/* -*- C++ -*-

 * src/lemon/concept/graph_component.h - Part of LEMON, a generic C++ optimization library

 *

 * Copyright (C) 2004 Egervary Jeno Kombinatorikus Optimalizalasi Kutatocsoport

 * (Egervary Combinatorial Optimization Research Group, EGRES).

 *

 * Permission to use, modify and distribute this software is granted

 * provided that this copyright notice appears in all copies. For

 * precise terms see the accompanying LICENSE file.

 *

 * This software is provided "AS IS" with no warranty of any kind,

 * express or implied, and with no claim as to its suitability for any

 * purpose.

 *

 */

///\ingroup graph_concepts

///\file

///\brief The graph components.

#ifndef LEMON_CONCEPT_GRAPH_COMPONENT_H

#define LEMON_CONCEPT_GRAPH_COMPONENT_H

#include <lemon/invalid.h>

#include <lemon/concept/maps.h>

#include <lemon/alteration_notifier.h>

namespace lemon {

  namespace concept {

    /// \addtogroup graph_concepts

    /// @{

    /****************   Graph iterator concepts   ****************/

    /// Skeleton class for graph Node and Edge types

    /// This class describes the interface of Node and Edge (and UndirEdge

    /// in undirected graphs) subtypes of graph types.

    ///

    /// \note This class is a template class so that we can use it to

    /// create graph skeleton classes. The reason for this is than Node

    /// and Edge types should \em not derive from the same base class.

    /// For Node you should instantiate it with character 'n' and for Edge

    /// with 'e'.

#ifndef DOXYGEN

    template <char _selector = '0'>

#endif

    class GraphItem {

    public:

      /// Default constructor.

      /// \warning The default constructor is not required to set

      /// the item to some well-defined value. So you should consider it

      /// as uninitialized.

      GraphItem() {}

      /// Copy constructor.

      /// Copy constructor.

      ///

      GraphItem(GraphItem const&) {}

      /// Invalid constructor \& conversion.

      /// This constructor initializes the item to be invalid.

      /// \sa Invalid for more details.

      GraphItem(Invalid) {}

      /// Assign operator for nodes.

      /// The nodes are assignable. 

      ///

      GraphItem& operator=(GraphItem const&) { return *this; }

      /// Equality operator.

      /// Two iterators are equal if and only if they represents the

      /// same node in the graph or both are invalid.

      bool operator==(GraphItem) const { return false; }

      /// Inequality operator.

      /// \sa operator==(const Node& n)

      ///

      bool operator!=(GraphItem) const { return false; }

      /// Artificial ordering operator.

      /// To allow the use of graph descriptors as key type in std::map or

      /// similar associative container we require this.

      ///

      /// \note This operator only have to define some strict ordering of

      /// the items; this order has nothing to do with the iteration

      /// ordering of the items.

      ///

      /// \bug This is a technical requirement. Do we really need this?

      bool operator<(GraphItem) const { return false; }

      template<typename _GraphItem>

      struct Constraints {

	void constraints() {

	  _GraphItem i1;

	  _GraphItem i2 = i1;

	  _GraphItem i3 = INVALID;

	  i1 = i2 = i3;

	  bool b;

	  //	  b = (ia == ib) && (ia != ib) && (ia < ib);

	  b = (ia == ib) && (ia != ib);

	  b = (ia == INVALID) && (ib != INVALID);

	  //	  b = (ia < ib);

	}

	const _GraphItem &ia;

	const _GraphItem &ib;

      };

    };

    /// A type describing the concept of graph node

    /// This is an instantiation of \ref GraphItem which can be used as a

    /// Node subtype in graph skeleton definitions

    typedef GraphItem<'n'> GraphNode;

    /// A type describing the concept of graph edge

    /// This is an instantiation of \ref GraphItem which can be used as a

    /// Edge subtype in graph skeleton definitions

    typedef GraphItem<'e'> GraphEdge;

    /**************** Basic features of graphs ****************/

    /// An empty base graph class.

    /// This class provides the minimal set of features needed for a graph

    /// structure. All graph concepts have to be conform to this base

    /// graph.

    ///

    /// \bug This is not true. The minimal graph concept is the

    /// BaseIterableGraphComponent.

    class BaseGraphComponent {

    public:

      typedef BaseGraphComponent Graph;

      /// Node class of the graph.

      /// This class represents the Nodes of the graph. 

      ///

      typedef GraphItem<'n'> Node;

      /// Edge class of the graph.

      /// This class represents the Edges of the graph. 

      ///

      typedef GraphItem<'e'> Edge;

      ///Gives back the target node of an edge.

      ///Gives back the target node of an edge.

      ///

      Node target(const Edge&) const { return INVALID;}

      ///Gives back the source node of an edge.

      ///Gives back the source node of an edge.

      ///

      Node source(const Edge&) const { return INVALID;}

      template <typename _Graph>

      struct Constraints {

	typedef typename _Graph::Node Node;

	typedef typename _Graph::Edge Edge;

	void constraints() {

	  checkConcept<GraphItem<'n'>, Node>();

	  checkConcept<GraphItem<'e'>, Edge>();

	  {

	    Node n;

	    Edge e;

	    n = graph.source(e);

	    n = graph.target(e);

	  }      

	}

	const _Graph& graph;

      };

    };

    /// An empty iterable base graph class.

    /// This class provides beside the core graph features

    /// core iterable interface for the graph structure.

    /// Most of the base graphs should be conform to this concept.

    class BaseIterableGraphComponent : virtual public BaseGraphComponent {

    public:

      typedef BaseGraphComponent::Node Node;

      typedef BaseGraphComponent::Edge Edge;

      /// Gives back the first Node in the iterating order.

      /// Gives back the first Node in the iterating order.

      ///     

      void first(Node&) const {}

      /// Gives back the next Node in the iterating order.

      /// Gives back the next Node in the iterating order.

      ///     

      void next(Node&) const {}

      /// Gives back the first Edge in the iterating order.

      /// Gives back the first Edge in the iterating order.

      ///     

      void first(Edge&) const {}

      /// Gives back the next Edge in the iterating order.

      /// Gives back the next Edge in the iterating order.

      ///     

      void next(Edge&) const {}

      /// Gives back the first of the Edges point to the given Node.

      /// Gives back the first of the Edges point to the given Node.

      ///     

      void firstIn(Edge&, const Node&) const {}

      /// Gives back the next of the Edges points to the given Node.

      /// Gives back the next of the Edges points to the given Node.

      ///

      void nextIn(Edge&) const {}

      /// Gives back the first of the Edges start from the given Node.

      /// Gives back the first of the Edges start from the given Node.

      ///     

      void firstOut(Edge&, const Node&) const {}

      /// Gives back the next of the Edges start from the given Node.

      /// Gives back the next of the Edges start from the given Node.

      ///     

      void nextOut(Edge&) const {}

      template <typename _Graph>

      struct Constraints {

	void constraints() {

	  checkConcept< BaseGraphComponent, _Graph >();

	  typename _Graph::Node node;      

	  typename _Graph::Edge edge;

	  {

	    graph.first(node);

	    graph.next(node);

	  }

	  {

	    graph.first(edge);

	    graph.next(edge);

	  }

	  {

	    graph.firstIn(edge, node);

	    graph.nextIn(edge);

	  }

	  {

	    graph.firstOut(edge, node);

	    graph.nextOut(edge);

	  }

	}

	const _Graph& graph;

      };

    };

    /// An empty idable base graph class.

    /// This class provides beside the core graph features

    /// core id functions for the graph structure.

    /// The most of the base graphs should be conform to this concept.

    /// The id's are unique and immutable.

    class IDableGraphComponent : virtual public BaseGraphComponent {

    public:

      typedef BaseGraphComponent::Node Node;

      typedef BaseGraphComponent::Edge Edge;

      /// Gives back an unique integer id for the Node. 

      /// Gives back an unique integer id for the Node. 

      ///

      int id(const Node&) const { return -1;}

      /// \brief Gives back the node by the unique id.

      ///

      /// Gives back the node by the unique id.

      /// If the graph does not contain node with the given id

      /// then the result of the function is undetermined. 

      Node fromId(int id, Node) const { return INVALID;}

      /// \brief Gives back an unique integer id for the Edge. 

      ///

      /// Gives back an unique integer id for the Edge. 

      ///

      int id(const Edge&) const { return -1;}

      /// \brief Gives back the edge by the unique id.

      ///

      /// Gives back the edge by the unique id.

      /// If the graph does not contain edge with the given id

      /// then the result of the function is undetermined. 

      Edge fromId(int id, Edge) const { return INVALID;}

      template <typename _Graph>

      struct Constraints {

	void constraints() {

	  checkConcept< BaseGraphComponent, _Graph >();

	  typename _Graph::Node node;

	  int nid = graph.id(node);

	  nid = graph.id(node);

	  node = graph.fromId(nid, Node());

	  typename _Graph::Edge edge;

	  int eid = graph.id(edge);

	  eid = graph.id(edge);

	  edge = graph.fromId(eid, Edge());

	}

	const _Graph& graph;

      };

    };

    /// An empty max-idable base graph class.

    /// This class provides beside the core graph features

    /// core max id functions for the graph structure.

    /// The most of the base graphs should be conform to this concept.

    /// The id's are unique and immutable.

    class MaxIDableGraphComponent : virtual public BaseGraphComponent {

    public:

      /// Gives back an integer greater or equal to the maximum Node id. 

      /// Gives back an integer greater or equal to the maximum Node id. 

      ///

      int maxId(Node = INVALID) const { return -1;}

      /// Gives back an integer greater or equal to the maximum Edge id. 

      /// Gives back an integer greater or equal to the maximum Edge id. 

      ///

      int maxId(Edge = INVALID) const { return -1;}

      template <typename _Graph>

      struct Constraints {

	void constraints() {

	  checkConcept<BaseGraphComponent, _Graph>();

	  int nid = graph.maxId(typename _Graph::Node());

	  ignore_unused_variable_warning(nid);

	  int eid = graph.maxId(typename _Graph::Edge());

	  ignore_unused_variable_warning(eid);

	}

	const _Graph& graph;

      };

    };

    /// An empty extendable base graph class.

    /// This class provides beside the core graph features

    /// core graph extend interface for the graph structure.

    /// The most of the base graphs should be conform to this concept.

    class BaseExtendableGraphComponent : virtual public BaseGraphComponent {

    public:

      typedef BaseGraphComponent::Node Node;

      typedef BaseGraphComponent::Edge Edge;

      /// Adds a new Node to the graph.

      /// Adds a new Node to the graph.

      ///

      Node addNode() {

	return INVALID;

      }

      /// Adds a new Edge connects the two Nodes to the graph.

      /// Adds a new Edge connects the two Nodes to the graph.

      ///

      Edge addEdge(const Node& from, const Node& to) {

	return INVALID;

      }

      template <typename _Graph>

      struct Constraints {

	void constraints() {

	  checkConcept<BaseGraphComponent, _Graph >();

	  typename _Graph::Node node_a, node_b;

	  node_a = graph.addNode();

	  typename _Graph::Edge edge;

	  edge = graph.addEdge(node_a, node_b);

	}

	_Graph& graph;

      };

    };

    /// An empty erasable base graph class.

    /// This class provides beside the core graph features

    /// core erase functions for the graph structure.

    /// The most of the base graphs should be conform to this concept.

    class BaseErasableGraphComponent : virtual public BaseGraphComponent {

    public:

      typedef BaseGraphComponent::Node Node;

      typedef BaseGraphComponent::Edge Edge;

      /// Erase a Node from the graph.

      /// Erase a Node from the graph. This function should not

      /// erase edges connecting to the Node.

      void erase(const Node&) {}    

      /// Erase an Edge from the graph.

      /// Erase an Edge from the graph.

      ///

      void erase(const Edge&) {}

      template <typename _Graph>

      struct Constraints {

	void constraints() {

	  checkConcept<BaseGraphComponent, _Graph>();

	  typename _Graph::Node node;

	  graph.erase(node);

	  typename _Graph::Edge edge;

	  graph.erase(edge);

	}

	_Graph& graph;

      };

    };

    /// An empty clearable base graph class.

    /// This class provides beside the core graph features

    /// core clear functions for the graph structure.

    /// The most of the base graphs should be conform to this concept.

    class ClearableGraphComponent : virtual public BaseGraphComponent {

    public:

      /// Erase all the Nodes and Edges from the graph.

      /// Erase all the Nodes and Edges from the graph.

      ///

      void clear() {}    

      template <typename _Graph>

      struct Constraints {

	void constraints() {

	  checkConcept<BaseGraphComponent, _Graph>();

	  graph.clear();

	}

	_Graph graph;

      };

    };

    /// Skeleton class for graph NodeIt and EdgeIt

    /// Skeleton class for graph NodeIt and EdgeIt.

    ///

    template <typename _Graph, typename _Item>

    class GraphIterator : public _Item {

    public:

      /// \todo Don't we need the Item type as typedef?

      /// Default constructor.

      /// @warning The default constructor sets the iterator

      /// to an undefined value.

      GraphIterator() {}

      /// Copy constructor.

      /// Copy constructor.

      ///

      GraphIterator(GraphIterator const&) {}

      /// Sets the iterator to the first item.

      /// Sets the iterator to the first item of \c the graph.

      ///

      explicit GraphIterator(const _Graph&) {}

      /// Invalid constructor \& conversion.

      /// This constructor initializes the item to be invalid.

      /// \sa Invalid for more details.

      GraphIterator(Invalid) {}

      /// Assign operator for items.

      /// The items are assignable. 

      ///

      GraphIterator& operator=(GraphIterator const&) { return *this; }      

      /// Next item.

      /// Assign the iterator to the next item.

      ///

      GraphIterator& operator++() { return *this; }

      //	Node operator*() const { return INVALID; }

      /// Equality operator

      /// Two iterators are equal if and only if they point to the

      /// same object or both are invalid.

      bool operator==(const GraphIterator&) const { return true;}

      /// Inequality operator

      /// \sa operator==(Node n)

      ///

      bool operator!=(const GraphIterator&) const { return true;}

      template<typename _GraphIterator>

      struct Constraints {

	void constraints() {

	  //	  checkConcept< Item, _GraphIterator >();

	  _GraphIterator it1(g);

	  /// \todo Do we need NodeIt(Node) kind of constructor?

	  //	_GraphIterator it2(bj);

	  _GraphIterator it2;

	  it2 = ++it1;

	  ++it2 = it1;

	  ++(++it1);

	  /// \bug This should be: is_base_and_derived<BaseItem, _GraphIterator>

	  _Item bi = it1;

	  bi = it2;

	}

	_Graph& g;

      };

    };

    /// Skeleton class for graph InEdgeIt and OutEdgeIt

    /// \note Because InEdgeIt and OutEdgeIt may not inherit from the same

    /// base class, the _selector is a additional template parameter. For 

    /// InEdgeIt you should instantiate it with character 'i' and for 

    /// OutEdgeIt with 'o'.

    /// \todo Is this a good name for this concept?

    template <typename Graph,

	      typename Edge = typename Graph::Edge,

	      char _selector = '0'>

    class GraphIncIterator : public Edge {

    public:

      /// Default constructor.

      /// @warning The default constructor sets the iterator

      /// to an undefined value.

      GraphIncIterator() {}

      /// Copy constructor.

      /// Copy constructor.

      ///

      GraphIncIterator(GraphIncIterator const&) {}

      /// Sets the iterator to the first edge incoming into or outgoing 

      /// from the node.

      /// Sets the iterator to the first edge incoming into or outgoing 

      /// from the node.

      ///

      explicit GraphIncIterator(const Graph&, const typename Graph::Node&) {}

      /// Invalid constructor \& conversion.

      /// This constructor initializes the item to be invalid.

      /// \sa Invalid for more details.

      GraphIncIterator(Invalid) {}

      /// Assign operator for nodes.

      /// The nodes are assignable. 

      ///

      GraphIncIterator& operator=(GraphIncIterator const&) { return *this; }      

      /// Next edge.

      /// Assign the iterator to the next node.

      ///

      GraphIncIterator& operator++() { return *this; }

      //	Node operator*() const { return INVALID; }

      /// Equality operator

      /// Two iterators are equal if and only if they point to the

      /// same object or both are invalid.

      bool operator==(const GraphIncIterator&) const { return true;}

      /// Inequality operator

      /// \sa operator==(Node n)

      ///

      bool operator!=(const GraphIncIterator&) const { return true;}

      template <typename _GraphIncIterator>

      struct Constraints {

	typedef typename Graph::Node Node;

	void constraints() {

	  checkConcept<GraphItem<'e'>, _GraphIncIterator>();

	  _GraphIncIterator it1(graph, node);

	  /// \todo Do we need OutEdgeIt(Edge) kind of constructor?

	  //	_GraphIncIterator it2(edge);

	  _GraphIncIterator it2;

	  it2 = ++it1;

	  ++it2 = it1;

	  ++(++it1);

	  Edge e = it1;

	  e = it2;

	  const_constraits();

	}

	void const_constraits() {

	  Node n = graph.baseNode(it);

	  n = graph.runningNode(it);

	}

	Edge edge;

	Node node;

	Graph graph;

	_GraphIncIterator it;

      };

    };

    /// An empty iterable base graph class.

    /// This class provides beside the core graph features

    /// iterator based iterable interface for the graph structure.

    /// This concept is part of the StaticGraphConcept.

    class IterableGraphComponent : virtual public BaseGraphComponent {

    public:

      typedef IterableGraphComponent Graph;

      typedef BaseGraphComponent::Node Node;

      typedef BaseGraphComponent::Edge Edge;

      /// This iterator goes through each node.

      /// This iterator goes through each node.

      ///

      typedef GraphIterator<Graph, Node> NodeIt;

      /// This iterator goes through each node.

      /// This iterator goes through each node.

      ///

      typedef GraphIterator<Graph, Edge> EdgeIt;

      /// This iterator goes trough the incoming edges of a node.

      /// This iterator goes trough the \e inccoming edges of a certain node

      /// of a graph.

      typedef GraphIncIterator<Graph, Edge, 'i'> InEdgeIt;

      /// This iterator goes trough the outgoing edges of a node.

      /// This iterator goes trough the \e outgoing edges of a certain node

      /// of a graph.

      typedef GraphIncIterator<Graph, Edge, 'o'> OutEdgeIt;

    };

    template <typename _Graph> 

    struct Constraints {

      void constraints() {

  	checkConcept< BaseGraphComponent, _Graph>();

	checkConcept<GraphIterator<_Graph, typename _Graph::Edge>,

	  typename _Graph::EdgeIt >();

	checkConcept<GraphIterator<_Graph, typename _Graph::Node>,

	  typename _Graph::NodeIt >();

	checkConcept<GraphIncIterator<_Graph>, typename _Graph::InEdgeIt >();

	checkConcept<GraphIncIterator<_Graph>, typename _Graph::OutEdgeIt >();

      }

    };

    /// An empty alteration notifier base graph class.

    /// This class provides beside the core graph features

    /// alteration notifier interface for the graph structure.

    /// This is an observer-notifier pattern. More Obsevers can

    /// be registered into the notifier and whenever an alteration

    /// occured in the graph all the observers will notified about it.

    class AlterableGraphComponent : virtual public BaseGraphComponent {

    public:

      /// The edge observer registry.

      typedef AlterationNotifier<Edge> EdgeNotifier;

      /// The node observer registry.

      typedef AlterationNotifier<Node> NodeNotifier;

      /// \brief Gives back the edge alteration notifier.

      ///

      /// Gives back the edge alteration notifier.

      EdgeNotifier getNotifier(Edge) const {

	return EdgeNotifier();

      }

      /// \brief Gives back the node alteration notifier.

      ///

      /// Gives back the node alteration notifier.

      NodeNotifier getNotifier(Node) const {

	return NodeNotifier();

      }

    };

    /// Class describing the concept of graph maps

    /// This class describes the common interface of the graph maps

    /// (NodeMap, EdgeMap), that is \ref maps-pages "maps" which can be used to

    /// associate data to graph descriptors (nodes or edges).

    template <typename Graph, typename Item, typename _Value>

    class GraphMap : public ReadWriteMap<Item, _Value> {

    protected:      

      GraphMap() {}

    public:

      /// \brief Construct a new map.

      ///

      /// Construct a new map for the graph.

      explicit GraphMap(const Graph&) {}

      /// \brief Construct a new map with default value.

      ///

      /// Construct a new map for the graph and initalise the values.

      GraphMap(const Graph&, const _Value&) {}

      /// \brief Copy constructor.

      ///

      /// Copy Constructor.

      GraphMap(const GraphMap&) {}

      /// \brief Assign operator.

      ///

      /// Assign operator.

      GraphMap& operator=(const GraphMap&) { return *this;}

      template<typename _Map>

      struct Constraints {

	void constraints() {

	  checkConcept<ReadWriteMap<Item, _Value>, _Map >();

	  // Construction with a graph parameter

	  _Map a(g);

	  // Constructor with a graph and a default value parameter

	  _Map a2(g,t);

	  // Copy constructor. Do we need it?

	  _Map b=c;

	  // Copy operator. Do we need it?

	  a=b;

	  ignore_unused_variable_warning(a2);

	}

	const _Map &c;

	const Graph &g;

	const typename GraphMap::Value &t;

      };

    };

    /// An empty mappable base graph class.

    /// This class provides beside the core graph features

    /// map interface for the graph structure.

    /// This concept is part of the StaticGraphConcept.

    class MappableGraphComponent : virtual public BaseGraphComponent {

    public:

      typedef MappableGraphComponent Graph;

      typedef BaseGraphComponent::Node Node;

      typedef BaseGraphComponent::Edge Edge;

      /// ReadWrite map of the nodes.

      /// ReadWrite map of the nodes.

      ///

      template <typename _Value>

      class NodeMap : public GraphMap<Graph, Node, _Value> {

      private:

	NodeMap();

      public:

	/// \brief Construct a new map.

	///

	/// Construct a new map for the graph.

	/// \todo call the right parent class constructor

	explicit NodeMap(const Graph&) {}

	/// \brief Construct a new map with default value.

	///

	/// Construct a new map for the graph and initalise the values.

	NodeMap(const Graph&, const _Value&) {}

	/// \brief Copy constructor.

	///

	/// Copy Constructor.

	NodeMap(const NodeMap&) {}

	/// \brief Assign operator.

	///

	/// Assign operator.

	NodeMap& operator=(const NodeMap&) { return *this;}

      };

      /// ReadWrite map of the edges.

      /// ReadWrite map of the edges.

      ///

      template <typename _Value>

      class EdgeMap : public GraphMap<Graph, Edge, _Value> {

      private:

	EdgeMap();

      public:

	/// \brief Construct a new map.

	///

	/// Construct a new map for the graph.

	/// \todo call the right parent class constructor

	explicit EdgeMap(const Graph&) {}

	/// \brief Construct a new map with default value.

	///

	/// Construct a new map for the graph and initalise the values.

	EdgeMap(const Graph&, const _Value&) {}

	/// \brief Copy constructor.

	///

	/// Copy Constructor.

	EdgeMap(const EdgeMap&) {}

	/// \brief Assign operator.

	///

	/// Assign operator.

	EdgeMap& operator=(const EdgeMap&) { return *this;}

      };

      template <typename _Graph>

      struct Constraints {

	struct Type {

	  int value;

	  Type() : value(0) {}

	  Type(int _v) : value(_v) {}

	};

	void constraints() {

	  checkConcept<BaseGraphComponent, _Graph>();

	  { // int map test

	    typedef typename _Graph::template NodeMap<int> IntNodeMap;

	    checkConcept<GraphMap<_Graph, typename _Graph::Node, int>, 

	      IntNodeMap >();

	  } { // bool map test

	    typedef typename _Graph::template NodeMap<bool> BoolNodeMap;

	    checkConcept<GraphMap<_Graph, typename _Graph::Node, bool>,

	      BoolNodeMap >();

	  } { // Type map test

	    typedef typename _Graph::template NodeMap<Type> TypeNodeMap;

	    checkConcept<GraphMap<_Graph, typename _Graph::Node, Type>,

	      TypeNodeMap >();

	  } 

	  { // int map test

	    typedef typename _Graph::template EdgeMap<int> IntEdgeMap;

	    checkConcept<GraphMap<_Graph, typename _Graph::Edge, int>,

	      IntEdgeMap >();

	  } { // bool map test

	    typedef typename _Graph::template EdgeMap<bool> BoolEdgeMap;

	    checkConcept<GraphMap<_Graph, typename _Graph::Edge, bool>,

	      BoolEdgeMap >();

	  } { // Type map test

	    typedef typename _Graph::template EdgeMap<Type> TypeEdgeMap;

	    checkConcept<GraphMap<_Graph, typename _Graph::Edge, Type>, 

	      TypeEdgeMap >();

	  } 

	}

	_Graph& graph;

      };

    };

    /// \brief An empty extendable extended graph class.

    ///

    /// This class provides beside the core graph features

    /// item addition interface for the graph structure.

    /// The difference between this class and the 

    /// \c BaseExtendableGraphComponent is that it should

    /// notify the item alteration observers.

    class ExtendableGraphComponent : virtual public BaseGraphComponent {

    public:

      typedef ExtendableGraphComponent Graph;

      typedef BaseGraphComponent::Node Node;

      typedef BaseGraphComponent::Edge Edge;

      /// \brief Add a node to the graph.

      ///

      /// Add a node to the graph and notify the observers.

      Node addNode() {

	return INVALID;

      }

      /// \brief Add an edge to the graph.

      ///

      /// Add an edge to the graph and notify the observers.

      Edge addEdge(const Node& from, const Node& to) {

	return INVALID;

      }

      template <typename _Graph>

      struct Constraints {

	void constraints() {

	  checkConcept<BaseGraphComponent, _Graph >();

	  typename _Graph::Node node_a, node_b;

	  node_a = graph.addNode();

	  typename _Graph::Edge edge;

	  edge = graph.addEdge(node_a, node_b);      

	}

	_Graph& graph;

      };

    };

    /// \brief An empty erasable extended graph class.

    ///

    /// This class provides beside the core graph features

    /// item erase interface for the graph structure.

    /// The difference between this class and the 

    /// \c BaseErasableGraphComponent is that it should

    /// notify the item alteration observers.

    class ErasableGraphComponent : virtual public BaseGraphComponent {

    public:

      typedef ErasableGraphComponent Graph;

      typedef BaseGraphComponent::Node Node;

      typedef BaseGraphComponent::Edge Edge;

      /// \brief Erase the Node and notify the node alteration observers.

      ///

      ///  Erase the Node and notify the node alteration observers.

      void erase(const Node&) {}    

      /// \brief Erase the Edge and notify the edge alteration observers.

      ///

      ///  Erase the Edge and notify the edge alteration observers.

      void erase(const Edge&) {}

      template <typename _Graph>

      struct Constraints {

	void constraints() {

	  checkConcept<BaseGraphComponent, _Graph >();

	  typename _Graph::Node node;

	  graph.erase(node);

	  typename _Graph::Edge edge;

	  graph.erase(edge);      

	}

	_Graph& graph;

      };

    };

    /// @}

  }

}

#endif