/* -*- C++ -*-

  *

  * This file is a part of LEMON, a generic C++ optimization library

  *

  * Copyright (C) 2003-2008

  * Egervary Jeno Kombinatorikus Optimalizalasi Kutatocsoport

  * (Egervary Research Group on Combinatorial Optimization, 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.

  *

  */

 #ifndef LEMON_LIST_GRAPH_H

 #define LEMON_LIST_GRAPH_H

 ///\ingroup graphs

 ///\file

 ///\brief ListDigraph, ListGraph classes.

 #include <lemon/bits/graph_extender.h>

 #include <vector>

 #include <list>

 namespace lemon {

   class ListDigraphBase {

   protected:

     struct NodeT {

       int first_in, first_out;

       int prev, next;

     };

     struct ArcT {

       int target, source;

       int prev_in, prev_out;

       int next_in, next_out;

     };

     std::vector<NodeT> nodes;

     int first_node;

     int first_free_node;

     std::vector<ArcT> arcs;

     int first_free_arc;

   public:

     typedef ListDigraphBase Digraph;

     class Node {

       friend class ListDigraphBase;

     protected:

       int id;

       explicit Node(int pid) { id = pid;}

     public:

       Node() {}

       Node (Invalid) { id = -1; }

       bool operator==(const Node& node) const {return id == node.id;}

       bool operator!=(const Node& node) const {return id != node.id;}

       bool operator<(const Node& node) const {return id < node.id;}

     };

     class Arc {

       friend class ListDigraphBase;

     protected:

       int id;

       explicit Arc(int pid) { id = pid;}

     public:

       Arc() {}

       Arc (Invalid) { id = -1; }

       bool operator==(const Arc& arc) const {return id == arc.id;}

       bool operator!=(const Arc& arc) const {return id != arc.id;}

       bool operator<(const Arc& arc) const {return id < arc.id;}

     };

     ListDigraphBase()

       : nodes(), first_node(-1),

 	first_free_node(-1), arcs(), first_free_arc(-1) {}

     int maxNodeId() const { return nodes.size()-1; } 

     int maxArcId() const { return arcs.size()-1; }

     Node source(Arc e) const { return Node(arcs[e.id].source); }

     Node target(Arc e) const { return Node(arcs[e.id].target); }

     void first(Node& node) const { 

       node.id = first_node;

     }

     void next(Node& node) const {

       node.id = nodes[node.id].next;

     }

     void first(Arc& e) const { 

       int n;

       for(n = first_node; 

 	  n!=-1 && nodes[n].first_in == -1; 

 	  n = nodes[n].next);

       e.id = (n == -1) ? -1 : nodes[n].first_in;

     }

     void next(Arc& arc) const {

       if (arcs[arc.id].next_in != -1) {

 	arc.id = arcs[arc.id].next_in;

       } else {

 	int n;

 	for(n = nodes[arcs[arc.id].target].next;

 	  n!=-1 && nodes[n].first_in == -1; 

 	  n = nodes[n].next);

 	arc.id = (n == -1) ? -1 : nodes[n].first_in;

       }      

     }

     void firstOut(Arc &e, const Node& v) const {

       e.id = nodes[v.id].first_out;

     }

     void nextOut(Arc &e) const {

       e.id=arcs[e.id].next_out;

     }

     void firstIn(Arc &e, const Node& v) const {

       e.id = nodes[v.id].first_in;

     }

     void nextIn(Arc &e) const {

       e.id=arcs[e.id].next_in;

     }

     static int id(Node v) { return v.id; }

     static int id(Arc e) { return e.id; }

     static Node nodeFromId(int id) { return Node(id);}

     static Arc arcFromId(int id) { return Arc(id);}

     Node addNode() {     

       int n;

       if(first_free_node==-1) {

 	n = nodes.size();

 	nodes.push_back(NodeT());

       } else {

 	n = first_free_node;

 	first_free_node = nodes[n].next;

       }

       nodes[n].next = first_node;

       if(first_node != -1) nodes[first_node].prev = n;

       first_node = n;

       nodes[n].prev = -1;

       nodes[n].first_in = nodes[n].first_out = -1;

       return Node(n);

     }

     Arc addArc(Node u, Node v) {

       int n;      

       if (first_free_arc == -1) {

 	n = arcs.size();

 	arcs.push_back(ArcT());

       } else {

 	n = first_free_arc;

 	first_free_arc = arcs[n].next_in;

       }

       arcs[n].source = u.id; 

       arcs[n].target = v.id;

       arcs[n].next_out = nodes[u.id].first_out;

       if(nodes[u.id].first_out != -1) {

 	arcs[nodes[u.id].first_out].prev_out = n;

       }

       arcs[n].next_in = nodes[v.id].first_in;

       if(nodes[v.id].first_in != -1) {

 	arcs[nodes[v.id].first_in].prev_in = n;

       }

       arcs[n].prev_in = arcs[n].prev_out = -1;

       nodes[u.id].first_out = nodes[v.id].first_in = n;

       return Arc(n);

     }

     void erase(const Node& node) {

       int n = node.id;

       if(nodes[n].next != -1) {

 	nodes[nodes[n].next].prev = nodes[n].prev;

       }

       if(nodes[n].prev != -1) {

 	nodes[nodes[n].prev].next = nodes[n].next;

       } else {

 	first_node = nodes[n].next;

       }

       nodes[n].next = first_free_node;

       first_free_node = n;

     }

     void erase(const Arc& arc) {

       int n = arc.id;

       if(arcs[n].next_in!=-1) {

 	arcs[arcs[n].next_in].prev_in = arcs[n].prev_in;

       }

       if(arcs[n].prev_in!=-1) {

 	arcs[arcs[n].prev_in].next_in = arcs[n].next_in;

       } else {

 	nodes[arcs[n].target].first_in = arcs[n].next_in;

       }

       if(arcs[n].next_out!=-1) {

 	arcs[arcs[n].next_out].prev_out = arcs[n].prev_out;

       } 

       if(arcs[n].prev_out!=-1) {

 	arcs[arcs[n].prev_out].next_out = arcs[n].next_out;

       } else {

 	nodes[arcs[n].source].first_out = arcs[n].next_out;

       }

       arcs[n].next_in = first_free_arc;

       first_free_arc = n;      

     }

     void clear() {

       arcs.clear();

       nodes.clear();

       first_node = first_free_node = first_free_arc = -1;

     }

   protected:

     void changeTarget(Arc e, Node n) 

     {

       if(arcs[e.id].next_in != -1)

 	arcs[arcs[e.id].next_in].prev_in = arcs[e.id].prev_in;

       if(arcs[e.id].prev_in != -1)

 	arcs[arcs[e.id].prev_in].next_in = arcs[e.id].next_in;

       else nodes[arcs[e.id].target].first_in = arcs[e.id].next_in;

       if (nodes[n.id].first_in != -1) {

 	arcs[nodes[n.id].first_in].prev_in = e.id;

       }

       arcs[e.id].target = n.id;

       arcs[e.id].prev_in = -1;

       arcs[e.id].next_in = nodes[n.id].first_in;

       nodes[n.id].first_in = e.id;

     }

     void changeSource(Arc e, Node n) 

     {

       if(arcs[e.id].next_out != -1)

 	arcs[arcs[e.id].next_out].prev_out = arcs[e.id].prev_out;

       if(arcs[e.id].prev_out != -1)

 	arcs[arcs[e.id].prev_out].next_out = arcs[e.id].next_out;

       else nodes[arcs[e.id].source].first_out = arcs[e.id].next_out;

       if (nodes[n.id].first_out != -1) {

 	arcs[nodes[n.id].first_out].prev_out = e.id;

       }

       arcs[e.id].source = n.id;

       arcs[e.id].prev_out = -1;

       arcs[e.id].next_out = nodes[n.id].first_out;

       nodes[n.id].first_out = e.id;

     }

   };

   typedef DigraphExtender<ListDigraphBase> ExtendedListDigraphBase;

   /// \addtogroup digraphs

   /// @{

   ///A list digraph class.

   ///This is a simple and fast digraph implementation.

   ///

   ///It conforms to the \ref concepts::Digraph "Digraph concept" and it

   ///also provides several additional useful extra functionalities.

   ///The most of the member functions and nested classes are

   ///documented only in the concept class.

   ///

   ///An important extra feature of this digraph implementation is that

   ///its maps are real \ref concepts::ReferenceMap "reference map"s.

   ///

   ///\sa concepts::Digraph.

   class ListDigraph : public ExtendedListDigraphBase {

   private:

     ///ListDigraph is \e not copy constructible. Use DigraphCopy() instead.

     ///ListDigraph is \e not copy constructible. Use DigraphCopy() instead.

     ///

     ListDigraph(const ListDigraph &) :ExtendedListDigraphBase() {};

     ///\brief Assignment of ListDigraph to another one is \e not allowed.

     ///Use DigraphCopy() instead.

     ///Assignment of ListDigraph to another one is \e not allowed.

     ///Use DigraphCopy() instead.

     void operator=(const ListDigraph &) {}

   public:

     typedef ExtendedListDigraphBase Parent;

     /// Constructor

     /// Constructor.

     ///

     ListDigraph() {}

     ///Add a new node to the digraph.

     /// \return the new node.

     ///

     Node addNode() { return Parent::addNode(); }

     ///Add a new arc to the digraph.

     ///Add a new arc to the digraph with source node \c s

     ///and target node \c t.

     ///\return the new arc.

     Arc addArc(const Node& s, const Node& t) { 

       return Parent::addArc(s, t); 

     }

     /// Changes the target of \c e to \c n

     /// Changes the target of \c e to \c n

     ///

     ///\note The <tt>ArcIt</tt>s and <tt>OutArcIt</tt>s referencing

     ///the changed arc remain valid. However <tt>InArcIt</tt>s are

     ///invalidated.

     ///\warning This functionality cannot be used together with the Snapshot

     ///feature.

     void changeTarget(Arc e, Node n) { 

       Parent::changeTarget(e,n); 

     }

     /// Changes the source of \c e to \c n

     /// Changes the source of \c e to \c n

     ///

     ///\note The <tt>ArcIt</tt>s and <tt>InArcIt</tt>s referencing

     ///the changed arc remain valid. However <tt>OutArcIt</tt>s are

     ///invalidated.

     ///\warning This functionality cannot be used together with the Snapshot

     ///feature.

     void changeSource(Arc e, Node n) { 

       Parent::changeSource(e,n);

     }

     /// Invert the direction of an arc.

     ///\note The <tt>ArcIt</tt>s referencing the changed arc remain

     ///valid. However <tt>OutArcIt</tt>s and <tt>InArcIt</tt>s are

     ///invalidated.

     ///\warning This functionality cannot be used together with the Snapshot

     ///feature.

     void reverseArc(Arc e) {

       Node t=target(e);

       changeTarget(e,source(e));

       changeSource(e,t);

     }

     /// Using this it is possible to avoid the superfluous memory

     /// allocation: if you know that the digraph you want to build will

     /// be very large (e.g. it will contain millions of nodes and/or arcs)

     /// then it is worth reserving space for this amount before starting

     /// to build the digraph.

     /// \sa reserveArc

     void reserveNode(int n) { nodes.reserve(n); };

     /// \brief Using this it is possible to avoid the superfluous memory

     /// allocation.

     /// Using this it is possible to avoid the superfluous memory

     /// allocation: if you know that the digraph you want to build will

     /// be very large (e.g. it will contain millions of nodes and/or arcs)

     /// then it is worth reserving space for this amount before starting

     /// to build the digraph.

     /// \sa reserveNode

     void reserveArc(int m) { arcs.reserve(m); };

     ///Contract two nodes.

     ///This function contracts two nodes.

     ///

     ///Node \p b will be removed but instead of deleting

     ///incident arcs, they will be joined to \p a.

     ///The last parameter \p r controls whether to remove loops. \c true

     ///means that loops will be removed.

     ///

     ///\note The <tt>ArcIt</tt>s

     ///referencing a moved arc remain

     ///valid. However <tt>InArcIt</tt>s and <tt>OutArcIt</tt>s

     ///may be invalidated.

     ///\warning This functionality cannot be used together with the Snapshot

     ///feature.

     void contract(Node a, Node b, bool r = true) 

     {

       for(OutArcIt e(*this,b);e!=INVALID;) {

 	OutArcIt f=e;

 	++f;

 	if(r && target(e)==a) erase(e);

 	else changeSource(e,a);

 	e=f;

       }

       for(InArcIt e(*this,b);e!=INVALID;) {

 	InArcIt f=e;

 	++f;

 	if(r && source(e)==a) erase(e);

 	else changeTarget(e,a);

 	e=f;

       }

       erase(b);

     }

     ///Split a node.

     ///This function splits a node. First a new node is added to the digraph,

     ///then the source of each outgoing arc of \c n is moved to this new node.

     ///If \c connect is \c true (this is the default value), then a new arc

     ///from \c n to the newly created node is also added.

     ///\return The newly created node.

     ///

     ///\note The <tt>ArcIt</tt>s referencing a moved arc remain

     ///valid. However <tt>InArcIt</tt>s and <tt>OutArcIt</tt>s may

     ///be invalidated.  

     ///

     ///\warning This functionality cannot be used together with the

     ///Snapshot feature.  \todo It could be implemented in a bit

     ///faster way.

     Node split(Node n, bool connect = true) {

       Node b = addNode();

       for(OutArcIt e(*this,n);e!=INVALID;) {

  	OutArcIt f=e;

 	++f;

 	changeSource(e,b);

 	e=f;

       }

       if (connect) addArc(n,b);

       return b;

     }

     ///Split an arc.

     ///This function splits an arc. First a new node \c b is added to

     ///the digraph, then the original arc is re-targeted to \c

     ///b. Finally an arc from \c b to the original target is added.

     ///\return The newly created node.  

     ///\warning This functionality

     ///cannot be used together with the Snapshot feature.

     Node split(Arc e) {

       Node b = addNode();

       addArc(b,target(e));

       changeTarget(e,b);

       return b;

     }

     /// \brief Class to make a snapshot of the digraph and restore

     /// to it later.

     ///

     /// Class to make a snapshot of the digraph and to restore it

     /// later.

     ///

     /// The newly added nodes and arcs can be removed using the

     /// restore() function.

     ///

     /// \warning Arc and node deletions cannot be restored. This

     /// events invalidate the snapshot. 

     class Snapshot {

     protected:

       typedef Parent::NodeNotifier NodeNotifier;

       class NodeObserverProxy : public NodeNotifier::ObserverBase {

       public:

         NodeObserverProxy(Snapshot& _snapshot)

           : snapshot(_snapshot) {}

         using NodeNotifier::ObserverBase::attach;

         using NodeNotifier::ObserverBase::detach;

         using NodeNotifier::ObserverBase::attached;

       protected:

         virtual void add(const Node& node) {

           snapshot.addNode(node);

         }

         virtual void add(const std::vector<Node>& nodes) {

           for (int i = nodes.size() - 1; i >= 0; ++i) {

             snapshot.addNode(nodes[i]);

           }

         }

         virtual void erase(const Node& node) {

           snapshot.eraseNode(node);

         }

         virtual void erase(const std::vector<Node>& nodes) {

           for (int i = 0; i < int(nodes.size()); ++i) {

             snapshot.eraseNode(nodes[i]);

           }

         }

         virtual void build() {

           Node node;

           std::vector<Node> nodes;

           for (notifier()->first(node); node != INVALID; 

                notifier()->next(node)) {

             nodes.push_back(node);

           }

           for (int i = nodes.size() - 1; i >= 0; --i) {

             snapshot.addNode(nodes[i]);

           }

         }

         virtual void clear() {

           Node node;

           for (notifier()->first(node); node != INVALID; 

                notifier()->next(node)) {

             snapshot.eraseNode(node);

           }

         }

         Snapshot& snapshot;

       };

       class ArcObserverProxy : public ArcNotifier::ObserverBase {

       public:

         ArcObserverProxy(Snapshot& _snapshot)

           : snapshot(_snapshot) {}

         using ArcNotifier::ObserverBase::attach;

         using ArcNotifier::ObserverBase::detach;

         using ArcNotifier::ObserverBase::attached;

       protected:

         virtual void add(const Arc& arc) {

           snapshot.addArc(arc);

         }

         virtual void add(const std::vector<Arc>& arcs) {

           for (int i = arcs.size() - 1; i >= 0; ++i) {

             snapshot.addArc(arcs[i]);

           }

         }

         virtual void erase(const Arc& arc) {

           snapshot.eraseArc(arc);

         }

         virtual void erase(const std::vector<Arc>& arcs) {

           for (int i = 0; i < int(arcs.size()); ++i) {

             snapshot.eraseArc(arcs[i]);

           }

         }

         virtual void build() {

           Arc arc;

           std::vector<Arc> arcs;

           for (notifier()->first(arc); arc != INVALID; 

                notifier()->next(arc)) {

             arcs.push_back(arc);

           }

           for (int i = arcs.size() - 1; i >= 0; --i) {

             snapshot.addArc(arcs[i]);

           }

         }

         virtual void clear() {

           Arc arc;

           for (notifier()->first(arc); arc != INVALID; 

                notifier()->next(arc)) {

             snapshot.eraseArc(arc);

           }

         }

         Snapshot& snapshot;

       };

       ListDigraph *digraph;

       NodeObserverProxy node_observer_proxy;

       ArcObserverProxy arc_observer_proxy;

       std::list<Node> added_nodes;

       std::list<Arc> added_arcs;

       void addNode(const Node& node) {

         added_nodes.push_front(node);        

       }

       void eraseNode(const Node& node) {

         std::list<Node>::iterator it = 

           std::find(added_nodes.begin(), added_nodes.end(), node);

         if (it == added_nodes.end()) {

           clear();

           arc_observer_proxy.detach();

           throw NodeNotifier::ImmediateDetach();

         } else {

           added_nodes.erase(it);

         }

       }

       void addArc(const Arc& arc) {

         added_arcs.push_front(arc);        

       }

       void eraseArc(const Arc& arc) {

         std::list<Arc>::iterator it = 

           std::find(added_arcs.begin(), added_arcs.end(), arc);

         if (it == added_arcs.end()) {

           clear();

           node_observer_proxy.detach(); 

           throw ArcNotifier::ImmediateDetach();

         } else {

           added_arcs.erase(it);

         }        

       }

       void attach(ListDigraph &_digraph) {

 	digraph = &_digraph;

 	node_observer_proxy.attach(digraph->notifier(Node()));

         arc_observer_proxy.attach(digraph->notifier(Arc()));

       }

       void detach() {

 	node_observer_proxy.detach();

 	arc_observer_proxy.detach();

       }

       bool attached() const {

         return node_observer_proxy.attached();

       }

       void clear() {

         added_nodes.clear();

         added_arcs.clear();        

       }

     public:

       /// \brief Default constructor.

       ///

       /// Default constructor.

       /// To actually make a snapshot you must call save().

       Snapshot() 

         : digraph(0), node_observer_proxy(*this), 

           arc_observer_proxy(*this) {}

       /// \brief Constructor that immediately makes a snapshot.

       ///      

       /// This constructor immediately makes a snapshot of the digraph.

       /// \param _digraph The digraph we make a snapshot of.

       Snapshot(ListDigraph &_digraph) 

         : node_observer_proxy(*this), 

           arc_observer_proxy(*this) {

 	attach(_digraph);

       }

       /// \brief Make a snapshot.

       ///

       /// Make a snapshot of the digraph.

       ///

       /// This function can be called more than once. In case of a repeated

       /// call, the previous snapshot gets lost.

       /// \param _digraph The digraph we make the snapshot of.

       void save(ListDigraph &_digraph) {

         if (attached()) {

           detach();

           clear();

         }

         attach(_digraph);

       }

       /// \brief Undo the changes until the last snapshot.

       // 

       /// Undo the changes until the last snapshot created by save().

       void restore() {

 	detach();

 	for(std::list<Arc>::iterator it = added_arcs.begin(); 

             it != added_arcs.end(); ++it) {

 	  digraph->erase(*it);

 	}

 	for(std::list<Node>::iterator it = added_nodes.begin(); 

             it != added_nodes.end(); ++it) {

 	  digraph->erase(*it);

 	}

         clear();

       }

       /// \brief Gives back true when the snapshot is valid.

       ///

       /// Gives back true when the snapshot is valid.

       bool valid() const {

         return attached();

       }

     };

   };

   ///@}

   class ListGraphBase {

   protected:

     struct NodeT {

       int first_out;

       int prev, next;

     };

     struct ArcT {

       int target;

       int prev_out, next_out;

     };

     std::vector<NodeT> nodes;

     int first_node;

     int first_free_node;

     std::vector<ArcT> arcs;

     int first_free_arc;

   public:

     typedef ListGraphBase Digraph;

     class Node;

     class Arc;

     class Edge;

     class Node {

       friend class ListGraphBase;

     protected:

       int id;

       explicit Node(int pid) { id = pid;}

     public:

       Node() {}

       Node (Invalid) { id = -1; }

       bool operator==(const Node& node) const {return id == node.id;}

       bool operator!=(const Node& node) const {return id != node.id;}

       bool operator<(const Node& node) const {return id < node.id;}

     };

     class Edge {

       friend class ListGraphBase;

     protected:

       int id;

       explicit Edge(int pid) { id = pid;}

     public:

       Edge() {}

       Edge (Invalid) { id = -1; }

       bool operator==(const Edge& arc) const {return id == arc.id;}

       bool operator!=(const Edge& arc) const {return id != arc.id;}

       bool operator<(const Edge& arc) const {return id < arc.id;}

     };

     class Arc {

       friend class ListGraphBase;

     protected:

       int id;

       explicit Arc(int pid) { id = pid;}

     public:

       operator Edge() const { return edgeFromId(id / 2); }

       Arc() {}

       Arc (Invalid) { id = -1; }

       bool operator==(const Arc& arc) const {return id == arc.id;}

       bool operator!=(const Arc& arc) const {return id != arc.id;}

       bool operator<(const Arc& arc) const {return id < arc.id;}

     };

     ListGraphBase()

       : nodes(), first_node(-1),

 	first_free_node(-1), arcs(), first_free_arc(-1) {}

     int maxNodeId() const { return nodes.size()-1; } 

     int maxEdgeId() const { return arcs.size() / 2 - 1; }

     int maxArcId() const { return arcs.size()-1; }

     Node source(Arc e) const { return Node(arcs[e.id ^ 1].target); }

     Node target(Arc e) const { return Node(arcs[e.id].target); }

     Node u(Edge e) const { return Node(arcs[2 * e.id].target); }

     Node v(Edge e) const { return Node(arcs[2 * e.id + 1].target); }

     static bool direction(Arc e) {

       return (e.id & 1) == 1;

     }

     static Arc direct(Edge e, bool d) {

       return Arc(e.id * 2 + (d ? 1 : 0));

     }

     void first(Node& node) const { 

       node.id = first_node;

     }

     void next(Node& node) const {

       node.id = nodes[node.id].next;

     }

     void first(Arc& e) const { 

       int n = first_node;

       while (n != -1 && nodes[n].first_out == -1) {

         n = nodes[n].next;

       }

       e.id = (n == -1) ? -1 : nodes[n].first_out;

     }

     void next(Arc& e) const {

       if (arcs[e.id].next_out != -1) {

 	e.id = arcs[e.id].next_out;

       } else {

 	int n = nodes[arcs[e.id ^ 1].target].next;

         while(n != -1 && nodes[n].first_out == -1) {

           n = nodes[n].next;

         }

 	e.id = (n == -1) ? -1 : nodes[n].first_out;

       }      

     }

     void first(Edge& e) const { 

       int n = first_node;

       while (n != -1) {

         e.id = nodes[n].first_out;

         while ((e.id & 1) != 1) {

           e.id = arcs[e.id].next_out;

         }

         if (e.id != -1) {

           e.id /= 2;

           return;

         } 

         n = nodes[n].next;

       }

       e.id = -1;

     }

     void next(Edge& e) const {

       int n = arcs[e.id * 2].target;

       e.id = arcs[(e.id * 2) | 1].next_out;

       while ((e.id & 1) != 1) {

         e.id = arcs[e.id].next_out;

       }

       if (e.id != -1) {

         e.id /= 2;

         return;

       } 

       n = nodes[n].next;

       while (n != -1) {

         e.id = nodes[n].first_out;

         while ((e.id & 1) != 1) {

           e.id = arcs[e.id].next_out;

         }

         if (e.id != -1) {

           e.id /= 2;

           return;

         } 

         n = nodes[n].next;

       }

       e.id = -1;

     }

     void firstOut(Arc &e, const Node& v) const {

       e.id = nodes[v.id].first_out;

     }

     void nextOut(Arc &e) const {

       e.id = arcs[e.id].next_out;

     }

     void firstIn(Arc &e, const Node& v) const {

       e.id = ((nodes[v.id].first_out) ^ 1);

       if (e.id == -2) e.id = -1;

     }

     void nextIn(Arc &e) const {

       e.id = ((arcs[e.id ^ 1].next_out) ^ 1);

       if (e.id == -2) e.id = -1;

     }

     void firstInc(Edge &e, bool& d, const Node& v) const {

       int de = nodes[v.id].first_out;

       if (de != -1 ) {

         e.id = de / 2;

         d = ((de & 1) == 1);

       } else {

         e.id = -1;

         d = true;

       }

     }

     void nextInc(Edge &e, bool& d) const {

       int de = (arcs[(e.id * 2) | (d ? 1 : 0)].next_out);

       if (de != -1 ) {

         e.id = de / 2;

         d = ((de & 1) == 1);

       } else {

         e.id = -1;

         d = true;

       }

     }

     static int id(Node v) { return v.id; }

     static int id(Arc e) { return e.id; }

     static int id(Edge e) { return e.id; }

     static Node nodeFromId(int id) { return Node(id);}

     static Arc arcFromId(int id) { return Arc(id);}

     static Edge edgeFromId(int id) { return Edge(id);}

     Node addNode() {     

       int n;

       if(first_free_node==-1) {

 	n = nodes.size();

 	nodes.push_back(NodeT());

       } else {

 	n = first_free_node;

 	first_free_node = nodes[n].next;

       }

       nodes[n].next = first_node;

       if (first_node != -1) nodes[first_node].prev = n;

       first_node = n;

       nodes[n].prev = -1;

       nodes[n].first_out = -1;

       return Node(n);

     }

     Edge addEdge(Node u, Node v) {

       int n;      

       if (first_free_arc == -1) {

 	n = arcs.size();

 	arcs.push_back(ArcT());

 	arcs.push_back(ArcT());

       } else {

 	n = first_free_arc;

 	first_free_arc = arcs[n].next_out;

       }

       arcs[n].target = u.id;

       arcs[n | 1].target = v.id;

       arcs[n].next_out = nodes[v.id].first_out;

       if (nodes[v.id].first_out != -1) {

 	arcs[nodes[v.id].first_out].prev_out = n;

       }      

       arcs[n].prev_out = -1;

       nodes[v.id].first_out = n;

       arcs[n | 1].next_out = nodes[u.id].first_out;

       if (nodes[u.id].first_out != -1) {

 	arcs[nodes[u.id].first_out].prev_out = (n | 1);

       }

       arcs[n | 1].prev_out = -1;      

       nodes[u.id].first_out = (n | 1);

       return Edge(n / 2);

     }

     void erase(const Node& node) {

       int n = node.id;

       if(nodes[n].next != -1) {

 	nodes[nodes[n].next].prev = nodes[n].prev;

       }

       if(nodes[n].prev != -1) {

 	nodes[nodes[n].prev].next = nodes[n].next;

       } else {

 	first_node = nodes[n].next;

       }

       nodes[n].next = first_free_node;

       first_free_node = n;

     }

     void erase(const Edge& arc) {

       int n = arc.id * 2;

       if (arcs[n].next_out != -1) {

 	arcs[arcs[n].next_out].prev_out = arcs[n].prev_out;

       } 

       if (arcs[n].prev_out != -1) {

 	arcs[arcs[n].prev_out].next_out = arcs[n].next_out;

       } else {

 	nodes[arcs[n | 1].target].first_out = arcs[n].next_out;

       }

       if (arcs[n | 1].next_out != -1) {

 	arcs[arcs[n | 1].next_out].prev_out = arcs[n | 1].prev_out;

       } 

       if (arcs[n | 1].prev_out != -1) {

 	arcs[arcs[n | 1].prev_out].next_out = arcs[n | 1].next_out;

       } else {

 	nodes[arcs[n].target].first_out = arcs[n | 1].next_out;

       }

       arcs[n].next_out = first_free_arc;

       first_free_arc = n;      

     }

     void clear() {

       arcs.clear();

       nodes.clear();

       first_node = first_free_node = first_free_arc = -1;

     }

   protected:

     void changeTarget(Edge e, Node n) {

       if(arcs[2 * e.id].next_out != -1) {

 	arcs[arcs[2 * e.id].next_out].prev_out = arcs[2 * e.id].prev_out;

       }

       if(arcs[2 * e.id].prev_out != -1) {

 	arcs[arcs[2 * e.id].prev_out].next_out = 

           arcs[2 * e.id].next_out;

       } else {

         nodes[arcs[(2 * e.id) | 1].target].first_out = 

           arcs[2 * e.id].next_out;

       }

       if (nodes[n.id].first_out != -1) {

 	arcs[nodes[n.id].first_out].prev_out = 2 * e.id;

       }

       arcs[(2 * e.id) | 1].target = n.id;

       arcs[2 * e.id].prev_out = -1;

       arcs[2 * e.id].next_out = nodes[n.id].first_out;

       nodes[n.id].first_out = 2 * e.id;

     }

     void changeSource(Edge e, Node n) {

       if(arcs[(2 * e.id) | 1].next_out != -1) {

 	arcs[arcs[(2 * e.id) | 1].next_out].prev_out = 

           arcs[(2 * e.id) | 1].prev_out;

       }

       if(arcs[(2 * e.id) | 1].prev_out != -1) {

 	arcs[arcs[(2 * e.id) | 1].prev_out].next_out = 

           arcs[(2 * e.id) | 1].next_out;

       } else {

         nodes[arcs[2 * e.id].target].first_out = 

           arcs[(2 * e.id) | 1].next_out;

       }

       if (nodes[n.id].first_out != -1) {

 	arcs[nodes[n.id].first_out].prev_out = ((2 * e.id) | 1);

       }

       arcs[2 * e.id].target = n.id;

       arcs[(2 * e.id) | 1].prev_out = -1;

       arcs[(2 * e.id) | 1].next_out = nodes[n.id].first_out;

       nodes[n.id].first_out = ((2 * e.id) | 1);

     }

   };

 //   typedef GraphExtender<UndirDigraphExtender<ListDigraphBase> > 

 //   ExtendedListGraphBase;

   typedef GraphExtender<ListGraphBase> ExtendedListGraphBase;

   /// \addtogroup digraphs

   /// @{

   ///An undirected list digraph class.

   ///This is a simple and fast undirected digraph implementation.

   ///

   ///An important extra feature of this digraph implementation is that

   ///its maps are real \ref concepts::ReferenceMap "reference map"s.

   ///

   ///It conforms to the

   ///\ref concepts::Graph "Graph concept".

   ///

   ///\sa concepts::Graph.

   ///

   class ListGraph : public ExtendedListGraphBase {

   private:

     ///ListGraph is \e not copy constructible. Use GraphCopy() instead.

     ///ListGraph is \e not copy constructible. Use GraphCopy() instead.

     ///

     ListGraph(const ListGraph &) :ExtendedListGraphBase()  {};

     ///\brief Assignment of ListGraph to another one is \e not allowed.

     ///Use GraphCopy() instead.

     ///Assignment of ListGraph to another one is \e not allowed.

     ///Use GraphCopy() instead.

     void operator=(const ListGraph &) {}

   public:

     /// Constructor

     /// Constructor.

     ///

     ListGraph() {}

     typedef ExtendedListGraphBase Parent;

     typedef Parent::OutArcIt IncArcIt;

     /// \brief Add a new node to the digraph.

     ///

     /// \return the new node.

     ///

     Node addNode() { return Parent::addNode(); }

     /// \brief Add a new edge to the digraph.

     ///

     /// Add a new arc to the digraph with source node \c s

     /// and target node \c t.

     /// \return the new edge.

     Edge addEdge(const Node& s, const Node& t) { 

       return Parent::addEdge(s, t); 

     }

     /// \brief Changes the source of \c e to \c n

     ///

     /// Changes the source of \c e to \c n

     ///

     ///\note The <tt>ArcIt</tt>s and <tt>InArcIt</tt>s

     ///referencing the changed arc remain

     ///valid. However <tt>OutArcIt</tt>s are invalidated.

     void changeSource(Edge e, Node n) { 

       Parent::changeSource(e,n); 

     }    

     /// \brief Changes the target of \c e to \c n

     ///

     /// Changes the target of \c e to \c n

     ///

     /// \note The <tt>ArcIt</tt>s referencing the changed arc remain

     /// valid. However the other iterators may be invalidated.

     void changeTarget(Edge e, Node n) { 

       Parent::changeTarget(e,n); 

     }

     /// \brief Changes the source of \c e to \c n

     ///

     /// Changes the source of \c e to \c n. It changes the proper

     /// node of the represented edge.

     ///

     ///\note The <tt>ArcIt</tt>s and <tt>InArcIt</tt>s

     ///referencing the changed arc remain

     ///valid. However <tt>OutArcIt</tt>s are invalidated.

     void changeSource(Arc e, Node n) { 

       if (Parent::direction(e)) {

         Parent::changeSource(e,n);

       } else {

         Parent::changeTarget(e,n);

       } 

     }

     /// \brief Changes the target of \c e to \c n

     ///

     /// Changes the target of \c e to \c n. It changes the proper

     /// node of the represented edge.

     ///

     ///\note The <tt>ArcIt</tt>s and <tt>OutArcIt</tt>s

     ///referencing the changed arc remain

     ///valid. However <tt>InArcIt</tt>s are invalidated.

     void changeTarget(Arc e, Node n) { 

       if (Parent::direction(e)) {

         Parent::changeTarget(e,n);

       } else {

         Parent::changeSource(e,n);

       } 

     }

     /// \brief Contract two nodes.

     ///

     /// This function contracts two nodes.

     ///

     /// Node \p b will be removed but instead of deleting

     /// its neighboring arcs, they will be joined to \p a.

     /// The last parameter \p r controls whether to remove loops. \c true

     /// means that loops will be removed.

     ///

     /// \note The <tt>ArcIt</tt>s referencing a moved arc remain

     /// valid.

     void contract(Node a, Node b, bool r = true) {

       for(IncArcIt e(*this, b); e!=INVALID;) {

 	IncArcIt f = e; ++f;

 	if (r && runningNode(e) == a) {

 	  erase(e);

 	} else if (source(e) == b) {

 	  changeSource(e, a);

 	} else {

 	  changeTarget(e, a);

 	}

 	e = f;

       }

       erase(b);

     }

     /// \brief Class to make a snapshot of the digraph and restore

     /// to it later.

     ///

     /// Class to make a snapshot of the digraph and to restore it

     /// later.

     ///

     /// The newly added nodes and edges can be removed

     /// using the restore() function.

     ///

     /// \warning Arc and node deletions cannot be restored. This

     /// events invalidate the snapshot. 

     class Snapshot {

     protected:

       typedef Parent::NodeNotifier NodeNotifier;

       class NodeObserverProxy : public NodeNotifier::ObserverBase {

       public:

         NodeObserverProxy(Snapshot& _snapshot)

           : snapshot(_snapshot) {}

         using NodeNotifier::ObserverBase::attach;

         using NodeNotifier::ObserverBase::detach;

         using NodeNotifier::ObserverBase::attached;

       protected:

         virtual void add(const Node& node) {

           snapshot.addNode(node);

         }

         virtual void add(const std::vector<Node>& nodes) {

           for (int i = nodes.size() - 1; i >= 0; ++i) {

             snapshot.addNode(nodes[i]);

           }

         }

         virtual void erase(const Node& node) {

           snapshot.eraseNode(node);

         }

         virtual void erase(const std::vector<Node>& nodes) {

           for (int i = 0; i < int(nodes.size()); ++i) {

             snapshot.eraseNode(nodes[i]);

           }

         }

         virtual void build() {

           Node node;

           std::vector<Node> nodes;

           for (notifier()->first(node); node != INVALID; 

                notifier()->next(node)) {

             nodes.push_back(node);

           }

           for (int i = nodes.size() - 1; i >= 0; --i) {

             snapshot.addNode(nodes[i]);

           }

         }

         virtual void clear() {

           Node node;

           for (notifier()->first(node); node != INVALID; 

                notifier()->next(node)) {

             snapshot.eraseNode(node);

           }

         }

         Snapshot& snapshot;

       };

       class EdgeObserverProxy : public EdgeNotifier::ObserverBase {

       public:

         EdgeObserverProxy(Snapshot& _snapshot)

           : snapshot(_snapshot) {}

         using EdgeNotifier::ObserverBase::attach;

         using EdgeNotifier::ObserverBase::detach;

         using EdgeNotifier::ObserverBase::attached;

       protected:

         virtual void add(const Edge& arc) {

           snapshot.addEdge(arc);

         }

         virtual void add(const std::vector<Edge>& arcs) {

           for (int i = arcs.size() - 1; i >= 0; ++i) {

             snapshot.addEdge(arcs[i]);

           }

         }

         virtual void erase(const Edge& arc) {

           snapshot.eraseEdge(arc);

         }

         virtual void erase(const std::vector<Edge>& arcs) {

           for (int i = 0; i < int(arcs.size()); ++i) {

             snapshot.eraseEdge(arcs[i]);

           }

         }

         virtual void build() {

           Edge arc;

           std::vector<Edge> arcs;

           for (notifier()->first(arc); arc != INVALID; 

                notifier()->next(arc)) {

             arcs.push_back(arc);

           }

           for (int i = arcs.size() - 1; i >= 0; --i) {

             snapshot.addEdge(arcs[i]);

           }

         }

         virtual void clear() {

           Edge arc;

           for (notifier()->first(arc); arc != INVALID; 

                notifier()->next(arc)) {

             snapshot.eraseEdge(arc);

           }

         }

         Snapshot& snapshot;

       };

       ListGraph *digraph;

       NodeObserverProxy node_observer_proxy;

       EdgeObserverProxy arc_observer_proxy;

       std::list<Node> added_nodes;

       std::list<Edge> added_arcs;

       void addNode(const Node& node) {

         added_nodes.push_front(node);        

       }

       void eraseNode(const Node& node) {

         std::list<Node>::iterator it = 

           std::find(added_nodes.begin(), added_nodes.end(), node);

         if (it == added_nodes.end()) {

           clear();

           arc_observer_proxy.detach();

           throw NodeNotifier::ImmediateDetach();

         } else {

           added_nodes.erase(it);

         }

       }

       void addEdge(const Edge& arc) {

         added_arcs.push_front(arc);        

       }

       void eraseEdge(const Edge& arc) {

         std::list<Edge>::iterator it = 

           std::find(added_arcs.begin(), added_arcs.end(), arc);

         if (it == added_arcs.end()) {

           clear();

           node_observer_proxy.detach();

           throw EdgeNotifier::ImmediateDetach();

         } else {

           added_arcs.erase(it);

         }        

       }

       void attach(ListGraph &_digraph) {

 	digraph = &_digraph;

 	node_observer_proxy.attach(digraph->notifier(Node()));

         arc_observer_proxy.attach(digraph->notifier(Edge()));

       }

       void detach() {

 	node_observer_proxy.detach();

 	arc_observer_proxy.detach();

       }

       bool attached() const {

         return node_observer_proxy.attached();

       }

       void clear() {

         added_nodes.clear();

         added_arcs.clear();        

       }

     public:

       /// \brief Default constructor.

       ///

       /// Default constructor.

       /// To actually make a snapshot you must call save().

       Snapshot() 

         : digraph(0), node_observer_proxy(*this), 

           arc_observer_proxy(*this) {}

       /// \brief Constructor that immediately makes a snapshot.

       ///      

       /// This constructor immediately makes a snapshot of the digraph.

       /// \param _digraph The digraph we make a snapshot of.

       Snapshot(ListGraph &_digraph) 

         : node_observer_proxy(*this), 

           arc_observer_proxy(*this) {

 	attach(_digraph);

       }

       /// \brief Make a snapshot.

       ///

       /// Make a snapshot of the digraph.

       ///

       /// This function can be called more than once. In case of a repeated

       /// call, the previous snapshot gets lost.

       /// \param _digraph The digraph we make the snapshot of.

       void save(ListGraph &_digraph) {

         if (attached()) {

           detach();

           clear();

         }

         attach(_digraph);

       }

       /// \brief Undo the changes until the last snapshot.

       // 

       /// Undo the changes until the last snapshot created by save().

       void restore() {

 	detach();

 	for(std::list<Edge>::iterator it = added_arcs.begin(); 

             it != added_arcs.end(); ++it) {

 	  digraph->erase(*it);

 	}

 	for(std::list<Node>::iterator it = added_nodes.begin(); 

             it != added_nodes.end(); ++it) {

 	  digraph->erase(*it);

 	}

         clear();

       }

       /// \brief Gives back true when the snapshot is valid.

       ///

       /// Gives back true when the snapshot is valid.

       bool valid() const {

         return attached();

       }

     };

   };

   /// @}  

 } //namespace lemon

 #endif