source: lemon-0.x/src/work/athos/minlengthpaths.h @ 389:770cc1f4861f

// -*- c++ -*-
#ifndef HUGO_MINLENGTHPATHS_H
#define HUGO_MINLENGTHPATHS_H

///\file
///\brief An algorithm for finding k paths of minimal total length.

#include <iostream>
#include <dijkstra.h>
#include <graph_wrapper.h>
#include <maps.h>
#include <vector>


namespace hugo {



  ///\brief Implementation of an algorithm for finding k paths between 2 nodes 
  /// of minimal total length 
  ///
  /// The class \ref hugo::MinLengthPaths "MinLengthPaths" implements
  /// an algorithm which finds k edge-disjoint paths
  /// from a given source node to a given target node in an
  /// edge-weighted directed graph having minimal total weigth (length).

  template <typename Graph, typename LengthMap>
  class MinLengthPaths {

    typedef typename LengthMap::ValueType Length;

    typedef typename Graph::Node Node;
    typedef typename Graph::NodeIt NodeIt;
    typedef typename Graph::Edge Edge;
    typedef typename Graph::OutEdgeIt OutEdgeIt;
    typedef typename Graph::EdgeMap<int> EdgeIntMap;

    typedef ConstMap<Edge,int> ConstMap;

    typedef ResGraphWrapper<const Graph,int,ConstMap,EdgeIntMap> ResGraphType;


    class ModLengthMap {   
      typedef typename ResGraphType::NodeMap<Length> NodeMap;
      const ResGraphType& G;
      const EdgeIntMap& rev;
      const LengthMap &ol;
      const NodeMap &pot;
    public :
      typedef typename LengthMap::KeyType KeyType;
      typedef typename LengthMap::ValueType ValueType;

      ValueType operator[](typename ResGraphType::Edge e) const {     
        //if ( (1-2*rev[e])*ol[e]-(pot[G.head(e)]-pot[G.tail(e)] ) <0 ){
        //  std::cout<<"Negative length!!"<<std::endl;
        //}
        return (1-2*rev[e])*ol[e]-(pot[G.head(e)]-pot[G.tail(e)]);   
      }     

      ModLengthMap(const ResGraphType& _G, const EdgeIntMap& _rev, 
                   const LengthMap &o,  const NodeMap &p) : 
        G(_G), rev(_rev), ol(o), pot(p){}; 
    };
    

    const Graph& G;
    const LengthMap& length;

    //auxiliary variables

    //The value is 1 iff the edge is reversed. 
    //If the algorithm has finished, the edges of the seeked paths are 
    //exactly those that are reversed 
    EdgeIntMap reversed; 
    
    //Container to store found paths
    std::vector< std::vector<Edge> > paths;

  public :


    MinLengthPaths(Graph& _G, LengthMap& _length) : G(_G), 
      length(_length), reversed(_G)/*, dijkstra_dist(_G)*/{ }

    
    ///Runs the algorithm.

    ///Runs the algorithm.
    ///Returns k if there are at least k edge-disjoint paths from s to t.
    ///Otherwise it returns the number of found edge-disjoint paths from s to t.
    int run(Node s, Node t, int k) {
      ConstMap const1map(1);

      //We need a residual graph, in which some of the edges are reversed
      ResGraphType res_graph(G, const1map, reversed);

      //Initialize the copy of the Dijkstra potential to zero
      typename ResGraphType::NodeMap<Length> dijkstra_dist(res_graph);
      ModLengthMap mod_length(res_graph, reversed, length, dijkstra_dist);

      Dijkstra<ResGraphType, ModLengthMap> dijkstra(res_graph, mod_length);

      int i;
      for (i=0; i<k; ++i){
        dijkstra.run(s);
        if (!dijkstra.reached(t)){
          //There are no k paths from s to t
          break;
        };
        
        {
          //We have to copy the potential
          typename ResGraphType::NodeIt n;
          for ( res_graph.first(n) ; res_graph.valid(n) ; res_graph.next(n) ) {
              dijkstra_dist[n] += dijkstra.distMap()[n];
          }
        }


        //Reversing the sortest path
        Node n=t;
        Edge e;
        while (n!=s){
          e = dijkstra.pred(n);
          n = dijkstra.predNode(n);
          reversed[e] = 1-reversed[e];
        }

          
      }
      
      //Let's find the paths
      //We put the paths into vectors (just for now). In the meantime we lose 
      //the information stored in 'reversed'
      //We suppose the lengths to be positive now.
      paths.clear();
      paths.resize(k);
      for (int j=0; j<i; ++j){
        Node n=s;
        OutEdgeIt e;

        while (n!=t){


          G.first(e,n);
          
          while (!reversed[e]){
            G.next(e);
          }
          n = G.head(e);
          paths[j].push_back(e);
          reversed[e] = 1-reversed[e];
        }
        
      }

      return i;
    }


  }; //class MinLengthPaths


} //namespace hugo

#endif //HUGO_MINLENGTHPATHS_H