//
// ********************************************************************
// * License and Disclaimer                                           *
// *                                                                  *
// * The  Geant4 software  is  copyright of the Copyright Holders  of *
// * the Geant4 Collaboration.  It is provided  under  the terms  and *
// * conditions of the Geant4 Software License,  included in the file *
// * LICENSE and available at  http://cern.ch/geant4/license .  These *
// * include a list of copyright holders.                             *
// *                                                                  *
// * Neither the authors of this software system, nor their employing *
// * institutes,nor the agencies providing financial support for this *
// * work  make  any representation or  warranty, express or implied, *
// * regarding  this  software system or assume any liability for its *
// * use.  Please see the license in the file  LICENSE  and URL above *
// * for the full disclaimer and the limitation of liability.         *
// *                                                                  *
// * This  code  implementation is the result of  the  scientific and *
// * technical work of the GEANT4 collaboration.                      *
// * By using,  copying,  modifying or  distributing the software (or *
// * any work based  on the software)  you  agree  to acknowledge its *
// * use  in  resulting  scientific  publications,  and indicate your *
// * acceptance of all terms of the Geant4 Software license.          *
// ********************************************************************
//
//
// $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; 
}
