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//
//
// $Id$
//
//
// class G4Field
//
// Class description:
//
// Abstract class for any kind of Field.
// It allows any kind of field (vector, scalar, tensor and any set of them)
// to be defined by implementing the inquiry function interface.
//
// The key method is  GetFieldValue( const  double Point[4],
//                    *************         double *fieldArr ) 
// Given an input position/time vector 'Point', 
// this method must return the value of the field in "fieldArr".
//
// A field must also specify whether it changes a track's energy:
//                    DoesFieldChangeEnergy() 
//                    *********************
// A field must co-work with a corresponding Equation of Motion, to
// enable the integration of a particle's position, momentum and, optionally, 
// spin.  For this a field and its equation of motion must follow the
// same convention for the order of field components in the array "fieldArr"
// -------------------------------------------------------------------
// History:
// - Created:  John Apostolakis, 10.03.1997
// - Modified: 
//   V. Grichine     8 Nov 2001: Extended "Point" arg to [4] array to add time
//   J. Apostolakis  5 Nov 2003: Added virtual method DoesFieldChangeEnergy()
//   J. Apostolakis 31 Aug 2004: Information on convention for components
// -------------------------------------------------------------------

#ifndef G4FIELD_HH
#define G4FIELD_HH

#include "G4Types.hh"
#include "globals.hh"

class G4Field
{
  public:  // with description

      virtual void  GetFieldValue( const  double Point[4],
                                          double *fieldArr ) const = 0;
       // Given the position time vector 'Point', 
       // return the value of the field in the array fieldArr.
       //  Notes: 
       //   1) The 'Point' vector has the following structure:
       //        Point[0]  is  x  ( position, in Geant4 units )
       //        Point[1]  is  y
       //        Point[2]  is  z
       //        Point[3]  is  t  ( time,  in Geant4 units )
       //   2) The convention for the components of the field
       //      array 'fieldArr' are determined by the type of field.
       //      See for example the class G4ElectroMagneticField.

      G4Field( G4bool gravityOn= false);
      G4Field( const G4Field & );
      virtual ~G4Field();
      G4Field& operator = (const G4Field &p); 

     // A field signature function that can be used to insure
     // that the Equation of motion object and the G4Field object
     // have the same "field signature"?
 
      virtual G4bool   DoesFieldChangeEnergy() const= 0 ;
       //  Each type/class of field should respond this accordingly
       //  For example:
       //    - an electric field     should return "true"
       //    - a pure magnetic field should return "false"

      G4bool   IsGravityActive() const { return fGravityActive;}
       //  Does this field include gravity?
      inline void SetGravityActive( G4bool OnOffFlag );
    
      virtual G4Field* Clone() const;
       // Implements cloning, needed by G4 MT

  private:

      G4bool  fGravityActive;
};

inline void  G4Field::SetGravityActive( G4bool OnOffFlag )
{ 
  fGravityActive= OnOffFlag; 
} 
#endif /* G4FIELD_HH */
