lemon/smart_graph.h
author Akos Ladanyi <ladanyi@tmit.bme.hu>
Wed, 10 Sep 2008 11:23:41 +0100
changeset 259 362415050b29
parent 220 a5d8c039f218
child 280 e7f8647ce760
permissions -rw-r--r--
Disable checks for ILP solvers.
     1 /* -*- mode: C++; indent-tabs-mode: nil; -*-
     2  *
     3  * This file is a part of LEMON, a generic C++ optimization library.
     4  *
     5  * Copyright (C) 2003-2008
     6  * Egervary Jeno Kombinatorikus Optimalizalasi Kutatocsoport
     7  * (Egervary Research Group on Combinatorial Optimization, EGRES).
     8  *
     9  * Permission to use, modify and distribute this software is granted
    10  * provided that this copyright notice appears in all copies. For
    11  * precise terms see the accompanying LICENSE file.
    12  *
    13  * This software is provided "AS IS" with no warranty of any kind,
    14  * express or implied, and with no claim as to its suitability for any
    15  * purpose.
    16  *
    17  */
    18 
    19 #ifndef LEMON_SMART_GRAPH_H
    20 #define LEMON_SMART_GRAPH_H
    21 
    22 ///\ingroup graphs
    23 ///\file
    24 ///\brief SmartDigraph and SmartGraph classes.
    25 
    26 #include <vector>
    27 
    28 #include <lemon/core.h>
    29 #include <lemon/error.h>
    30 #include <lemon/bits/graph_extender.h>
    31 
    32 namespace lemon {
    33 
    34   class SmartDigraph;
    35   ///Base of SmartDigraph
    36 
    37   ///Base of SmartDigraph
    38   ///
    39   class SmartDigraphBase {
    40   protected:
    41 
    42     struct NodeT
    43     {
    44       int first_in, first_out;
    45       NodeT() {}
    46     };
    47     struct ArcT
    48     {
    49       int target, source, next_in, next_out;
    50       ArcT() {}
    51     };
    52 
    53     std::vector<NodeT> nodes;
    54     std::vector<ArcT> arcs;
    55 
    56   public:
    57 
    58     typedef SmartDigraphBase Graph;
    59 
    60     class Node;
    61     class Arc;
    62 
    63   public:
    64 
    65     SmartDigraphBase() : nodes(), arcs() { }
    66     SmartDigraphBase(const SmartDigraphBase &_g)
    67       : nodes(_g.nodes), arcs(_g.arcs) { }
    68 
    69     typedef True NodeNumTag;
    70     typedef True EdgeNumTag;
    71 
    72     int nodeNum() const { return nodes.size(); }
    73     int arcNum() const { return arcs.size(); }
    74 
    75     int maxNodeId() const { return nodes.size()-1; }
    76     int maxArcId() const { return arcs.size()-1; }
    77 
    78     Node addNode() {
    79       int n = nodes.size();
    80       nodes.push_back(NodeT());
    81       nodes[n].first_in = -1;
    82       nodes[n].first_out = -1;
    83       return Node(n);
    84     }
    85 
    86     Arc addArc(Node u, Node v) {
    87       int n = arcs.size();
    88       arcs.push_back(ArcT());
    89       arcs[n].source = u._id;
    90       arcs[n].target = v._id;
    91       arcs[n].next_out = nodes[u._id].first_out;
    92       arcs[n].next_in = nodes[v._id].first_in;
    93       nodes[u._id].first_out = nodes[v._id].first_in = n;
    94 
    95       return Arc(n);
    96     }
    97 
    98     void clear() {
    99       arcs.clear();
   100       nodes.clear();
   101     }
   102 
   103     Node source(Arc a) const { return Node(arcs[a._id].source); }
   104     Node target(Arc a) const { return Node(arcs[a._id].target); }
   105 
   106     static int id(Node v) { return v._id; }
   107     static int id(Arc a) { return a._id; }
   108 
   109     static Node nodeFromId(int id) { return Node(id);}
   110     static Arc arcFromId(int id) { return Arc(id);}
   111 
   112     bool valid(Node n) const {
   113       return n._id >= 0 && n._id < static_cast<int>(nodes.size());
   114     }
   115     bool valid(Arc a) const {
   116       return a._id >= 0 && a._id < static_cast<int>(arcs.size());
   117     }
   118 
   119     class Node {
   120       friend class SmartDigraphBase;
   121       friend class SmartDigraph;
   122 
   123     protected:
   124       int _id;
   125       explicit Node(int id) : _id(id) {}
   126     public:
   127       Node() {}
   128       Node (Invalid) : _id(-1) {}
   129       bool operator==(const Node i) const {return _id == i._id;}
   130       bool operator!=(const Node i) const {return _id != i._id;}
   131       bool operator<(const Node i) const {return _id < i._id;}
   132     };
   133 
   134 
   135     class Arc {
   136       friend class SmartDigraphBase;
   137       friend class SmartDigraph;
   138 
   139     protected:
   140       int _id;
   141       explicit Arc(int id) : _id(id) {}
   142     public:
   143       Arc() { }
   144       Arc (Invalid) : _id(-1) {}
   145       bool operator==(const Arc i) const {return _id == i._id;}
   146       bool operator!=(const Arc i) const {return _id != i._id;}
   147       bool operator<(const Arc i) const {return _id < i._id;}
   148     };
   149 
   150     void first(Node& node) const {
   151       node._id = nodes.size() - 1;
   152     }
   153 
   154     static void next(Node& node) {
   155       --node._id;
   156     }
   157 
   158     void first(Arc& arc) const {
   159       arc._id = arcs.size() - 1;
   160     }
   161 
   162     static void next(Arc& arc) {
   163       --arc._id;
   164     }
   165 
   166     void firstOut(Arc& arc, const Node& node) const {
   167       arc._id = nodes[node._id].first_out;
   168     }
   169 
   170     void nextOut(Arc& arc) const {
   171       arc._id = arcs[arc._id].next_out;
   172     }
   173 
   174     void firstIn(Arc& arc, const Node& node) const {
   175       arc._id = nodes[node._id].first_in;
   176     }
   177 
   178     void nextIn(Arc& arc) const {
   179       arc._id = arcs[arc._id].next_in;
   180     }
   181 
   182   };
   183 
   184   typedef DigraphExtender<SmartDigraphBase> ExtendedSmartDigraphBase;
   185 
   186   ///\ingroup graphs
   187   ///
   188   ///\brief A smart directed graph class.
   189   ///
   190   ///This is a simple and fast digraph implementation.
   191   ///It is also quite memory efficient, but at the price
   192   ///that <b> it does support only limited (only stack-like)
   193   ///node and arc deletions</b>.
   194   ///It conforms to the \ref concepts::Digraph "Digraph concept" with
   195   ///an important extra feature that its maps are real \ref
   196   ///concepts::ReferenceMap "reference map"s.
   197   ///
   198   ///\sa concepts::Digraph.
   199   class SmartDigraph : public ExtendedSmartDigraphBase {
   200   public:
   201 
   202     typedef ExtendedSmartDigraphBase Parent;
   203 
   204   private:
   205 
   206     ///SmartDigraph is \e not copy constructible. Use DigraphCopy() instead.
   207 
   208     ///SmartDigraph is \e not copy constructible. Use DigraphCopy() instead.
   209     ///
   210     SmartDigraph(const SmartDigraph &) : ExtendedSmartDigraphBase() {};
   211     ///\brief Assignment of SmartDigraph to another one is \e not allowed.
   212     ///Use DigraphCopy() instead.
   213 
   214     ///Assignment of SmartDigraph to another one is \e not allowed.
   215     ///Use DigraphCopy() instead.
   216     void operator=(const SmartDigraph &) {}
   217 
   218   public:
   219 
   220     /// Constructor
   221 
   222     /// Constructor.
   223     ///
   224     SmartDigraph() {};
   225 
   226     ///Add a new node to the digraph.
   227 
   228     /// \return the new node.
   229     ///
   230     Node addNode() { return Parent::addNode(); }
   231 
   232     ///Add a new arc to the digraph.
   233 
   234     ///Add a new arc to the digraph with source node \c s
   235     ///and target node \c t.
   236     ///\return the new arc.
   237     Arc addArc(const Node& s, const Node& t) {
   238       return Parent::addArc(s, t);
   239     }
   240 
   241     /// \brief Using this it is possible to avoid the superfluous memory
   242     /// allocation.
   243 
   244     /// Using this it is possible to avoid the superfluous memory
   245     /// allocation: if you know that the digraph you want to build will
   246     /// be very large (e.g. it will contain millions of nodes and/or arcs)
   247     /// then it is worth reserving space for this amount before starting
   248     /// to build the digraph.
   249     /// \sa reserveArc
   250     void reserveNode(int n) { nodes.reserve(n); };
   251 
   252     /// \brief Using this it is possible to avoid the superfluous memory
   253     /// allocation.
   254 
   255     /// Using this it is possible to avoid the superfluous memory
   256     /// allocation: if you know that the digraph you want to build will
   257     /// be very large (e.g. it will contain millions of nodes and/or arcs)
   258     /// then it is worth reserving space for this amount before starting
   259     /// to build the digraph.
   260     /// \sa reserveNode
   261     void reserveArc(int m) { arcs.reserve(m); };
   262 
   263     /// \brief Node validity check
   264     ///
   265     /// This function gives back true if the given node is valid,
   266     /// ie. it is a real node of the graph.
   267     ///
   268     /// \warning A removed node (using Snapshot) could become valid again
   269     /// when new nodes are added to the graph.
   270     bool valid(Node n) const { return Parent::valid(n); }
   271 
   272     /// \brief Arc validity check
   273     ///
   274     /// This function gives back true if the given arc is valid,
   275     /// ie. it is a real arc of the graph.
   276     ///
   277     /// \warning A removed arc (using Snapshot) could become valid again
   278     /// when new arcs are added to the graph.
   279     bool valid(Arc a) const { return Parent::valid(a); }
   280 
   281     ///Clear the digraph.
   282 
   283     ///Erase all the nodes and arcs from the digraph.
   284     ///
   285     void clear() {
   286       Parent::clear();
   287     }
   288 
   289     ///Split a node.
   290 
   291     ///This function splits a node. First a new node is added to the digraph,
   292     ///then the source of each outgoing arc of \c n is moved to this new node.
   293     ///If \c connect is \c true (this is the default value), then a new arc
   294     ///from \c n to the newly created node is also added.
   295     ///\return The newly created node.
   296     ///
   297     ///\note The <tt>Arc</tt>s
   298     ///referencing a moved arc remain
   299     ///valid. However <tt>InArc</tt>'s and <tt>OutArc</tt>'s
   300     ///may be invalidated.
   301     ///\warning This functionality cannot be used together with the Snapshot
   302     ///feature.
   303     ///\todo It could be implemented in a bit faster way.
   304     Node split(Node n, bool connect = true)
   305     {
   306       Node b = addNode();
   307       nodes[b._id].first_out=nodes[n._id].first_out;
   308       nodes[n._id].first_out=-1;
   309       for(int i=nodes[b._id].first_out;i!=-1;i++) arcs[i].source=b._id;
   310       if(connect) addArc(n,b);
   311       return b;
   312     }
   313 
   314   public:
   315 
   316     class Snapshot;
   317 
   318   protected:
   319 
   320     void restoreSnapshot(const Snapshot &s)
   321     {
   322       while(s.arc_num<arcs.size()) {
   323         Arc arc = arcFromId(arcs.size()-1);
   324         Parent::notifier(Arc()).erase(arc);
   325         nodes[arcs.back().source].first_out=arcs.back().next_out;
   326         nodes[arcs.back().target].first_in=arcs.back().next_in;
   327         arcs.pop_back();
   328       }
   329       while(s.node_num<nodes.size()) {
   330         Node node = nodeFromId(nodes.size()-1);
   331         Parent::notifier(Node()).erase(node);
   332         nodes.pop_back();
   333       }
   334     }
   335 
   336   public:
   337 
   338     ///Class to make a snapshot of the digraph and to restrore to it later.
   339 
   340     ///Class to make a snapshot of the digraph and to restrore to it later.
   341     ///
   342     ///The newly added nodes and arcs can be removed using the
   343     ///restore() function.
   344     ///\note After you restore a state, you cannot restore
   345     ///a later state, in other word you cannot add again the arcs deleted
   346     ///by restore() using another one Snapshot instance.
   347     ///
   348     ///\warning If you do not use correctly the snapshot that can cause
   349     ///either broken program, invalid state of the digraph, valid but
   350     ///not the restored digraph or no change. Because the runtime performance
   351     ///the validity of the snapshot is not stored.
   352     class Snapshot
   353     {
   354       SmartDigraph *_graph;
   355     protected:
   356       friend class SmartDigraph;
   357       unsigned int node_num;
   358       unsigned int arc_num;
   359     public:
   360       ///Default constructor.
   361 
   362       ///Default constructor.
   363       ///To actually make a snapshot you must call save().
   364       ///
   365       Snapshot() : _graph(0) {}
   366       ///Constructor that immediately makes a snapshot
   367 
   368       ///This constructor immediately makes a snapshot of the digraph.
   369       ///\param _g The digraph we make a snapshot of.
   370       Snapshot(SmartDigraph &graph) : _graph(&graph) {
   371         node_num=_graph->nodes.size();
   372         arc_num=_graph->arcs.size();
   373       }
   374 
   375       ///Make a snapshot.
   376 
   377       ///Make a snapshot of the digraph.
   378       ///
   379       ///This function can be called more than once. In case of a repeated
   380       ///call, the previous snapshot gets lost.
   381       ///\param _g The digraph we make the snapshot of.
   382       void save(SmartDigraph &graph)
   383       {
   384         _graph=&graph;
   385         node_num=_graph->nodes.size();
   386         arc_num=_graph->arcs.size();
   387       }
   388 
   389       ///Undo the changes until a snapshot.
   390 
   391       ///Undo the changes until a snapshot created by save().
   392       ///
   393       ///\note After you restored a state, you cannot restore
   394       ///a later state, in other word you cannot add again the arcs deleted
   395       ///by restore().
   396       void restore()
   397       {
   398         _graph->restoreSnapshot(*this);
   399       }
   400     };
   401   };
   402 
   403 
   404   class SmartGraphBase {
   405 
   406   protected:
   407 
   408     struct NodeT {
   409       int first_out;
   410     };
   411 
   412     struct ArcT {
   413       int target;
   414       int next_out;
   415     };
   416 
   417     std::vector<NodeT> nodes;
   418     std::vector<ArcT> arcs;
   419 
   420     int first_free_arc;
   421 
   422   public:
   423 
   424     typedef SmartGraphBase Digraph;
   425 
   426     class Node;
   427     class Arc;
   428     class Edge;
   429 
   430     class Node {
   431       friend class SmartGraphBase;
   432     protected:
   433 
   434       int _id;
   435       explicit Node(int id) { _id = id;}
   436 
   437     public:
   438       Node() {}
   439       Node (Invalid) { _id = -1; }
   440       bool operator==(const Node& node) const {return _id == node._id;}
   441       bool operator!=(const Node& node) const {return _id != node._id;}
   442       bool operator<(const Node& node) const {return _id < node._id;}
   443     };
   444 
   445     class Edge {
   446       friend class SmartGraphBase;
   447     protected:
   448 
   449       int _id;
   450       explicit Edge(int id) { _id = id;}
   451 
   452     public:
   453       Edge() {}
   454       Edge (Invalid) { _id = -1; }
   455       bool operator==(const Edge& arc) const {return _id == arc._id;}
   456       bool operator!=(const Edge& arc) const {return _id != arc._id;}
   457       bool operator<(const Edge& arc) const {return _id < arc._id;}
   458     };
   459 
   460     class Arc {
   461       friend class SmartGraphBase;
   462     protected:
   463 
   464       int _id;
   465       explicit Arc(int id) { _id = id;}
   466 
   467     public:
   468       operator Edge() const { 
   469         return _id != -1 ? edgeFromId(_id / 2) : INVALID; 
   470       }
   471 
   472       Arc() {}
   473       Arc (Invalid) { _id = -1; }
   474       bool operator==(const Arc& arc) const {return _id == arc._id;}
   475       bool operator!=(const Arc& arc) const {return _id != arc._id;}
   476       bool operator<(const Arc& arc) const {return _id < arc._id;}
   477     };
   478 
   479 
   480 
   481     SmartGraphBase()
   482       : nodes(), arcs() {}
   483 
   484 
   485     int maxNodeId() const { return nodes.size()-1; }
   486     int maxEdgeId() const { return arcs.size() / 2 - 1; }
   487     int maxArcId() const { return arcs.size()-1; }
   488 
   489     Node source(Arc e) const { return Node(arcs[e._id ^ 1].target); }
   490     Node target(Arc e) const { return Node(arcs[e._id].target); }
   491 
   492     Node u(Edge e) const { return Node(arcs[2 * e._id].target); }
   493     Node v(Edge e) const { return Node(arcs[2 * e._id + 1].target); }
   494 
   495     static bool direction(Arc e) {
   496       return (e._id & 1) == 1;
   497     }
   498 
   499     static Arc direct(Edge e, bool d) {
   500       return Arc(e._id * 2 + (d ? 1 : 0));
   501     }
   502 
   503     void first(Node& node) const {
   504       node._id = nodes.size() - 1;
   505     }
   506 
   507     void next(Node& node) const {
   508       --node._id;
   509     }
   510 
   511     void first(Arc& arc) const {
   512       arc._id = arcs.size() - 1;
   513     }
   514 
   515     void next(Arc& arc) const {
   516       --arc._id;
   517     }
   518 
   519     void first(Edge& arc) const {
   520       arc._id = arcs.size() / 2 - 1;
   521     }
   522 
   523     void next(Edge& arc) const {
   524       --arc._id;
   525     }
   526 
   527     void firstOut(Arc &arc, const Node& v) const {
   528       arc._id = nodes[v._id].first_out;
   529     }
   530     void nextOut(Arc &arc) const {
   531       arc._id = arcs[arc._id].next_out;
   532     }
   533 
   534     void firstIn(Arc &arc, const Node& v) const {
   535       arc._id = ((nodes[v._id].first_out) ^ 1);
   536       if (arc._id == -2) arc._id = -1;
   537     }
   538     void nextIn(Arc &arc) const {
   539       arc._id = ((arcs[arc._id ^ 1].next_out) ^ 1);
   540       if (arc._id == -2) arc._id = -1;
   541     }
   542 
   543     void firstInc(Edge &arc, bool& d, const Node& v) const {
   544       int de = nodes[v._id].first_out;
   545       if (de != -1) {
   546         arc._id = de / 2;
   547         d = ((de & 1) == 1);
   548       } else {
   549         arc._id = -1;
   550         d = true;
   551       }
   552     }
   553     void nextInc(Edge &arc, bool& d) const {
   554       int de = (arcs[(arc._id * 2) | (d ? 1 : 0)].next_out);
   555       if (de != -1) {
   556         arc._id = de / 2;
   557         d = ((de & 1) == 1);
   558       } else {
   559         arc._id = -1;
   560         d = true;
   561       }
   562     }
   563 
   564     static int id(Node v) { return v._id; }
   565     static int id(Arc e) { return e._id; }
   566     static int id(Edge e) { return e._id; }
   567 
   568     static Node nodeFromId(int id) { return Node(id);}
   569     static Arc arcFromId(int id) { return Arc(id);}
   570     static Edge edgeFromId(int id) { return Edge(id);}
   571 
   572     bool valid(Node n) const {
   573       return n._id >= 0 && n._id < static_cast<int>(nodes.size());
   574     }
   575     bool valid(Arc a) const {
   576       return a._id >= 0 && a._id < static_cast<int>(arcs.size());
   577     }
   578     bool valid(Edge e) const {
   579       return e._id >= 0 && 2 * e._id < static_cast<int>(arcs.size());
   580     }
   581 
   582     Node addNode() {
   583       int n = nodes.size();
   584       nodes.push_back(NodeT());
   585       nodes[n].first_out = -1;
   586 
   587       return Node(n);
   588     }
   589 
   590     Edge addEdge(Node u, Node v) {
   591       int n = arcs.size();
   592       arcs.push_back(ArcT());
   593       arcs.push_back(ArcT());
   594 
   595       arcs[n].target = u._id;
   596       arcs[n | 1].target = v._id;
   597 
   598       arcs[n].next_out = nodes[v._id].first_out;
   599       nodes[v._id].first_out = n;
   600 
   601       arcs[n | 1].next_out = nodes[u._id].first_out;
   602       nodes[u._id].first_out = (n | 1);
   603 
   604       return Edge(n / 2);
   605     }
   606 
   607     void clear() {
   608       arcs.clear();
   609       nodes.clear();
   610     }
   611 
   612   };
   613 
   614   typedef GraphExtender<SmartGraphBase> ExtendedSmartGraphBase;
   615 
   616   /// \ingroup graphs
   617   ///
   618   /// \brief A smart undirected graph class.
   619   ///
   620   /// This is a simple and fast graph implementation.
   621   /// It is also quite memory efficient, but at the price
   622   /// that <b> it does support only limited (only stack-like)
   623   /// node and arc deletions</b>.
   624   /// Except from this it conforms to
   625   /// the \ref concepts::Graph "Graph concept".
   626   ///
   627   /// It also has an
   628   /// important extra feature that
   629   /// its maps are real \ref concepts::ReferenceMap "reference map"s.
   630   ///
   631   /// \sa concepts::Graph.
   632   ///
   633   class SmartGraph : public ExtendedSmartGraphBase {
   634   private:
   635 
   636     ///SmartGraph is \e not copy constructible. Use GraphCopy() instead.
   637 
   638     ///SmartGraph is \e not copy constructible. Use GraphCopy() instead.
   639     ///
   640     SmartGraph(const SmartGraph &) : ExtendedSmartGraphBase() {};
   641 
   642     ///\brief Assignment of SmartGraph to another one is \e not allowed.
   643     ///Use GraphCopy() instead.
   644 
   645     ///Assignment of SmartGraph to another one is \e not allowed.
   646     ///Use GraphCopy() instead.
   647     void operator=(const SmartGraph &) {}
   648 
   649   public:
   650 
   651     typedef ExtendedSmartGraphBase Parent;
   652 
   653     /// Constructor
   654 
   655     /// Constructor.
   656     ///
   657     SmartGraph() {}
   658 
   659     ///Add a new node to the graph.
   660 
   661     /// \return the new node.
   662     ///
   663     Node addNode() { return Parent::addNode(); }
   664 
   665     ///Add a new edge to the graph.
   666 
   667     ///Add a new edge to the graph with node \c s
   668     ///and \c t.
   669     ///\return the new edge.
   670     Edge addEdge(const Node& s, const Node& t) {
   671       return Parent::addEdge(s, t);
   672     }
   673 
   674     /// \brief Node validity check
   675     ///
   676     /// This function gives back true if the given node is valid,
   677     /// ie. it is a real node of the graph.
   678     ///
   679     /// \warning A removed node (using Snapshot) could become valid again
   680     /// when new nodes are added to the graph.
   681     bool valid(Node n) const { return Parent::valid(n); }
   682 
   683     /// \brief Arc validity check
   684     ///
   685     /// This function gives back true if the given arc is valid,
   686     /// ie. it is a real arc of the graph.
   687     ///
   688     /// \warning A removed arc (using Snapshot) could become valid again
   689     /// when new edges are added to the graph.
   690     bool valid(Arc a) const { return Parent::valid(a); }
   691 
   692     /// \brief Edge validity check
   693     ///
   694     /// This function gives back true if the given edge is valid,
   695     /// ie. it is a real edge of the graph.
   696     ///
   697     /// \warning A removed edge (using Snapshot) could become valid again
   698     /// when new edges are added to the graph.
   699     bool valid(Edge e) const { return Parent::valid(e); }
   700 
   701     ///Clear the graph.
   702 
   703     ///Erase all the nodes and edges from the graph.
   704     ///
   705     void clear() {
   706       Parent::clear();
   707     }
   708 
   709   public:
   710 
   711     class Snapshot;
   712 
   713   protected:
   714 
   715     void saveSnapshot(Snapshot &s)
   716     {
   717       s._graph = this;
   718       s.node_num = nodes.size();
   719       s.arc_num = arcs.size();
   720     }
   721 
   722     void restoreSnapshot(const Snapshot &s)
   723     {
   724       while(s.arc_num<arcs.size()) {
   725         int n=arcs.size()-1;
   726         Edge arc=edgeFromId(n/2);
   727         Parent::notifier(Edge()).erase(arc);
   728         std::vector<Arc> dir;
   729         dir.push_back(arcFromId(n));
   730         dir.push_back(arcFromId(n-1));
   731         Parent::notifier(Arc()).erase(dir);
   732         nodes[arcs[n].target].first_out=arcs[n].next_out;
   733         nodes[arcs[n-1].target].first_out=arcs[n-1].next_out;
   734         arcs.pop_back();
   735         arcs.pop_back();
   736       }
   737       while(s.node_num<nodes.size()) {
   738         int n=nodes.size()-1;
   739         Node node = nodeFromId(n);
   740         Parent::notifier(Node()).erase(node);
   741         nodes.pop_back();
   742       }
   743     }
   744 
   745   public:
   746 
   747     ///Class to make a snapshot of the digraph and to restrore to it later.
   748 
   749     ///Class to make a snapshot of the digraph and to restrore to it later.
   750     ///
   751     ///The newly added nodes and arcs can be removed using the
   752     ///restore() function.
   753     ///
   754     ///\note After you restore a state, you cannot restore
   755     ///a later state, in other word you cannot add again the arcs deleted
   756     ///by restore() using another one Snapshot instance.
   757     ///
   758     ///\warning If you do not use correctly the snapshot that can cause
   759     ///either broken program, invalid state of the digraph, valid but
   760     ///not the restored digraph or no change. Because the runtime performance
   761     ///the validity of the snapshot is not stored.
   762     class Snapshot
   763     {
   764       SmartGraph *_graph;
   765     protected:
   766       friend class SmartGraph;
   767       unsigned int node_num;
   768       unsigned int arc_num;
   769     public:
   770       ///Default constructor.
   771 
   772       ///Default constructor.
   773       ///To actually make a snapshot you must call save().
   774       ///
   775       Snapshot() : _graph(0) {}
   776       ///Constructor that immediately makes a snapshot
   777 
   778       ///This constructor immediately makes a snapshot of the digraph.
   779       ///\param g The digraph we make a snapshot of.
   780       Snapshot(SmartGraph &graph) {
   781         graph.saveSnapshot(*this);
   782       }
   783 
   784       ///Make a snapshot.
   785 
   786       ///Make a snapshot of the graph.
   787       ///
   788       ///This function can be called more than once. In case of a repeated
   789       ///call, the previous snapshot gets lost.
   790       ///\param g The digraph we make the snapshot of.
   791       void save(SmartGraph &graph)
   792       {
   793         graph.saveSnapshot(*this);
   794       }
   795 
   796       ///Undo the changes until a snapshot.
   797 
   798       ///Undo the changes until a snapshot created by save().
   799       ///
   800       ///\note After you restored a state, you cannot restore
   801       ///a later state, in other word you cannot add again the arcs deleted
   802       ///by restore().
   803       void restore()
   804       {
   805         _graph->restoreSnapshot(*this);
   806       }
   807     };
   808   };
   809 
   810 } //namespace lemon
   811 
   812 
   813 #endif //LEMON_SMART_GRAPH_H