/* -*- C++ -*-

 *

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

 *

 * Copyright (C) 2003-2008

 * 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_MIN_MEAN_CYCLE_H

#define LEMON_MIN_MEAN_CYCLE_H

/// \ingroup shortest_path

///

/// \file

/// \brief Howard's algorithm for finding a minimum mean cycle.

#include <vector>

#include <lemon/core.h>

#include <lemon/path.h>

#include <lemon/tolerance.h>

#include <lemon/connectivity.h>

namespace lemon {

  /// \addtogroup shortest_path

  /// @{

  /// \brief Implementation of Howard's algorithm for finding a minimum

  /// mean cycle.

  ///

  /// \ref MinMeanCycle implements Howard's algorithm for finding a

  /// directed cycle of minimum mean length (cost) in a digraph.

  ///

  /// \tparam GR The type of the digraph the algorithm runs on.

  /// \tparam LEN The type of the length map. The default

  /// map type is \ref concepts::Digraph::ArcMap "GR::ArcMap<int>".

  ///

  /// \warning \c LEN::Value must be convertible to \c double.

#ifdef DOXYGEN

  template <typename GR, typename LEN>

#else

  template < typename GR,

             typename LEN = typename GR::template ArcMap<int> >

#endif

  class MinMeanCycle

  {

  public:

    /// The type of the digraph the algorithm runs on

    typedef GR Digraph;

    /// The type of the length map

    typedef LEN LengthMap;

    /// The type of the arc lengths

    typedef typename LengthMap::Value Value;

    /// The type of the paths

    typedef lemon::Path<Digraph> Path;

  private:

    TEMPLATE_DIGRAPH_TYPEDEFS(Digraph);

    // The digraph the algorithm runs on

    const Digraph &_gr;

    // The length of the arcs

    const LengthMap &_length;

    // Data for the found cycles

    bool _curr_found, _best_found;

    Value _curr_length, _best_length;

    int _curr_size, _best_size;

    Node _curr_node, _best_node;

    Path *_cycle_path;

    bool _local_path;

    // Internal data used by the algorithm

    typename Digraph::template NodeMap<Arc> _policy;

    typename Digraph::template NodeMap<bool> _reached;

    typename Digraph::template NodeMap<int> _level;

    typename Digraph::template NodeMap<double> _dist;

    // Data for storing the strongly connected components

    int _comp_num;

    typename Digraph::template NodeMap<int> _comp;

    std::vector<std::vector<Node> > _comp_nodes;

    std::vector<Node>* _nodes;

    typename Digraph::template NodeMap<std::vector<Arc> > _in_arcs;

    // Queue used for BFS search

    std::vector<Node> _queue;

    int _qfront, _qback;

    Tolerance<double> _tol;

  public:

    /// \brief Constructor.

    ///

    /// The constructor of the class.

    ///

    /// \param digraph The digraph the algorithm runs on.

    /// \param length The lengths (costs) of the arcs.

    MinMeanCycle( const Digraph &digraph,

                  const LengthMap &length ) :

      _gr(digraph), _length(length), _cycle_path(NULL), _local_path(false),

      _policy(digraph), _reached(digraph), _level(digraph), _dist(digraph),

      _comp(digraph), _in_arcs(digraph)

    {}

    /// Destructor.

    ~MinMeanCycle() {

      if (_local_path) delete _cycle_path;

    }

    /// \brief Set the path structure for storing the found cycle.

    ///

    /// This function sets an external path structure for storing the

    /// found cycle.

    ///

    /// If you don't call this function before calling \ref run() or

    /// \ref findMinMean(), it will allocate a local \ref Path "path"

    /// structure. The destuctor deallocates this automatically

    /// allocated object, of course.

    ///

    /// \note The algorithm calls only the \ref lemon::Path::addBack()

    /// "addBack()" function of the given path structure.

    ///

    /// \return <tt>(*this)</tt>

    ///

    /// \sa cycle()

    MinMeanCycle& cyclePath(Path &path) {

      if (_local_path) {

        delete _cycle_path;

        _local_path = false;

      }

      _cycle_path = &path;

      return *this;

    }

    /// \name Execution control

    /// The simplest way to execute the algorithm is to call the \ref run()

    /// function.\n

    /// If you only need the minimum mean length, you may call

    /// \ref findMinMean().

    /// @{

    /// \brief Run the algorithm.

    ///

    /// This function runs the algorithm.

    /// It can be called more than once (e.g. if the underlying digraph

    /// and/or the arc lengths have been modified).

    ///

    /// \return \c true if a directed cycle exists in the digraph.

    ///

    /// \note <tt>mmc.run()</tt> is just a shortcut of the following code.

    /// \code

    ///   return mmc.findMinMean() && mmc.findCycle();

    /// \endcode

    bool run() {

      return findMinMean() && findCycle();

    }

    /// \brief Find the minimum cycle mean.

    ///

    /// This function finds the minimum mean length of the directed

    /// cycles in the digraph.

    ///

    /// \return \c true if a directed cycle exists in the digraph.

    bool findMinMean() {

      // Initialize and find strongly connected components

      init();

      findComponents();

      // Find the minimum cycle mean in the components

      for (int comp = 0; comp < _comp_num; ++comp) {

        // Find the minimum mean cycle in the current component

        if (!buildPolicyGraph(comp)) continue;

        while (true) {

          findPolicyCycle();

          if (!computeNodeDistances()) break;

        }

        // Update the best cycle (global minimum mean cycle)

        if ( !_best_found || (_curr_found &&

             _curr_length * _best_size < _best_length * _curr_size) ) {

          _best_found = true;

          _best_length = _curr_length;

          _best_size = _curr_size;

          _best_node = _curr_node;

        }

      }

      return _best_found;

    }

    /// \brief Find a minimum mean directed cycle.

    ///

    /// This function finds a directed cycle of minimum mean length

    /// in the digraph using the data computed by findMinMean().

    ///

    /// \return \c true if a directed cycle exists in the digraph.

    ///

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

    bool findCycle() {

      if (!_best_found) return false;

      _cycle_path->addBack(_policy[_best_node]);

      for ( Node v = _best_node;

            (v = _gr.target(_policy[v])) != _best_node; ) {

        _cycle_path->addBack(_policy[v]);

      }

      return true;

    }

    /// @}

    /// \name Query Functions

    /// The results of the algorithm can be obtained using these

    /// functions.\n

    /// The algorithm should be executed before using them.

    /// @{

    /// \brief Return the total length of the found cycle.

    ///

    /// This function returns the total length of the found cycle.

    ///

    /// \pre \ref run() or \ref findMinMean() must be called before

    /// using this function.

    Value cycleLength() const {

      return _best_length;

    }

    /// \brief Return the number of arcs on the found cycle.

    ///

    /// This function returns the number of arcs on the found cycle.

    ///

    /// \pre \ref run() or \ref findMinMean() must be called before

    /// using this function.

    int cycleArcNum() const {

      return _best_size;

    }

    /// \brief Return the mean length of the found cycle.

    ///

    /// This function returns the mean length of the found cycle.

    ///

    /// \note <tt>alg.cycleMean()</tt> is just a shortcut of the

    /// following code.

    /// \code

    ///   return static_cast<double>(alg.cycleLength()) / alg.cycleArcNum();

    /// \endcode

    ///

    /// \pre \ref run() or \ref findMinMean() must be called before

    /// using this function.

    double cycleMean() const {

      return static_cast<double>(_best_length) / _best_size;

    }

    /// \brief Return the found cycle.

    ///

    /// This function returns a const reference to the path structure

    /// storing the found cycle.

    ///

    /// \pre \ref run() or \ref findCycle() must be called before using

    /// this function.

    ///

    /// \sa cyclePath()

    const Path& cycle() const {

      return *_cycle_path;

    }

    ///@}

  private:

    // Initialize

    void init() {

      _tol.epsilon(1e-6);

      if (!_cycle_path) {

        _local_path = true;

        _cycle_path = new Path;

      }

      _queue.resize(countNodes(_gr));

      _best_found = false;

      _best_length = 0;

      _best_size = 1;

      _cycle_path->clear();

    }

    // Find strongly connected components and initialize _comp_nodes

    // and _in_arcs

    void findComponents() {

      _comp_num = stronglyConnectedComponents(_gr, _comp);

      _comp_nodes.resize(_comp_num);

      if (_comp_num == 1) {

        _comp_nodes[0].clear();

        for (NodeIt n(_gr); n != INVALID; ++n) {

          _comp_nodes[0].push_back(n);

          _in_arcs[n].clear();

          for (InArcIt a(_gr, n); a != INVALID; ++a) {

            _in_arcs[n].push_back(a);

          }

        }

      } else {

        for (int i = 0; i < _comp_num; ++i)

          _comp_nodes[i].clear();

        for (NodeIt n(_gr); n != INVALID; ++n) {

          int k = _comp[n];

          _comp_nodes[k].push_back(n);

          _in_arcs[n].clear();

          for (InArcIt a(_gr, n); a != INVALID; ++a) {

            if (_comp[_gr.source(a)] == k) _in_arcs[n].push_back(a);

          }

        }

      }

    }

    // Build the policy graph in the given strongly connected component

    // (the out-degree of every node is 1)

    bool buildPolicyGraph(int comp) {

      _nodes = &(_comp_nodes[comp]);

      if (_nodes->size() < 1 ||

          (_nodes->size() == 1 && _in_arcs[(*_nodes)[0]].size() == 0)) {

        return false;

      }

      for (int i = 0; i < int(_nodes->size()); ++i) {

        _dist[(*_nodes)[i]] = std::numeric_limits<double>::max();

      }

      Node u, v;

      Arc e;

      for (int i = 0; i < int(_nodes->size()); ++i) {

        v = (*_nodes)[i];

        for (int j = 0; j < int(_in_arcs[v].size()); ++j) {

          e = _in_arcs[v][j];

          u = _gr.source(e);

          if (_length[e] < _dist[u]) {

            _dist[u] = _length[e];

            _policy[u] = e;

          }

        }

      }

      return true;

    }

    // Find the minimum mean cycle in the policy graph

    void findPolicyCycle() {

      for (int i = 0; i < int(_nodes->size()); ++i) {

        _level[(*_nodes)[i]] = -1;

      }

      Value clength;

      int csize;

      Node u, v;

      _curr_found = false;

      for (int i = 0; i < int(_nodes->size()); ++i) {

        u = (*_nodes)[i];

        if (_level[u] >= 0) continue;

        for (; _level[u] < 0; u = _gr.target(_policy[u])) {

          _level[u] = i;

        }

        if (_level[u] == i) {

          // A cycle is found

          clength = _length[_policy[u]];

          csize = 1;

          for (v = u; (v = _gr.target(_policy[v])) != u; ) {

            clength += _length[_policy[v]];

            ++csize;

          }

          if ( !_curr_found ||

               (clength * _curr_size < _curr_length * csize) ) {

            _curr_found = true;

            _curr_length = clength;

            _curr_size = csize;

            _curr_node = u;

          }

        }

      }

    }

    // Contract the policy graph and compute node distances

    bool computeNodeDistances() {

      // Find the component of the main cycle and compute node distances

      // using reverse BFS

      for (int i = 0; i < int(_nodes->size()); ++i) {

        _reached[(*_nodes)[i]] = false;

      }

      double curr_mean = double(_curr_length) / _curr_size;

      _qfront = _qback = 0;

      _queue[0] = _curr_node;

      _reached[_curr_node] = true;

      _dist[_curr_node] = 0;

      Node u, v;

      Arc e;

      while (_qfront <= _qback) {

        v = _queue[_qfront++];

        for (int j = 0; j < int(_in_arcs[v].size()); ++j) {

          e = _in_arcs[v][j];

          u = _gr.source(e);

          if (_policy[u] == e && !_reached[u]) {

            _reached[u] = true;

            _dist[u] = _dist[v] + _length[e] - curr_mean;

            _queue[++_qback] = u;

          }

        }

      }

      // Connect all other nodes to this component and compute node

      // distances using reverse BFS

      _qfront = 0;

      while (_qback < int(_nodes->size())-1) {

        v = _queue[_qfront++];

        for (int j = 0; j < int(_in_arcs[v].size()); ++j) {

          e = _in_arcs[v][j];

          u = _gr.source(e);

          if (!_reached[u]) {

            _reached[u] = true;

            _policy[u] = e;

            _dist[u] = _dist[v] + _length[e] - curr_mean;

            _queue[++_qback] = u;

          }

        }

      }

      // Improve node distances

      bool improved = false;

      for (int i = 0; i < int(_nodes->size()); ++i) {

        v = (*_nodes)[i];

        for (int j = 0; j < int(_in_arcs[v].size()); ++j) {

          e = _in_arcs[v][j];

          u = _gr.source(e);

          double delta = _dist[v] + _length[e] - curr_mean;

          if (_tol.less(delta, _dist[u])) {

            _dist[u] = delta;

            _policy[u] = e;

            improved = true;

          }

        }

      }

      return improved;

    }

  }; //class MinMeanCycle

  ///@}

} //namespace lemon

#endif //LEMON_MIN_MEAN_CYCLE_H