=     yInput[i];
      fYInSav[i]= yInput[i];
   }

   MagFieldEvaluate(yInput, Bfld_value) ;        
     
   // DumbStepper(yIn, Bfld_value, hstep, yTemp);
   AdvanceHelix(yInput, Bfld_value, hstep, yOut);

   // We are assuming a constant field: helix is exact.
   for(i=0;i<nvar;i++) {
     yErr[i] = 0.0 ;
   }

    yInitialEHS = G4ThreeVector( yInput[0],   yInput[1],   yInput[2]); 
    yFinalEHS   = G4ThreeVector( yOut[0],  yOut[1],  yOut[2]); 
    fBfieldValue=Bfld_value;
    
}

void
G4ExactHelixStepper::DumbStepper( const G4double  yIn[],
                                   G4ThreeVector   Bfld,
                                   G4double  h,
                                   G4double  yOut[])
{
  // Assuming a constant field: solution is a helix
  AdvanceHelix(yIn, Bfld, h, yOut);

  G4Exception("G4ExactHelixStepper::DumbStepper should not be called.",
              "EHS:NoDumbStepper", FatalException, "Stepper must do all the work." ); 
}  


// ---------------------------------------------------------------------------

G4double G4ExactHelixStepper::DistChord()   const 
{
  // Implementation : must check whether h/R >  pi  !!
  //   If( h/R <  pi)   DistChord=h/2*std::tan(Ang_curve/4)
  //   Else             DistChord=R_helix
  //
  G4double distChord;
  G4double Ang_curve=GetAngCurve();

      
         if(Ang_curve<=pi){
           distChord=GetRadHelix()*(1-std::cos(0.5*Ang_curve));
         }
         else 
         if(Ang_curve<twopi){
           distChord=GetRadHelix()*(1+std::cos(0.5*(twopi-Ang_curve)));
         }
         else{
          distChord=2.*GetRadHelix();  
         }

   

  return distChord;
  
}   

G4int
G4ExactHelixStepper::IntegratorOrder() const 
{
  return 1; 
}