CPP Boost Visitor Multi Target

Question

I have the following problem. I would like to use the DIJKSTRA algorithm from boost. Furthermore, I would like to stop the search when I hit a certain target node. My first implementation works fine for a single target node:

vertex_descriptor src = vertex(start_vertex, g); //start_node
vertex_descriptor targ = vertex(end_vertex, g); //end_node
try {
dijkstra_shortest_paths(g, src,predecessor_map(&p[0]).distance_map(&d[0]).visitor(target_visit(targ,on_examine_vertex())));
}
catch( ... ) {}

with following visitor:

template <class Vertex, class Tag>struct target_visitor : public default_dijkstra_visitor{
target_visitor(Vertex u) : v(u) { }
template <class Graph>
void examine_vertex(Vertex u, Graph& g)
{
    if( u == v ) {
        throw(-1);
    }
}
private:
    Vertex v;
};
template <class Vertex, class Tag>
target_visitor<Vertex, Tag>target_visit(Vertex u, Tag) {
return target_visitor<Vertex, Tag>(u);
}

At the moment I can only treat single end nodes (vertex_descriptor targ). I would like to change my code that a vector of end nodes is allowed. Then the visitor should stop if one of the end_nodes is reached. Can someone help me to modify this? Everytime I tried to change the type of targ into something like vector I get a problem with the vistor template?

Regards, Chris

Answer 1

Instead of holding a single target Vertex make target_visitor hold a std::set<Vertex> and check if the Vertex is within that set.

I modified the code from http://lists.boost.org/boost-users/2006/03/18043.php to demonstrate this:

#include <iostream>

#include <boost/graph/graph_traits.hpp>
#include <boost/graph/adjacency_list.hpp>
#include <boost/graph/dijkstra_shortest_paths.hpp>

#include <set>

using namespace boost;

template <class Vertex, class Tag>
struct target_visitor : public default_dijkstra_visitor
{
    target_visitor(const std::set<Vertex>& targets) : targets(targets) { }

    template <class Graph>
    void examine_vertex(Vertex v, Graph& g)
    {
        if(targets.find(v) != targets.end())
        {
            std::cout << "found target: " << v << std::endl;
            throw(-1);
        }
    }
private:
    std::set<Vertex> targets;
};

template <class Vertex, class Tag>
target_visitor<Vertex, Tag>
target_visit(const std::set<Vertex>& targets, Tag)
{
    return target_visitor<Vertex, Tag>(targets);
}


int main(int argc, char** argv)
{
    typedef adjacency_list < listS, vecS, directedS,
            no_property, property < edge_weight_t, int > > graph_t;
    typedef graph_traits < graph_t >::vertex_descriptor vertex_descriptor;
    typedef graph_traits < graph_t >::edge_descriptor edge_descriptor;
    typedef std::pair<int, int> Edge;

    const int num_nodes = 5;
    enum nodes { A, B, C, D, E };
    char name[] = "ABCDE";
    Edge edge_array[] =
    {
        Edge(A, C), Edge(B, B), Edge(B, D), Edge(B, E),
        Edge(C, B), Edge(C, D), Edge(D, E), Edge(E, A), Edge(E, B)
    };
    int weights[] = { 1, 2, 1, 2, 7, 3, 1, 1, 1 };
    int num_arcs = sizeof(edge_array) / sizeof(Edge);

    graph_t g(edge_array, edge_array + num_arcs, weights, num_nodes);
    property_map<graph_t, edge_weight_t>::type weightmap = get(edge_weight, g);

    std::vector<vertex_descriptor> p(num_vertices(g));
    std::vector<int> d(num_vertices(g));

    vertex_descriptor src = vertex(A, g);
    std::set<vertex_descriptor> targets = {vertex(E, g), vertex(D, g)};

    try {
    dijkstra_shortest_paths(g, src,
                            predecessor_map(&p[0]).distance_map(&d[0]).visitor(target_visit(targets,
                                                                                            on_examine_vertex())));
    }
    catch( ... ) {
    }

    std::cout << "distances and parents:" << std::endl;
    graph_traits < graph_t >::vertex_iterator vi, vend;
    for (tie(vi, vend) = vertices(g); vi != vend; ++vi)
    {
        std::cout << "distance(" << name[*vi] << ") = " << d[*vi] << ", ";
        std::cout << "parent(" << name[*vi] << ") = " << name[p[*vi]] << std::endl; 
    }
    return 0;
}