/* -*- mode: C++; indent-tabs-mode: nil; -*-

  *

  * This file is a part of LEMON, a generic C++ optimization library.

  *

  * Copyright (C) 2003-2010

  * 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_OPT2_TSP_H

 #define LEMON_OPT2_TSP_H

 /// \ingroup tsp

 /// \file

 /// \brief 2-opt algorithm for symmetric TSP

 #include <vector>

 #include <lemon/full_graph.h>

 namespace lemon {

   /// \brief 2-opt algorithm for symmetric TSP.

   ///

   /// Opt2Tsp implements the 2-opt heuristic for solving

   /// symmetric \ref tsp "TSP".

   ///

   /// This algorithm starts with an initial tour and iteratively improves it.

   /// At each step, it removes two edges and the reconnects the created two

   /// paths in the other way if the resulting tour is shorter.

   /// The algorithm finishes when no such 2-opt move can be applied, and so

   /// the tour is 2-optimal.

   ///

   /// If no starting tour is given to the \ref run() function, then the

   /// algorithm uses the node sequence determined by the node IDs.

   /// Oherwise, it starts with the given tour.

   ///

   /// This is a relatively slow but powerful method. 

   /// A typical usage of it is the improvement of a solution that is resulted

   /// by a fast tour construction heuristic (e.g. the InsertionTsp algorithm).

   ///

   /// \tparam CM Type of the cost map.

   template <typename CM>

   class Opt2Tsp

   {

     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<int> _plist;

       std::vector<Node> _path;

     public:

       /// \brief Constructor

       ///

       /// Constructor.

       /// \param gr The \ref FullGraph "full graph" the algorithm runs on.

       /// \param cost The cost map.

       Opt2Tsp(const FullGraph &gr, const CostMap &cost)

         : _gr(gr), _cost(cost) {}

       /// \name Execution Control

       /// @{

       /// \brief Runs the algorithm from scratch.

       ///

       /// This function runs the algorithm starting from the tour that is

       /// determined by the node ID sequence.

       ///

       /// \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))];

         }

         _plist.resize(2*_gr.nodeNum());

         for (int i = 1; i < _gr.nodeNum()-1; ++i) {

           _plist[2*i] = i-1;

           _plist[2*i+1] = i+1;

         }

         _plist[0] = _gr.nodeNum()-1;

         _plist[1] = 1;

         _plist[2*_gr.nodeNum()-2] = _gr.nodeNum()-2;

         _plist[2*_gr.nodeNum()-1] = 0;

         return start();

       }

       /// \brief Runs the algorithm from the given tour.

       ///

       /// This function runs the algorithm starting from the given tour.

       ///

       /// \param tour The tour as a path structure. It must be a

       /// \ref checkPath() "valid path" containing excactly n arcs.

       ///

       /// \return The total cost of the found tour.

       template <typename Path>

       Cost run(const Path& tour) {

         _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))];

         }

         _plist.resize(2*_gr.nodeNum());

         typename Path::ArcIt it(tour);

         int first = _gr.id(_gr.source(it)),

             prev = first,

             curr = _gr.id(_gr.target(it)),

             next = -1;

         _plist[2*first+1] = curr;

         for (++it; it != INVALID; ++it) {

           next = _gr.id(_gr.target(it));

           _plist[2*curr] = prev;

           _plist[2*curr+1] = next;

           prev = curr;

           curr = next;

         }

         _plist[2*first] = prev;

         return start();

       }

       /// \brief Runs the algorithm from the given tour.

       ///

       /// This function runs the algorithm starting from the given tour.

       ///

       /// \param tour The tour as a node sequence. It must be a standard

       /// sequence container storing all <tt>Node</tt>s in the desired order.

       ///

       /// \return The total cost of the found tour.

       template <template <typename> class Container>

       Cost run(const Container<Node>& tour) {

         _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))];

         }

         _plist.resize(2*_gr.nodeNum());

         typename Container<Node>::const_iterator it = tour.begin();

         int first = _gr.id(*it),

             prev = first,

             curr = _gr.id(*(++it)),

             next = -1;

         _plist[2*first+1] = curr;

         for (++it; it != tour.end(); ++it) {

           next = _gr.id(*it);

           _plist[2*curr] = prev;

           _plist[2*curr+1] = next;

           prev = curr;

           curr = next;

         }

         _plist[2*first] = curr;

         _plist[2*curr] = prev;

         _plist[2*curr+1] = first;

         return start();

       }

       /// @}

       /// \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 the internal structure

       /// 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

       /// the given standard container.

       ///

       /// \pre run() must be called before using this function.

       template <typename Container>

       void tourNodes(Container &container) const {

         container.assign(_path.begin(), _path.end());

       }

       /// \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 concept::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()));

         }

       }

       /// @}

     private:

       // Iterator class for the linked list storage of the tour

       class PathListIt {

         public:

           PathListIt(const std::vector<int> &pl, int i=0)

             : plist(&pl), act(i), last(pl[2*act]) {}

           PathListIt(const std::vector<int> &pl, int i, int l)

             : plist(&pl), act(i), last(l) {}

           int nextIndex() const {

             return (*plist)[2*act] == last ? 2*act+1 : 2*act;

           }

           int prevIndex() const {

             return (*plist)[2*act] == last ? 2*act : 2*act+1;

           }

           int next() const {

             int x = (*plist)[2*act];

             return x == last ? (*plist)[2*act+1] : x;

           }

           int prev() const {

             return last;

           }

           PathListIt& operator++() {

             int tmp = act;

             act = next();

             last = tmp;

             return *this;

           }

           operator int() const {

             return act;

           }

         private:

           const std::vector<int> *plist;

           int act;

           int last;

       };

       // Checks and applies 2-opt move (if it improves the tour)

       bool checkOpt2(const PathListIt& i, const PathListIt& j) {

         Node u  = _gr.nodeFromId(i),

              un = _gr.nodeFromId(i.next()),

              v  = _gr.nodeFromId(j),

              vn = _gr.nodeFromId(j.next());

         if (_cost[_gr.edge(u, un)] + _cost[_gr.edge(v, vn)] >

             _cost[_gr.edge(u, v)] + _cost[_gr.edge(un, vn)])

         {

           _plist[PathListIt(_plist, i.next(), i).prevIndex()] = j.next();

           _plist[PathListIt(_plist, j.next(), j).prevIndex()] = i.next();

           _plist[i.nextIndex()] = j;

           _plist[j.nextIndex()] = i;

           return true;

         }

         return false;

      }

       // Executes the algorithm from the initial tour

       Cost start() {

       restart_search:

         for (PathListIt i(_plist); true; ++i) {

           PathListIt j = i;

           if (++j == 0 || ++j == 0) break;

           for (; j != 0 && j != i.prev(); ++j) {

             if (checkOpt2(i, j))

               goto restart_search;

           }

         }

         PathListIt i(_plist);

         _path.push_back(_gr.nodeFromId(i));

         for (++i; i != 0; ++i)

           _path.push_back(_gr.nodeFromId(i));

         _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;

       }

   };

 }; // namespace lemon

 #endif