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// $Id: G4ExactHelixStepper.cc,v 1.11 2010/07/21 13:46:01 tnikitin Exp $ 
// GEANT4 tag $Name: field-V09-03-03 $
//
//  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),
    fPtrMagEqOfMot(EqRhs)
{
  ;
}

G4ExactHelixStepper::~G4ExactHelixStepper() {} 

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

   G4int i;
   G4ThreeVector Bfld_value;

   MagFieldEvaluate(yInput, Bfld_value);        
   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 ;
  }

  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",
              "EHS:NoDumbStepper", FatalException,
              "Should not be called. 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; 
}