// -*- tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 2 -*-
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#ifndef DUNE_GRID_INTERSECTIONITERATOR_HH
#define DUNE_GRID_INTERSECTIONITERATOR_HH

#include <dune/common/iteratorfacades.hh>
#include <dune/common/proxymemberaccess.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 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, class IntersectionIteratorImp, 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 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< GridImp, IntersectionImp > Intersection;

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

    // The behavior when dereferencing the IntersectionIterator facade depends on
    // the way the grid implementation handles returning intersections. The implementation
    // may either return a reference to an intersection stored inside the IntersectionIterator
    // implementation or a temporary Intersection object. This object has to be forwarded through
    // the facade to the user, which requires a little trickery, especially for operator->().
    //
    // In order to avoid confusing users reading the Doxygen documentation, we provide "clean"
    // function signatures to Doxygen and hide the actual implementations.

#ifdef DOXYGEN

    /** \brief Dereferencing operator. */
    Intersection operator*() const;

    /** \brief Pointer operator. */
    const Intersection* operator->() const;

#else // DOXYGEN

    /** \brief Dereferencing operator. */
    typename std::conditional<
      std::is_lvalue_reference<
        decltype(realIterator.dereference())
        >::value,
      const Intersection&,
      Intersection
      >::type
    operator*() const
    {
      return this->realIterator.dereference();
    }

    /** \brief Pointer operator. */
    decltype(handle_proxy_member_access(realIterator.dereference()))
    operator->() const
    {
      return handle_proxy_member_access(realIterator.dereference());
    }

#endif // DOXYGEN

    //@}


    //===========================================================
    /** @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;
    }

    /** @brief Default constructor. */
    IntersectionIterator()
    {}

    //===========================================================
    /** @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 Implementation &impl )
      : realIterator( impl )
    {}

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

} // namespace Dune

#include "intersection.hh"

#endif // DUNE_GRID_INTERSECTIONITERATOR_HH
