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PONegative edge weight check in boost's dijkstra_shortest_paths
 

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  1. PONegative edge weight check in boost's dijkstra_shortest_paths
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    <p>I am using the boost graph library to make a call to <code>dijkstra_shortest_paths</code>. However, I have something special setup in that the <code>weight_map</code> is actually a functor. Hence, everytime the boost library requires the weight of an edge, my functor is called, makes a complicated computation and delivers the result back to boost.</p> <p>Unfortunately, in <code>dijkstra_shortest_paths.hpp</code> the struct <code>dijkstra_bfs_visitor</code>'s method <code>examine_edge</code> has a <code>get</code> call to the weightmap, only to check if the returned value is negative. I am fully aware that I cannot use Dijkstra's algorithm with negative values and I am certain that my functor only returns positive values. However, this check causes my functor to be called twice for each edge. As it performs a complicated computation I'd like to avoid performing it twice (the results don't change between calls.. each edge gets the same wait during a <code>dijkstra_shortest_paths</code> run).</p> <p>So far, I am manually checking the edge passed to the functor and in case of the duplicate call I am returning the previous remembered result. This clearly is more of a workaround than a solution.</p> <p>I tried to pass my own visitor that overwrites <code>examine_edge</code>, however, the original method defined by boost's <code>dijkstra_bfs_visitor</code> is still applied.</p> <p>Does anyone know if there is a better way to handle this situation and somehow avoid the negate edge weight check?</p>
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    1. USFrank
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    1. COif it's expensive, caching this edge weight calculation seems like a good idea anyways. other than that...maybe you can provide a combine functor that always returns false?
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      1. USfearlesstost
    2. COI do cache it already, of course. I just don't like the functor being called twice for no reason. The combine functor should return a distance value, so what do you mean with returning false?
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      1. USFrank
    3. COCan you derive from the `struct DijkstraVisitorConcept` and overload your own `void constraints()` function, where you can remove the `examine_edge()` check altogether.
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