/* -*- 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_NEAREST_NEIGHBOUR_TSP_H
#define LEMON_NEAREST_NEIGHBOUR_TSP_H

/// \ingroup tsp
/// \file
/// \brief Nearest neighbor algorithm for symmetric TSP

#include <deque>
#include <lemon/full_graph.h>
#include <lemon/maps.h>
#include <limits>
#include <vector>

namespace lemon {

/// \ingroup tsp
///
/// \brief Nearest neighbor algorithm for symmetric TSP.
///
/// NearestNeighborTsp implements the nearest neighbor heuristic for solving
/// symmetric \ref tsp "TSP".
///
/// This is probably the simplest TSP heuristic.
/// It starts with a minimum cost edge and at each step, it connects the
/// nearest unvisited node to the current path.
/// Finally, it connects the two end points of the path to form a tour.
///
/// 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 NearestNeighborTsp {
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;

public:
  /// \brief Constructor
  ///
  /// Constructor.
  /// \param gr The \ref FullGraph "full graph" the algorithm runs on.
  /// \param cost The cost map.
  NearestNeighborTsp(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::deque<Node> path_dq;
    Edge min_edge1 = INVALID, min_edge2 = INVALID;

    min_edge1 = mapMin(_gr, _cost);
    Node n1 = _gr.u(min_edge1), n2 = _gr.v(min_edge1);
    path_dq.push_back(n1);
    path_dq.push_back(n2);

    FullGraph::NodeMap<bool> used(_gr, false);
    used[n1] = true;
    used[n2] = true;

    min_edge1 = INVALID;
    while (int(path_dq.size()) != _gr.nodeNum()) {
      if (min_edge1 == INVALID) {
        for (IncEdgeIt e(_gr, n1); e != INVALID; ++e) {
          if (!used[_gr.runningNode(e)] &&
              (min_edge1 == INVALID || _cost[e] < _cost[min_edge1])) {
            min_edge1 = e;
          }
        }
      }

      if (min_edge2 == INVALID) {
        for (IncEdgeIt e(_gr, n2); e != INVALID; ++e) {
          if (!used[_gr.runningNode(e)] &&
              (min_edge2 == INVALID || _cost[e] < _cost[min_edge2])) {
            min_edge2 = e;
          }
        }
      }

      if (_cost[min_edge1] < _cost[min_edge2]) {
        n1 = _gr.oppositeNode(n1, min_edge1);
        path_dq.push_front(n1);

        used[n1] = true;
        min_edge1 = INVALID;

        if (_gr.u(min_edge2) == n1 || _gr.v(min_edge2) == n1)
          min_edge2 = INVALID;
      } else {
        n2 = _gr.oppositeNode(n2, min_edge2);
        path_dq.push_back(n2);

        used[n2] = true;
        min_edge2 = INVALID;

        if (_gr.u(min_edge1) == n2 || _gr.v(min_edge1) == n2)
          min_edge1 = INVALID;
      }
    }

    n1 = path_dq.back();
    n2 = path_dq.front();
    _path.push_back(n2);
    _sum = _cost[_gr.edge(n1, n2)];
    for (int i = 1; i < int(path_dq.size()); ++i) {
      n1 = n2;
      n2 = path_dq[i];
      _path.push_back(n2);
      _sum += _cost[_gr.edge(n1, n2)];
    }

    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
