RhodeCode

  /// \ingroup graph_adaptors

  ///

  /// \brief Adaptor class for hiding nodes in a digraph or a graph.

  ///

  /// FilterNodes adaptor can be used for hiding nodes in a digraph or a

  /// graph. A \c bool node map must be specified, which defines the filter

  /// for the nodes. Only the nodes with \c true filter value and the

  /// arcs/edges incident to nodes both with \c true filter value are shown

  /// in the subgraph. This adaptor conforms to the \ref concepts::Digraph

  /// "Digraph" concept or the \ref concepts::Graph "Graph" concept

  /// depending on the \c GR template parameter.

  ///

  /// The adapted (di)graph can also be modified through this adaptor

  /// by adding or removing nodes or arcs/edges, unless the \c GR template

  /// parameter is set to be \c const.

  ///

  /// \tparam GR The type of the adapted digraph or graph.

  /// It must conform to the \ref concepts::Digraph "Digraph" concept

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

  /// It can also be specified to be \c const.

  /// \tparam NF The type of the node filter map.

  /// It must be a \c bool (or convertible) node map of the

  /// adapted (di)graph. The default type is

  /// \ref concepts::Graph::NodeMap "GR::NodeMap<bool>".

  ///

  /// \note The \c Node and <tt>Arc/Edge</tt> types of this adaptor and the

  /// adapted (di)graph are convertible to each other.

#ifdef DOXYGEN

  template<typename GR, typename NF>

  class FilterNodes {

#else

  template<typename GR,

           typename NF = typename GR::template NodeMap<bool>,

           typename Enable = void>

  class FilterNodes :

    public DigraphAdaptorExtender<

      SubDigraphBase<GR, NF, ConstMap<typename GR::Arc, Const<bool, true> >,

                     true> > {

#endif

    typedef DigraphAdaptorExtender<

      SubDigraphBase<GR, NF, ConstMap<typename GR::Arc, Const<bool, true> >, 

                     true> > Parent;

  public:

    typedef GR Digraph;

    typedef NF NodeFilterMap;

    typedef typename Parent::Node Node;

  protected:

    ConstMap<typename Digraph::Arc, Const<bool, true> > const_true_map;

    FilterNodes() : const_true_map() {}

  public:

    /// \brief Constructor

    ///

    /// Creates a subgraph for the given digraph or graph with the

    /// given node filter map.

    FilterNodes(GR& graph, NF& node_filter) 

      : Parent(), const_true_map()

    {

      Parent::initialize(graph, node_filter, const_true_map);

    }

    /// \brief Sets the status of the given node

    ///

    /// This function sets the status of the given node.

    /// It is done by simply setting the assigned value of \c n

    /// to \c v in the node filter map.

    void status(const Node& n, bool v) const { Parent::status(n, v); }

    /// \brief Returns the status of the given node

    ///

    /// This function returns the status of the given node.

    /// It is \c true if the given node is enabled (i.e. not hidden).

    bool status(const Node& n) const { return Parent::status(n); }

    /// \brief Disables the given node

    ///

    /// This function disables the given node, so the iteration

    /// jumps over it.

    /// It is the same as \ref status() "status(n, false)".

    void disable(const Node& n) const { Parent::status(n, false); }

    /// \brief Enables the given node

    ///

    /// This function enables the given node.

    /// It is the same as \ref status() "status(n, true)".

    void enable(const Node& n) const { Parent::status(n, true); }

  };

  template<typename GR, typename NF>

  class FilterNodes<GR, NF,

                    typename enable_if<UndirectedTagIndicator<GR> >::type> :

    public GraphAdaptorExtender<

      SubGraphBase<GR, NF, ConstMap<typename GR::Edge, Const<bool, true> >, 

                   true> > {

    typedef GraphAdaptorExtender<

      SubGraphBase<GR, NF, ConstMap<typename GR::Edge, Const<bool, true> >, 

                   true> > Parent;

  public:

    typedef GR Graph;

    typedef NF NodeFilterMap;

    typedef typename Parent::Node Node;

  protected:

    ConstMap<typename GR::Edge, Const<bool, true> > const_true_map;

    FilterNodes() : const_true_map() {}

  public:

    FilterNodes(GR& graph, NodeFilterMap& node_filter) :

      Parent(), const_true_map() {

      Parent::initialize(graph, node_filter, const_true_map);

    }

    void status(const Node& n, bool v) const { Parent::status(n, v); }

    bool status(const Node& n) const { return Parent::status(n); }

    void disable(const Node& n) const { Parent::status(n, false); }

    void enable(const Node& n) const { Parent::status(n, true); }

  };

  /// \brief Returns a read-only FilterNodes adaptor

  ///

  /// This function just returns a read-only \ref FilterNodes adaptor.

  /// \ingroup graph_adaptors

  /// \relates FilterNodes

  template<typename GR, typename NF>

  FilterNodes<const GR, NF>

  filterNodes(const GR& graph, NF& node_filter) {

    return FilterNodes<const GR, NF>(graph, node_filter);

  }

  template<typename GR, typename NF>

  FilterNodes<const GR, const NF>

  filterNodes(const GR& graph, const NF& node_filter) {

    return FilterNodes<const GR, const NF>(graph, node_filter);

  }

  /// \ingroup graph_adaptors

  ///

  /// \brief Adaptor class for hiding arcs in a digraph.

  ///

  /// FilterArcs adaptor can be used for hiding arcs in a digraph.

  /// A \c bool arc map must be specified, which defines the filter for

  /// the arcs. Only the arcs with \c true filter value are shown in the

  /// subdigraph. This adaptor conforms to the \ref concepts::Digraph

  /// "Digraph" concept.

  ///

  /// The adapted digraph can also be modified through this adaptor

  /// by adding or removing nodes or arcs, unless the \c GR template

  /// parameter is set to be \c const.

  ///

  /// \tparam DGR The type of the adapted digraph.

  /// It must conform to the \ref concepts::Digraph "Digraph" concept.

  /// It can also be specified to be \c const.

  /// \tparam AF The type of the arc filter map.

  /// It must be a \c bool (or convertible) arc map of the

  /// adapted digraph. The default type is

  /// \ref concepts::Digraph::ArcMap "DGR::ArcMap<bool>".

  ///

  /// \note The \c Node and \c Arc types of this adaptor and the adapted

  /// digraph are convertible to each other.

#ifdef DOXYGEN

  template<typename DGR,

           typename AF>

  class FilterArcs {

#else

  template<typename DGR,

           typename AF = typename DGR::template ArcMap<bool> >

  class FilterArcs :

    public DigraphAdaptorExtender<

      SubDigraphBase<DGR, ConstMap<typename DGR::Node, Const<bool, true> >,

                     AF, false> > {

#endif

    typedef DigraphAdaptorExtender<

      SubDigraphBase<DGR, ConstMap<typename DGR::Node, Const<bool, true> >, 

                     AF, false> > Parent;

  public:

    /// The type of the adapted digraph.

    typedef DGR Digraph;

    /// The type of the arc filter map.

    typedef AF ArcFilterMap;

    typedef typename Parent::Arc Arc;

  protected:

    ConstMap<typename DGR::Node, Const<bool, true> > const_true_map;

    FilterArcs() : const_true_map() {}

  public:

    /// \brief Constructor

    ///

    /// Creates a subdigraph for the given digraph with the given arc

    /// filter map.

    FilterArcs(DGR& digraph, ArcFilterMap& arc_filter)

      : Parent(), const_true_map() {

      Parent::initialize(digraph, const_true_map, arc_filter);

    }

    /// \brief Sets the status of the given arc

    ///

    /// This function sets the status of the given arc.

    /// It is done by simply setting the assigned value of \c a

    /// to \c v in the arc filter map.

    void status(const Arc& a, bool v) const { Parent::status(a, v); }

    /// \brief Returns the status of the given arc

    ///

    /// This function returns the status of the given arc.

    /// It is \c true if the given arc is enabled (i.e. not hidden).

    bool status(const Arc& a) const { return Parent::status(a); }

    /// \brief Disables the given arc

    ///

    /// This function disables the given arc in the subdigraph,

    /// so the iteration jumps over it.

    /// It is the same as \ref status() "status(a, false)".

    void disable(const Arc& a) const { Parent::status(a, false); }

    /// \brief Enables the given arc

    ///

    /// This function enables the given arc in the subdigraph.

    /// It is the same as \ref status() "status(a, true)".

    void enable(const Arc& a) const { Parent::status(a, true); }

  };

  /// \brief Returns a read-only FilterArcs adaptor

  ///

  /// This function just returns a read-only \ref FilterArcs adaptor.

  /// \ingroup graph_adaptors

  /// \relates FilterArcs

  template<typename DGR, typename AF>

  FilterArcs<const DGR, AF>

  filterArcs(const DGR& digraph, AF& arc_filter) {

    return FilterArcs<const DGR, AF>(digraph, arc_filter);

  }

  template<typename DGR, typename AF>

  FilterArcs<const DGR, const AF>

  filterArcs(const DGR& digraph, const AF& arc_filter) {

    return FilterArcs<const DGR, const AF>(digraph, arc_filter);

  }

  /// \ingroup graph_adaptors

  ///

  /// \brief Adaptor class for hiding edges in a graph.

  ///

  /// FilterEdges adaptor can be used for hiding edges in a graph.

  /// A \c bool edge map must be specified, which defines the filter for

  /// the edges. Only the edges with \c true filter value are shown in the

  /// subgraph. This adaptor conforms to the \ref concepts::Graph

  /// "Graph" concept.

  ///

  /// The adapted graph can also be modified through this adaptor

  /// by adding or removing nodes or edges, unless the \c GR template

  /// parameter is set to be \c const.

  ///

  /// \tparam GR The type of the adapted graph.

  /// It must conform to the \ref concepts::Graph "Graph" concept.

  /// It can also be specified to be \c const.

  /// \tparam EF The type of the edge filter map.

  /// It must be a \c bool (or convertible) edge map of the

  /// adapted graph. The default type is

  /// \ref concepts::Graph::EdgeMap "GR::EdgeMap<bool>".

  ///

  /// \note The \c Node, \c Edge and \c Arc types of this adaptor and the

  /// adapted graph are convertible to each other.

#ifdef DOXYGEN

  template<typename GR,

           typename EF>

  class FilterEdges {

#else

  template<typename GR,

           typename EF = typename GR::template EdgeMap<bool> >

  class FilterEdges :

    public GraphAdaptorExtender<

      SubGraphBase<GR, ConstMap<typename GR::Node, Const<bool, true> >, 

                   EF, false> > {

#endif

    typedef GraphAdaptorExtender<

      SubGraphBase<GR, ConstMap<typename GR::Node, Const<bool, true > >, 

                   EF, false> > Parent;

  public:

    /// The type of the adapted graph.

    typedef GR Graph;

    /// The type of the edge filter map.

    typedef EF EdgeFilterMap;

    typedef typename Parent::Edge Edge;

  protected:

    ConstMap<typename GR::Node, Const<bool, true> > const_true_map;

    FilterEdges() : const_true_map(true) {

      Parent::setNodeFilterMap(const_true_map);

    }

  public:

    /// \brief Constructor

    ///

    /// Creates a subgraph for the given graph with the given edge

    /// filter map.

    FilterEdges(GR& graph, EF& edge_filter) 

      : Parent(), const_true_map() {

      Parent::initialize(graph, const_true_map, edge_filter);

    }

    /// \brief Sets the status of the given edge

    ///

    /// This function sets the status of the given edge.

    /// It is done by simply setting the assigned value of \c e

    /// to \c v in the edge filter map.

    void status(const Edge& e, bool v) const { Parent::status(e, v); }

    /// \brief Returns the status of the given edge

    ///

    /// This function returns the status of the given edge.

    /// It is \c true if the given edge is enabled (i.e. not hidden).

    bool status(const Edge& e) const { return Parent::status(e); }

    /// \brief Disables the given edge

    ///

    /// This function disables the given edge in the subgraph,

    /// so the iteration jumps over it.

    /// It is the same as \ref status() "status(e, false)".

    void disable(const Edge& e) const { Parent::status(e, false); }

    /// \brief Enables the given edge

    ///

    /// This function enables the given edge in the subgraph.

    /// It is the same as \ref status() "status(e, true)".

    void enable(const Edge& e) const { Parent::status(e, true); }

  };

  /// \brief Returns a read-only FilterEdges adaptor

  ///

  /// This function just returns a read-only \ref FilterEdges adaptor.

  /// \ingroup graph_adaptors

  /// \relates FilterEdges

  template<typename GR, typename EF>

  FilterEdges<const GR, EF>

  filterEdges(const GR& graph, EF& edge_filter) {

    return FilterEdges<const GR, EF>(graph, edge_filter);

  }

  template<typename GR, typename EF>

  FilterEdges<const GR, const EF>

  filterEdges(const GR& graph, const EF& edge_filter) {

    return FilterEdges<const GR, const EF>(graph, edge_filter);

  }

  template <typename DGR>

  class UndirectorBase {

  public:

    typedef DGR Digraph;

    typedef UndirectorBase Adaptor;

    typedef True UndirectedTag;

    typedef typename Digraph::Arc Edge;

    typedef typename Digraph::Node Node;

      friend class UndirectorBase;

    protected:

      bool _forward;

    public:

      Arc() {}

      bool operator==(const Arc &other) const {

      }

      bool operator!=(const Arc &other) const {

      }

      bool operator<(const Arc &other) const {

        return _forward < other._forward ||

      }

    };

    void first(Node& n) const {

      _digraph->first(n);

    }

    void next(Node& n) const {

      _digraph->next(n);

    }

    void first(Arc& a) const {

      a._forward = true;

    }

    void next(Arc& a) const {

      if (a._forward) {

        a._forward = false;

      } else {

        a._forward = true;

      }

    }

    void first(Edge& e) const {

      _digraph->first(e);

    }

    void next(Edge& e) const {

      _digraph->next(e);

    }

    void firstOut(Arc& a, const Node& n) const {

        a._forward = false;

      } else {

        a._forward = true;

      }

    }

    void nextOut(Arc &a) const {

      if (!a._forward) {

          a._forward = true;

        }

      }

      else {

      }

    }

    void firstIn(Arc &a, const Node &n) const {

        a._forward = false;

      } else {

        a._forward = true;

      }

    }

    void nextIn(Arc &a) const {

      if (!a._forward) {

          a._forward = true;

        }

      }

      else {

      }

    }

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

      d = true;

      _digraph->firstOut(e, n);

      if (e != INVALID) return;

      d = false;

      _digraph->firstIn(e, n);

    }

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

      if (d) {

        Node s = _digraph->source(e);

        _digraph->nextOut(e);

        if (e != INVALID) return;

        d = false;

        _digraph->firstIn(e, s);

      } else {

        _digraph->nextIn(e);

      }

    }

    Node u(const Edge& e) const {

      return _digraph->source(e);

    }

    Node v(const Edge& e) const {

      return _digraph->target(e);

    }

    Node source(const Arc &a) const {

    }

    Node target(const Arc &a) const {

    }

    static Arc direct(const Edge &e, bool d) {

      return Arc(e, d);

    }

    static bool direction(const Arc &a) { return a._forward; }

    Node nodeFromId(int ix) const { return _digraph->nodeFromId(ix); }

    Arc arcFromId(int ix) const {

      return direct(_digraph->arcFromId(ix >> 1), bool(ix & 1));

    }

    Edge edgeFromId(int ix) const { return _digraph->arcFromId(ix); }

    int id(const Node &n) const { return _digraph->id(n); }

    int id(const Arc &a) const {

      return  (_digraph->id(a) << 1) | (a._forward ? 1 : 0);

    }

    int id(const Edge &e) const { return _digraph->id(e); }

    int maxNodeId() const { return _digraph->maxNodeId(); }

    int maxArcId() const { return (_digraph->maxArcId() << 1) | 1; }

    int maxEdgeId() const { return _digraph->maxArcId(); }

    Node addNode() { return _digraph->addNode(); }

    Edge addEdge(const Node& u, const Node& v) {

      return _digraph->addArc(u, v);

    }

    void erase(const Node& i) { _digraph->erase(i); }

    void erase(const Edge& i) { _digraph->erase(i); }

    void clear() { _digraph->clear(); }

    typedef NodeNumTagIndicator<Digraph> NodeNumTag;

    int nodeNum() const { return _digraph->nodeNum(); }

    typedef ArcNumTagIndicator<Digraph> ArcNumTag;

    int arcNum() const { return 2 * _digraph->arcNum(); }

    typedef ArcNumTag EdgeNumTag;

    int edgeNum() const { return _digraph->arcNum(); }

    typedef FindArcTagIndicator<Digraph> FindArcTag;

    Arc findArc(Node s, Node t, Arc p = INVALID) const {

      if (p == INVALID) {

        Edge arc = _digraph->findArc(s, t);

        if (arc != INVALID) return direct(arc, true);

        arc = _digraph->findArc(t, s);

        if (arc != INVALID) return direct(arc, false);

      } else if (direction(p)) {

        Edge arc = _digraph->findArc(s, t, p);

        if (arc != INVALID) return direct(arc, true);

        arc = _digraph->findArc(t, s);

        if (arc != INVALID) return direct(arc, false);

      } else {

        Edge arc = _digraph->findArc(t, s, p);

        if (arc != INVALID) return direct(arc, false);

      }

      return INVALID;

    }

    typedef FindArcTag FindEdgeTag;

    Edge findEdge(Node s, Node t, Edge p = INVALID) const {

      if (s != t) {

        if (p == INVALID) {

          Edge arc = _digraph->findArc(s, t);

          if (arc != INVALID) return arc;

          arc = _digraph->findArc(t, s);

          if (arc != INVALID) return arc;

        } else if (_digraph->source(p) == s) {

          Edge arc = _digraph->findArc(s, t, p);

          if (arc != INVALID) return arc;

          arc = _digraph->findArc(t, s);

          if (arc != INVALID) return arc;

        } else {

          Edge arc = _digraph->findArc(t, s, p);

          if (arc != INVALID) return arc;

        }

      } else {

        return _digraph->findArc(s, t, p);

      }

      return INVALID;

    }

  private:

    template <typename V>

    class ArcMapBase {

    private:

      typedef typename DGR::template ArcMap<V> MapImpl;

    public:

      typedef typename MapTraits<MapImpl>::ReferenceMapTag ReferenceMapTag;

      typedef V Value;

      typedef Arc Key;

      typedef typename MapTraits<MapImpl>::ConstReturnValue ConstReturnValue;

      typedef typename MapTraits<MapImpl>::ReturnValue ReturnValue;

      typedef typename MapTraits<MapImpl>::ConstReturnValue ConstReference;

      typedef typename MapTraits<MapImpl>::ReturnValue Reference;

      ArcMapBase(const UndirectorBase<DGR>& adaptor) :

        _forward(*adaptor._digraph), _backward(*adaptor._digraph) {}

      ArcMapBase(const UndirectorBase<DGR>& adaptor, const V& value)

        : _forward(*adaptor._digraph, value), 

          _backward(*adaptor._digraph, value) {}

      void set(const Arc& a, const V& value) {

        if (direction(a)) {

          _forward.set(a, value);

        } else {

          _backward.set(a, value);

        }

      }

      ConstReturnValue operator[](const Arc& a) const {

        if (direction(a)) {

          return _forward[a];

        } else {

          return _backward[a];

        }

      }

      ReturnValue operator[](const Arc& a) {

        if (direction(a)) {

          return _forward[a];

        } else {

          return _backward[a];

        }

      }

    protected:

      MapImpl _forward, _backward;

    };

  public:

    template <typename V>

    class NodeMap : public DGR::template NodeMap<V> {

      typedef typename DGR::template NodeMap<V> Parent;

    public:

      typedef V Value;

      explicit NodeMap(const UndirectorBase<DGR>& adaptor)

        : Parent(*adaptor._digraph) {}

      NodeMap(const UndirectorBase<DGR>& adaptor, const V& value)

        : Parent(*adaptor._digraph, value) { }

    private:

      NodeMap& operator=(const NodeMap& cmap) {

        return operator=<NodeMap>(cmap);

      }

      template <typename CMap>

      NodeMap& operator=(const CMap& cmap) {

        Parent::operator=(cmap);

        return *this;

      }

    };

    template <typename V>

    class ArcMap

      : public SubMapExtender<UndirectorBase<DGR>, ArcMapBase<V> > {

      typedef SubMapExtender<UndirectorBase<DGR>, ArcMapBase<V> > Parent;

    public:

      typedef V Value;

      explicit ArcMap(const UndirectorBase<DGR>& adaptor)

        : Parent(adaptor) {}

      ArcMap(const UndirectorBase<DGR>& adaptor, const V& value)

        : Parent(adaptor, value) {}

    private:

      ArcMap& operator=(const ArcMap& cmap) {

        return operator=<ArcMap>(cmap);

      }

      template <typename CMap>

      ArcMap& operator=(const CMap& cmap) {

        Parent::operator=(cmap);

        return *this;

      }

    };

    template <typename V>

    class EdgeMap : public Digraph::template ArcMap<V> {

      typedef typename Digraph::template ArcMap<V> Parent;

    public:

      typedef V Value;

      explicit EdgeMap(const UndirectorBase<DGR>& adaptor)

        : Parent(*adaptor._digraph) {}

      EdgeMap(const UndirectorBase<DGR>& adaptor, const V& value)

        : Parent(*adaptor._digraph, value) {}

    private:

      EdgeMap& operator=(const EdgeMap& cmap) {

        return operator=<EdgeMap>(cmap);

      }

      template <typename CMap>

      EdgeMap& operator=(const CMap& cmap) {

        Parent::operator=(cmap);

        return *this;

      }

    };

    typedef typename ItemSetTraits<DGR, Node>::ItemNotifier NodeNotifier;

    NodeNotifier& notifier(Node) const { return _digraph->notifier(Node()); }

    typedef typename ItemSetTraits<DGR, Edge>::ItemNotifier EdgeNotifier;

    EdgeNotifier& notifier(Edge) const { return _digraph->notifier(Edge()); }

    typedef EdgeNotifier ArcNotifier;

    ArcNotifier& notifier(Arc) const { return _digraph->notifier(Edge()); }

  protected:

    UndirectorBase() : _digraph(0) {}

    DGR* _digraph;

    void initialize(DGR& digraph) {

      _digraph = &digraph;

    }

  };

  /// \ingroup graph_adaptors

  ///

  /// \brief Adaptor class for viewing a digraph as an undirected graph.

  ///

  /// Undirector adaptor can be used for viewing a digraph as an undirected

  /// graph. All arcs of the underlying digraph are showed in the

  /// adaptor as an edge (and also as a pair of arcs, of course).

  /// This adaptor conforms to the \ref concepts::Graph "Graph" concept.

  ///

  /// The adapted digraph can also be modified through this adaptor

  /// by adding or removing nodes or edges, unless the \c GR template

  /// parameter is set to be \c const.

  ///

  /// \tparam DGR The type of the adapted digraph.

  /// It must conform to the \ref concepts::Digraph "Digraph" concept.

  /// It can also be specified to be \c const.

  ///

  /// \note The \c Node type of this adaptor and the adapted digraph are

  /// convertible to each other, moreover the \c Edge type of the adaptor

  /// and the \c Arc type of the adapted digraph are also convertible to

  /// each other.

  /// (Thus the \c Arc type of the adaptor is convertible to the \c Arc type

  /// of the adapted digraph.)

  template<typename DGR>

#ifdef DOXYGEN

  class Undirector {

#else

  class Undirector :

    public GraphAdaptorExtender<UndirectorBase<DGR> > {

#endif

    typedef GraphAdaptorExtender<UndirectorBase<DGR> > Parent;

  public:

    /// The type of the adapted digraph.

    typedef DGR Digraph;

  protected:

    Undirector() { }

  public:

    /// \brief Constructor

    ///

    /// Creates an undirected graph from the given digraph.

    Undirector(DGR& digraph) {

      initialize(digraph);

    }

    /// \brief Arc map combined from two original arc maps

    ///

    /// This map adaptor class adapts two arc maps of the underlying

    /// digraph to get an arc map of the undirected graph.

    /// Its value type is inherited from the first arc map type (\c FW).

    /// \tparam FW The type of the "foward" arc map.

    /// \tparam BK The type of the "backward" arc map.

    template <typename FW, typename BK>

    class CombinedArcMap {

    public:

      /// The key type of the map

      typedef typename Parent::Arc Key;

      /// The value type of the map

      typedef typename FW::Value Value;

      typedef typename MapTraits<FW>::ReferenceMapTag ReferenceMapTag;

      typedef typename MapTraits<FW>::ReturnValue ReturnValue;

      typedef typename MapTraits<FW>::ConstReturnValue ConstReturnValue;

      typedef typename MapTraits<FW>::ReturnValue Reference;

      typedef typename MapTraits<FW>::ConstReturnValue ConstReference;

      /// Constructor

      CombinedArcMap(FW& forward, BK& backward)

        : _forward(&forward), _backward(&backward) {}

      /// Sets the value associated with the given key.

      void set(const Key& e, const Value& a) {

        if (Parent::direction(e)) {

          _forward->set(e, a);

        } else {

          _backward->set(e, a);

        }

      }

      /// Returns the value associated with the given key.

      ConstReturnValue operator[](const Key& e) const {

        if (Parent::direction(e)) {

          return (*_forward)[e];

        } else {

          return (*_backward)[e];

        }

      }

      /// Returns a reference to the value associated with the given key.

      ReturnValue operator[](const Key& e) {

        if (Parent::direction(e)) {

          return (*_forward)[e];

        } else {

          return (*_backward)[e];

        }

      }

    protected:

      FW* _forward;

      BK* _backward;

    };

    /// \brief Returns a combined arc map

    ///

    /// This function just returns a combined arc map.

    template <typename FW, typename BK>

    static CombinedArcMap<FW, BK>

    combinedArcMap(FW& forward, BK& backward) {

      return CombinedArcMap<FW, BK>(forward, backward);

    }

    template <typename FW, typename BK>

    static CombinedArcMap<const FW, BK>

    combinedArcMap(const FW& forward, BK& backward) {

      return CombinedArcMap<const FW, BK>(forward, backward);

    }

    template <typename FW, typename BK>

    static CombinedArcMap<FW, const BK>

    combinedArcMap(FW& forward, const BK& backward) {

      return CombinedArcMap<FW, const BK>(forward, backward);

    }

    template <typename FW, typename BK>

    static CombinedArcMap<const FW, const BK>

    combinedArcMap(const FW& forward, const BK& backward) {

      return CombinedArcMap<const FW, const BK>(forward, backward);

    }

  };

  /// \brief Returns a read-only Undirector adaptor

  ///

  /// This function just returns a read-only \ref Undirector adaptor.

  /// \ingroup graph_adaptors

  /// \relates Undirector

  template<typename DGR>

  Undirector<const DGR> undirector(const DGR& digraph) {

    return Undirector<const DGR>(digraph);

  }

  template <typename GR, typename DM>