source: lemon/lemon/greedy_tsp.h @ 1270:dceba191c00d

/* -*- 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_GREEDY_TSP_H
#define LEMON_GREEDY_TSP_H

/// \ingroup tsp
/// \file
/// \brief Greedy algorithm for symmetric TSP

#include <vector>
#include <algorithm>
#include <lemon/full_graph.h>
#include <lemon/unionfind.h>

namespace lemon {

  /// \ingroup tsp
  ///
  /// \brief Greedy algorithm for symmetric TSP.
  ///
  /// GreedyTsp implements the greedy heuristic for solving
  /// symmetric \ref tsp "TSP".
  ///
  /// This algorithm is quite similar to the \ref NearestNeighborTsp
  /// "nearest neighbor" heuristic, but it maintains a set of disjoint paths.
  /// At each step, the shortest possible edge is added to these paths
  /// as long as it does not create a cycle of less than n edges and it does
  /// not increase the degree of any node above two.
  ///
  /// This method runs in O(n<sup>2</sup>) time.
  /// It quickly finds a relatively short tour for most TSP instances,
  /// but it could also yield a really bad (or even the worst) solution
  /// in special cases.
  ///
  /// \tparam CM Type of the cost map.
  template <typename CM>
  class GreedyTsp
  {
    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;
      Cost _sum;
      std::vector<Node> _path;

    private:

      // Functor class to compare edges by their costs
      class EdgeComp {
      private:
        const CostMap &_cost;

      public:
        EdgeComp(const CostMap &cost) : _cost(cost) {}

        bool operator()(const Edge &a, const Edge &b) const {
          return _cost[a] < _cost[b];
        }
      };

    public:

      /// \brief Constructor
      ///
      /// Constructor.
      /// \param gr The \ref FullGraph "full graph" the algorithm runs on.
      /// \param cost The cost map.
      GreedyTsp(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;
        }

        std::vector<int> plist;
        plist.resize(_gr.nodeNum()*2, -1);

        std::vector<Edge> sorted_edges;
        sorted_edges.reserve(_gr.edgeNum());
        for (EdgeIt e(_gr); e != INVALID; ++e)
          sorted_edges.push_back(e);
        std::sort(sorted_edges.begin(), sorted_edges.end(), EdgeComp(_cost));

        FullGraph::NodeMap<int> item_int_map(_gr);
        UnionFind<FullGraph::NodeMap<int> > union_find(item_int_map);
        for (NodeIt n(_gr); n != INVALID; ++n)
          union_find.insert(n);

        FullGraph::NodeMap<int> degree(_gr, 0);

        int nodesNum = 0, i = 0;
        while (nodesNum != _gr.nodeNum()-1) {
          Edge e = sorted_edges[i++];
          Node u = _gr.u(e),
               v = _gr.v(e);

          if (degree[u] <= 1 && degree[v] <= 1) {
            if (union_find.join(u, v)) {
              const int uid = _gr.id(u),
                        vid = _gr.id(v);

              plist[uid*2 + degree[u]] = vid;
              plist[vid*2 + degree[v]] = uid;

              ++degree[u];
              ++degree[v];
              ++nodesNum;
            }
          }
        }

        for (int i=0, n=-1; i<_gr.nodeNum()*2; ++i) {
          if (plist[i] == -1) {
            if (n==-1) {
              n = i;
            } else {
              plist[n] = i/2;
              plist[i] = n/2;
              break;
            }
          }
        }

        for (int i=0, next=0, last=-1; i!=_gr.nodeNum(); ++i) {
          _path.push_back(_gr.nodeFromId(next));
          if (plist[2*next] != last) {
            last = next;
            next = plist[2*next];
          } else {
            last = next;
            next = plist[2*next+1];
          }
        }

        _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