Index: src/hugo/Makefile.am
===================================================================
--- src/hugo/Makefile.am	(revision 795)
+++ src/hugo/Makefile.am	(revision 810)
@@ -12,4 +12,5 @@
 	graph_wrapper.h							\
 	invalid.h							\
+	kruskal.h							\
 	list_graph.h							\
 	map_defines.h                                                   \
Index: src/hugo/kruskal.h
===================================================================
--- src/hugo/kruskal.h	(revision 810)
+++ src/hugo/kruskal.h	(revision 810)
@@ -0,0 +1,304 @@
+// -*- c++ -*- //
+#ifndef HUGO_KRUSKAL_H
+#define HUGO_KRUSKAL_H
+
+#include <algorithm>
+#include <hugo/unionfind.h>
+
+/**
+@defgroup spantree Minimum Cost Spanning Tree Algorithms
+@ingroup galgs
+\brief This group containes the algorithms for finding a minimum cost spanning
+tree in a graph
+
+This group containes the algorithms for finding a minimum cost spanning
+tree in a graph
+*/
+
+///\ingroup spantree
+///\file
+///\brief Kruskal's algorithm to compute a minimum cost tree
+///
+///Kruskal's algorithm to compute a minimum cost tree.
+
+namespace hugo {
+
+  /// \addtogroup spantree
+  /// @{
+
+  /// Kruskal's algorithm to find a minimum cost tree of a graph.
+
+  /// This function runs Kruskal's algorithm to find a minimum cost tree.
+  /// \param G The graph the algorithm runs on. The algorithm considers the
+  /// graph to be undirected, the direction of the edges are not used.
+  ///
+  /// \param in This object is used to describe the edge costs. It must
+  /// be an STL compatible 'Forward Container'
+  /// with <tt>std::pair<Graph::Edge,X></tt> as its <tt>value_type</tt>,
+  /// where X is the type of the costs. It must contain every edge in
+  /// cost-ascending order.
+  ///\par
+  /// For the sake of simplicity, there is a helper class KruskalMapInput,
+  /// which converts a
+  /// simple edge map to an input of this form. Alternatively, you can use
+  /// the function \ref kruskalEdgeMap to compute the minimum cost tree if
+  /// the edge costs are given by an edge map.
+  ///
+  /// \retval out This must be a writable \c bool edge map.
+  /// After running the algorithm
+  /// this will contain the found minimum cost spanning tree: the value of an
+  /// edge will be set to \c true if it belongs to the tree, otherwise it will
+  /// be set to \c false. The value of each edge will be set exactly once.
+  ///
+  /// \return The cost of the found tree.
+
+  template <typename Graph, typename InputEdgeOrder, typename OutBoolMap>
+  typename InputEdgeOrder::value_type::second_type
+  kruskal(Graph const& G, InputEdgeOrder const& in, 
+		 OutBoolMap& out)
+  {
+    typedef typename InputEdgeOrder::value_type::second_type EdgeCost;
+    typedef typename Graph::template NodeMap<int> NodeIntMap;
+    typedef typename Graph::Node Node;
+
+    NodeIntMap comp(G, -1);
+    UnionFind<Node,NodeIntMap> uf(comp); 
+      
+    EdgeCost tot_cost = 0;
+    for (typename InputEdgeOrder::const_iterator p = in.begin(); 
+	 p!=in.end(); ++p ) {
+      if ( uf.join(G.head((*p).first),
+		   G.tail((*p).first)) ) {
+	out.set((*p).first, true);
+	tot_cost += (*p).second;
+      }
+      else {
+	out.set((*p).first, false);
+      }
+    }
+    return tot_cost;
+  }
+
+  /* A work-around for running Kruskal with const-reference bool maps... */
+
+  ///\bug What is this? Or why doesn't it works?
+  ///
+  template<typename Map>
+  class NonConstMapWr {
+    const Map &m;
+  public:
+    typedef typename Map::ValueType ValueType;
+
+    NonConstMapWr(const Map &_m) : m(_m) {}
+
+    template<typename KeyType>
+    void set(KeyType const& k, ValueType const &v) const { m.set(k,v); }
+  };
+
+  template <typename Graph, typename InputEdgeOrder, typename OutBoolMap>
+  inline
+  typename InputEdgeOrder::ValueType
+  kruskal(Graph const& G, InputEdgeOrder const& edges, 
+	  OutBoolMap const& out_map)
+  {
+    NonConstMapWr<OutBoolMap> map_wr(out_map);
+    return kruskal(G, edges, map_wr);
+  }  
+
+  /* ** ** Input-objects ** ** */
+
+  /// Kruskal input source.
+
+  /// Kruskal input source.
+  ///
+  /// In most cases you possibly want to use the \ref kruskalEdgeMap() instead.
+  ///
+  /// \sa makeKruskalMapInput()
+  ///
+  ///\param Graph The type of the graph the algorithm runs on.
+  ///\param Map An edge map containing the cost of the edges.
+  ///\par
+  ///The cost type can be any type satisfying
+  ///the STL 'LessThan comparable'
+  ///concept if it also has an operator+() implemented. (It is necessary for
+  ///computing the total cost of the tree).
+  ///
+  template<typename Graph, typename Map>
+  class KruskalMapInput
+    : public std::vector< std::pair<typename Graph::Edge,
+				    typename Map::ValueType> > {
+    
+  public:
+    typedef std::vector< std::pair<typename Graph::Edge,
+				   typename Map::ValueType> > Parent;
+    typedef typename Parent::value_type value_type;
+
+  private:
+    class comparePair {
+    public:
+      bool operator()(const value_type& a,
+		      const value_type& b) {
+	return a.second < b.second;
+      }
+    };
+
+  public:
+
+    void sort() {
+      std::sort(this->begin(), this->end(), comparePair());
+    }
+
+    KruskalMapInput(Graph const& G, Map const& m) {
+      typedef typename Graph::EdgeIt EdgeIt;
+      
+      this->clear();
+      for(EdgeIt e(G);e!=INVALID;++e) push_back(make_pair(e, m[e]));
+      sort();
+    }
+  };
+
+  /// Creates a KruskalMapInput object for \ref kruskal()
+
+  /// It makes is easier to use 
+  /// \ref KruskalMapInput by making it unnecessary 
+  /// to explicitly give the type of the parameters.
+  ///
+  /// In most cases you possibly
+  /// want to use the function kruskalEdgeMap() instead.
+  ///
+  ///\param G The type of the graph the algorithm runs on.
+  ///\param m An edge map containing the cost of the edges.
+  ///\par
+  ///The cost type can be any type satisfying the
+  ///STL 'LessThan Comparable'
+  ///concept if it also has an operator+() implemented. (It is necessary for
+  ///computing the total cost of the tree).
+  ///
+  ///\return An appropriate input source for \ref kruskal().
+  ///
+  template<typename Graph, typename Map>
+  inline
+  KruskalMapInput<Graph,Map> makeKruskalMapInput(const Graph &G,const Map &m)
+  {
+    return KruskalMapInput<Graph,Map>(G,m);
+  }
+  
+  
+  /* ** ** Output-objects: simple writable bool maps** ** */
+  
+  /// A writable bool-map that makes a sequence of "true" keys
+
+  /// A writable bool-map that creates a sequence out of keys that receives
+  /// the value "true".
+  /// \warning Not a regular property map, as it doesn't know its KeyType
+  /// \bug Missing documentation.
+  /// \todo This class may be of wider usage, therefore it could move to
+  /// <tt>maps.h</tt>
+  template<typename Iterator>
+  class SequenceOutput {
+    mutable Iterator it;
+
+  public:
+    typedef bool ValueType;
+
+    SequenceOutput(Iterator const &_it) : it(_it) {}
+
+    template<typename KeyType>
+    void set(KeyType const& k, bool v) const { if(v) {*it=k; ++it;} }
+  };
+
+  template<typename Iterator>
+  inline
+  SequenceOutput<Iterator>
+  makeSequenceOutput(Iterator it) {
+    return SequenceOutput<Iterator>(it);
+  }
+
+  /* ** ** Wrapper funtions ** ** */
+
+
+  /// \brief Wrapper function to kruskal().
+  /// Input is from an edge map, output is a plain bool map.
+  ///
+  /// Wrapper function to kruskal().
+  /// Input is from an edge map, output is a plain bool map.
+  ///
+  ///\param G The type of the graph the algorithm runs on.
+  ///\param in An edge map containing the cost of the edges.
+  ///\par
+  ///The cost type can be any type satisfying the
+  ///STL 'LessThan Comparable'
+  ///concept if it also has an operator+() implemented. (It is necessary for
+  ///computing the total cost of the tree).
+  ///
+  /// \retval out This must be a writable \c bool edge map.
+  /// After running the algorithm
+  /// this will contain the found minimum cost spanning tree: the value of an
+  /// edge will be set to \c true if it belongs to the tree, otherwise it will
+  /// be set to \c false. The value of each edge will be set exactly once.
+  ///
+  /// \return The cost of the found tree.
+
+
+  template <typename Graph, typename EdgeCostMap, typename RetEdgeBoolMap>
+  inline
+  typename EdgeCostMap::ValueType
+  kruskalEdgeMap(Graph const& G,
+		 EdgeCostMap const& in,
+		 RetEdgeBoolMap &out) {
+    return kruskal(G,
+		   KruskalMapInput<Graph,EdgeCostMap>(G,in),
+		   out);
+  }
+
+  /// \brief Wrapper function to kruskal().
+  /// Input is from an edge map, output is an STL Sequence.
+  ///
+  /// Wrapper function to kruskal().
+  /// Input is from an edge map, output is an STL Sequence.
+  ///
+  ///\param G The type of the graph the algorithm runs on.
+  ///\param in An edge map containing the cost of the edges.
+  ///\par
+  ///The cost type can be any type satisfying the
+  ///STL 'LessThan Comparable'
+  ///concept if it also has an operator+() implemented. (It is necessary for
+  ///computing the total cost of the tree).
+  ///
+  /// \retval out This must be an iteraror of an STL Container with
+  /// <tt>Graph::Edge</tt> as its <tt>value_type</tt>.
+  /// The algorithm copies the elements of the found tree into this sequence.
+  /// For example, if we know that the spanning tree of the graph \c G has
+  /// say 53 edges then
+  /// we can put its edges into a vector \c tree with a code like this.
+  /// \code
+  /// std::vector<Edge> tree(53);
+  /// kruskalEdgeMap_IteratorOut(G,cost,tree.begin());
+  /// \endcode
+  /// Or if we don't know in advance the size of the tree, we can write this.
+  /// \code
+  /// std::vector<Edge> tree;
+  /// kruskalEdgeMap_IteratorOut(G,cost,std::back_inserter(tree));
+  /// \endcode
+  ///
+  /// \return The cost of the found tree.
+  ///
+  /// \bug its name does not follow the coding style.
+  template <typename Graph, typename EdgeCostMap, typename RetIterator>
+  inline
+  typename EdgeCostMap::ValueType
+  kruskalEdgeMap_IteratorOut(const Graph& G,
+			     const EdgeCostMap& in,
+			     RetIterator out)
+  {
+    SequenceOutput<RetIterator> _out(out);
+    return kruskal(G,
+		   KruskalMapInput<Graph, EdgeCostMap>(G, in),
+		   _out);
+  }
+
+  /// @}
+
+} //namespace hugo
+
+#endif //HUGO_KRUSKAL_H
Index: src/hugo/unionfind.h
===================================================================
--- src/hugo/unionfind.h	(revision 774)
+++ src/hugo/unionfind.h	(revision 810)
@@ -33,7 +33,12 @@
    * For more features see the \ref UnionFindEnum class.
    *
+   * It is primarily used in Kruskal algorithm for finding minimal
+   * cost spanning tree in a graph.
+   * \sa kruskal()
+   *
    * \pre The elements are automatically added only if the map 
    * given to the constructor was filled with -1's. Otherwise you
    * need to add all the elements by the \ref insert() method.
+   * \bug It is not clear what the constructor parameter is used for.
    */