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// $Id: G4Mag_UsualEqRhs.cc,v 1.13 2010/07/14 10:00:36 gcosmo Exp $
// GEANT4 tag $Name: field-V09-03-03 $
//
//
//  This is the 'standard' right-hand side for the equation of motion
//  of a charged particle in a magnetic field.
//
//  Initial version: J. Apostolakis, January 13th, 1997
//
// --------------------------------------------------------------------

#include "G4Mag_UsualEqRhs.hh"
#include "G4MagneticField.hh"

G4Mag_UsualEqRhs::G4Mag_UsualEqRhs( G4MagneticField* MagField )
  : G4Mag_EqRhs( MagField ), fInvCurrentMomentumXc(0.) {}

G4Mag_UsualEqRhs::~G4Mag_UsualEqRhs() {}

void
G4Mag_UsualEqRhs::EvaluateRhsGivenB( const G4double y[],
			             const G4double B[3],
				           G4double dydx[] ) const
{
   G4double momentum_mag_square = y[3]*y[3] + y[4]*y[4] + y[5]*y[5];
   G4double inv_momentum_magnitude = 1.0 / std::sqrt( momentum_mag_square );

   G4double cof = FCof()*inv_momentum_magnitude;

   dydx[0] = y[3]*inv_momentum_magnitude;       //  (d/ds)x = Vx/V
   dydx[1] = y[4]*inv_momentum_magnitude;       //  (d/ds)y = Vy/V
   dydx[2] = y[5]*inv_momentum_magnitude;       //  (d/ds)z = Vz/V

   dydx[3] = cof*(y[4]*B[2] - y[5]*B[1]) ;   // Ax = a*(Vy*Bz - Vz*By)
   dydx[4] = cof*(y[5]*B[0] - y[3]*B[2]) ;   // Ay = a*(Vz*Bx - Vx*Bz)
   dydx[5] = cof*(y[3]*B[1] - y[4]*B[0]) ;   // Az = a*(Vx*By - Vy*Bx)

   return ;
}

void
G4Mag_UsualEqRhs::
 SetChargeMomentumMass( G4double particleCharge, // in e+ units
			                 G4double MomentumXc,
			                 G4double mass)

{
   fInvCurrentMomentumXc= 1.0 / MomentumXc;
   G4Mag_EqRhs::SetChargeMomentumMass( particleCharge, MomentumXc, mass);
}