diff -r cd72eae05bdf -r 3c00344f49c9 lemon/christofides_tsp.h
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/lemon/christofides_tsp.h	Wed Oct 17 19:14:07 2018 +0200
@@ -0,0 +1,254 @@
+/* -*- mode: C++; indent-tabs-mode: nil; -*-
+ *
+ * This file is a part of LEMON, a generic C++ optimization library.
+ *
+ * Copyright (C) 2003-2013
+ * 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_CHRISTOFIDES_TSP_H
+#define LEMON_CHRISTOFIDES_TSP_H
+
+/// \ingroup tsp
+/// \file
+/// \brief Christofides algorithm for symmetric TSP
+
+#include <lemon/full_graph.h>
+#include <lemon/smart_graph.h>
+#include <lemon/kruskal.h>
+#include <lemon/matching.h>
+#include <lemon/euler.h>
+
+namespace lemon {
+
+  /// \ingroup tsp
+  ///
+  /// \brief Christofides algorithm for symmetric TSP.
+  ///
+  /// ChristofidesTsp implements Christofides' heuristic for solving
+  /// symmetric \ref tsp "TSP".
+  ///
+  /// This a well-known approximation method for the TSP problem with
+  /// metric cost function.
+  /// It has a guaranteed approximation factor of 3/2 (i.e. it finds a tour
+  /// whose total cost is at most 3/2 of the optimum), but it usually
+  /// provides better solutions in practice.
+  /// This implementation runs in O(n<sup>3</sup>log(n)) time.
+  ///
+  /// The algorithm starts with a \ref spantree "minimum cost spanning tree" and
+  /// finds a \ref MaxWeightedPerfectMatching "minimum cost perfect matching"
+  /// in the subgraph induced by the nodes that have odd degree in the
+  /// spanning tree.
+  /// Finally, it constructs the tour from the \ref EulerIt "Euler traversal"
+  /// of the union of the spanning tree and the matching.
+  /// During this last step, the algorithm simply skips the visited nodes
+  /// (i.e. creates shortcuts) assuming that the triangle inequality holds
+  /// for the cost function.
+  ///
+  /// \tparam CM Type of the cost map.
+  ///
+  /// \warning CM::Value must be a signed number type.
+  template <typename CM>
+  class ChristofidesTsp
+  {
+    public:
+
+      /// Type of the cost map
+      typedef CM CostMap;
+      /// Type of the edge costs
+      typedef typename CM::Value Cost;
+
+    private:
+
+      GRAPH_TYPEDEFS(FullGraph);
+
+      const FullGraph &_gr;
+      const CostMap &_cost;
+      std::vector<Node> _path;
+      Cost _sum;
+
+    public:
+
+      /// \brief Constructor
+      ///
+      /// Constructor.
+      /// \param gr The \ref FullGraph "full graph" the algorithm runs on.
+      /// \param cost The cost map.
+      ChristofidesTsp(const FullGraph &gr, const CostMap &cost)
+        : _gr(gr), _cost(cost) {}
+
+      /// \name Execution Control
+      /// @{
+
+      /// \brief Runs the algorithm.
+      ///
+      /// This function runs the algorithm.
+      ///
+      /// \return The total cost of the found tour.
+      Cost run() {
+        _path.clear();
+
+        if (_gr.nodeNum() == 0) return _sum = 0;
+        else if (_gr.nodeNum() == 1) {
+          _path.push_back(_gr(0));
+          return _sum = 0;
+        }
+        else if (_gr.nodeNum() == 2) {
+          _path.push_back(_gr(0));
+          _path.push_back(_gr(1));
+          return _sum = 2 * _cost[_gr.edge(_gr(0), _gr(1))];
+        }
+
+        // Compute min. cost spanning tree
+        std::vector<Edge> tree;
+        kruskal(_gr, _cost, std::back_inserter(tree));
+
+        FullGraph::NodeMap<int> deg(_gr, 0);
+        for (int i = 0; i != int(tree.size()); ++i) {
+          Edge e = tree[i];
+          ++deg[_gr.u(e)];
+          ++deg[_gr.v(e)];
+        }
+
+        // Copy the induced subgraph of odd nodes
+        std::vector<Node> odd_nodes;
+        for (NodeIt u(_gr); u != INVALID; ++u) {
+          if (deg[u] % 2 == 1) odd_nodes.push_back(u);
+        }
+
+        SmartGraph sgr;
+        SmartGraph::EdgeMap<Cost> scost(sgr);
+        for (int i = 0; i != int(odd_nodes.size()); ++i) {
+          sgr.addNode();
+        }
+        for (int i = 0; i != int(odd_nodes.size()); ++i) {
+          for (int j = 0; j != int(odd_nodes.size()); ++j) {
+            if (j == i) continue;
+            SmartGraph::Edge e =
+              sgr.addEdge(sgr.nodeFromId(i), sgr.nodeFromId(j));
+            scost[e] = -_cost[_gr.edge(odd_nodes[i], odd_nodes[j])];
+          }
+        }
+
+        // Compute min. cost perfect matching
+        MaxWeightedPerfectMatching<SmartGraph, SmartGraph::EdgeMap<Cost> >
+          mwpm(sgr, scost);
+        mwpm.run();
+
+        for (SmartGraph::EdgeIt e(sgr); e != INVALID; ++e) {
+          if (mwpm.matching(e)) {
+            tree.push_back( _gr.edge(odd_nodes[sgr.id(sgr.u(e))],
+                                     odd_nodes[sgr.id(sgr.v(e))]) );
+          }
+        }
+
+        // Join the spanning tree and the matching
+        sgr.clear();
+        for (int i = 0; i != _gr.nodeNum(); ++i) {
+          sgr.addNode();
+        }
+        for (int i = 0; i != int(tree.size()); ++i) {
+          int ui = _gr.id(_gr.u(tree[i])),
+              vi = _gr.id(_gr.v(tree[i]));
+          sgr.addEdge(sgr.nodeFromId(ui), sgr.nodeFromId(vi));
+        }
+
+        // Compute the tour from the Euler traversal
+        SmartGraph::NodeMap<bool> visited(sgr, false);
+        for (EulerIt<SmartGraph> e(sgr); e != INVALID; ++e) {
+          SmartGraph::Node n = sgr.target(e);
+          if (!visited[n]) {
+            _path.push_back(_gr(sgr.id(n)));
+            visited[n] = true;
+          }
+        }
+
+        _sum = _cost[_gr.edge(_path.back(), _path.front())];
+        for (int i = 0; i < int(_path.size())-1; ++i) {
+          _sum += _cost[_gr.edge(_path[i], _path[i+1])];
+        }
+
+        return _sum;
+      }
+
+      /// @}
+
+      /// \name Query Functions
+      /// @{
+
+      /// \brief The total cost of the found tour.
+      ///
+      /// This function returns the total cost of the found tour.
+      ///
+      /// \pre run() must be called before using this function.
+      Cost tourCost() const {
+        return _sum;
+      }
+
+      /// \brief Returns a const reference to the node sequence of the
+      /// found tour.
+      ///
+      /// This function returns a const reference to a vector
+      /// that stores the node sequence of the found tour.
+      ///
+      /// \pre run() must be called before using this function.
+      const std::vector<Node>& tourNodes() const {
+        return _path;
+      }
+
+      /// \brief Gives back the node sequence of the found tour.
+      ///
+      /// This function copies the node sequence of the found tour into
+      /// an STL container through the given output iterator. The
+      /// <tt>value_type</tt> of the container must be <tt>FullGraph::Node</tt>.
+      /// For example,
+      /// \code
+      /// std::vector<FullGraph::Node> nodes(countNodes(graph));
+      /// tsp.tourNodes(nodes.begin());
+      /// \endcode
+      /// or
+      /// \code
+      /// std::list<FullGraph::Node> nodes;
+      /// tsp.tourNodes(std::back_inserter(nodes));
+      /// \endcode
+      ///
+      /// \pre run() must be called before using this function.
+      template <typename Iterator>
+      void tourNodes(Iterator out) const {
+        std::copy(_path.begin(), _path.end(), out);
+      }
+
+      /// \brief Gives back the found tour as a path.
+      ///
+      /// This function copies the found tour as a list of arcs/edges into
+      /// the given \ref lemon::concepts::Path "path structure".
+      ///
+      /// \pre run() must be called before using this function.
+      template <typename Path>
+      void tour(Path &path) const {
+        path.clear();
+        for (int i = 0; i < int(_path.size()) - 1; ++i) {
+          path.addBack(_gr.arc(_path[i], _path[i+1]));
+        }
+        if (int(_path.size()) >= 2) {
+          path.addBack(_gr.arc(_path.back(), _path.front()));
+        }
+      }
+
+      /// @}
+
+  };
+
+}; // namespace lemon
+
+#endif