#ifndef DUNE_GRID_INTERSECTIONITERATOR_HH
#define DUNE_GRID_INTERSECTIONITERATOR_HH

#include <dune/common/iteratorfacades.hh>

#include <dune/grid/common/intersection.hh>

namespace Dune
{

/** \brief Mesh entities of codimension 0 ("elements") allow to visit all
   intersections with "neighboring" elements and with the domain
   boundary.  

   Template parameters are:

   - <tt>GridImp</tt> Type that is a model of Dune::Grid
   - <tt>IntersectionIteratorImp</tt> Class template that is a model of 
   Dune::IntersectionIterator

   @warning the IntersectionIterator used to be both, Intersection and IntersectionIterator,
   at the same time. The two concepts are now properly separated. The IntersectionIterator
   still offers the old methods, but these are forwarded to the Intersection. All these methods
   are now marked deprecated.

   \deprecated All Intersection methods on the IntersectionIterator are deprecated,
     dereference IntersectionIterator to get the Intersection and call methods there.
   
   @warning The number of neigbors may be different from the number of
   faces/edges of an element!

   <h2>Overview</h2>
   
   Intersections are codimension 1 objects. These
   intersections are accessed via an IntersectionIterator. This allows
   the implementation of non-matching grids, as one face can now
   consist of several intersections.
   In a conforming mesh such an intersection corresponds to an entity of
   codimension 1 but in the general non-conforming case there will be no entity
   in the mesh that directly corresponds to the intersection. Thus, the
   IntersectionIterator describes these intersections implicitly.

   <H2>Engine Concept</H2>

   The IntersectionIterator class template wraps an object of type IntersectionIteratorImp
   and forwards all member 
   function calls to corresponding members of this class. In that sense IntersectionIterator
   defines the interface and IntersectionIteratorImp supplies the implementation.

   <h2>Intersections, leaf grid and level grid</h2>
   
   On an entity \b e of codimension zero there are two ways to create
   IntersectionIterators by either using ilevelbegin() / ilevelend() or
   ileafbegin()/ileafend(). In the first case intersections with
   neighboring entities having the same level as \b e are traversed; in
   the second case  ileafbegin()==ileafend() if \b e is not a leaf otherwise
   all intersections with neighboring leaf entities are traversed.

   Consider a situation where two elements \b a and \b b have a common intersection.
   %Element \b b has been refined into an element \b c and \b d, while \b a has not
   been refined.
   In one space dimension this situation is depicted in the figure below.

   \image html  islocalref.png "IntersectionIterator in a locally refined mesh."
   \image latex islocalref.eps "IntersectionIterator in a locally refined mesh." width=\textwidth
 
   Here the rule is the following: The %LevelIntersectionIterator 
   delivers all intersections
   with elements on the same level, the %LeafIntersectionIterator delivers
   the intersections with all leaf elements
   if it has been started on a leaf element.  Both iterators also stop at intersections
   with the grid boundary.
   According to this rule the level intersection iterator started at element \b a 
   in the example above delivers an intersection with \b b and the left grid boundary,
   whereas the leaf intersection iterator returns \b c instead of \b b. 
   Starting on entity \b c the level intersection iterator returns \b d and the
   intersection with the left boundary of the level 1 grid,
   but the leaf intersection iterator returns both \b d and \b a. 
   Finally, starting on \b b the level intersection
   iterator returns \b a and the right boundary, but the leaf intersection iterator is empty since
   \b b is not a leaf entity of the grid. Starting on \b d both the 
   level and the leaf intersection iterators will return the element \b c
   together with the right grid boundary.

   @ingroup GIIntersectionIterator
 */
template<class GridImp, template<class> class IntersectionIteratorImp, template<class> class IntersectionImp>
class IntersectionIterator
{
#if DUNE_GRID_EXPERIMENTAL_GRID_EXTENSIONS
public:
#else
protected:
  // give the GridDefaultImplementation class access to the realImp 
  friend class GridDefaultImplementation< 
            GridImp::dimension, GridImp::dimensionworld,
            typename GridImp::ctype,
            typename GridImp::GridFamily> ;
#endif
  // type of underlying implementation, for internal use only 
  typedef IntersectionIteratorImp< const GridImp > Implementation;

  //! return reference to the real implementation 
  Implementation &impl () { return realIterator; }
  //! return reference to the real implementation 
  const Implementation &impl () const { return realIterator; }

protected:
  Implementation realIterator;

public:
    /** \brief Type of Intersection this IntersectionIterator points to */
  typedef Dune::Intersection< const GridImp, IntersectionImp > Intersection;

  //===========================================================
  /** @name Dereferencing
   */
  //@{
  //===========================================================

  /** \brief Dereferencing operator. */
  const Intersection & operator*() const
    {
      return this->realIterator.dereference();
    }

  /** \brief Pointer operator. */
  const Intersection * operator->() const
    {
      return & this->realIterator.dereference();
    }
  //@}


  //===========================================================
  /** @name Compare methods
   */
  //@{
  //===========================================================

  /** @brief Checks for equality.     
      Only Iterators pointing to the same intersection from the same Entity
      are equal. Pointing to the same intersection from neighbor is
      unequal as inside and outside are permuted.
   */
  bool operator==(const IntersectionIterator& rhs) const
    {
      return rhs.equals(*this);
    }

  /** @brief Checks for inequality.     
      Only Iterators pointing to the same intersection from the same Entity
      are equal. Pointing to the same intersection from neighbor is
      unequal as inside and outside are permuted.
  */
  bool operator!=(const IntersectionIterator& rhs) const
    {
      return ! rhs.equals(*this);
    }
  //@}

  /** @brief Preincrement operator. Proceed to next intersection.*/
  IntersectionIterator& operator++()
    {
      this->realIterator.increment();
      return *this;
    }
  
  //===========================================================
  /** @name Implementor interface
   */
  //@{
  //===========================================================

  /** @brief forward equality check to realIterator */
  bool equals(const IntersectionIterator& rhs) const
    {
      return this->realIterator.equals(rhs.realIterator);
    }

  /** Copy Constructor from IntersectionIteratorImp */
  IntersectionIterator(const IntersectionIteratorImp<const GridImp> & i) :
    realIterator(i) {}

  /** Copy constructor */
  IntersectionIterator(const IntersectionIterator& i) :
    realIterator(i.realIterator) {}
  //@}

  typedef typename remove_const<GridImp>::type mutableGridImp;
};

} // namespace Dune

#include "intersection.hh"

#endif // DUNE_GRID_INTERSECTIONITERATOR_HH
