Question

I am running an astar algorithm on a graph that is partially (?) implicit – it is built from a large paging data source, but the graph is persistent. I need to handle paging in new parts of the graph whenever the astar algorithm gets to an area that is not fully paged in – ideally without starting the astar search over entirely.

I have tried a couple solutions but hit some road blocks, and I’m wondering if I’m missing something obvious or just approaching the problem wrong.

I am currently using boost 1.45 but plan to upgrade to 1.51.

First, I tried to modify the astar visitor such that when it determines it needs to page in new data, it calls a function on the graph and loads it in – however, since the graph is const, this is not possible. I have looked around and found this other question boost implicit graph and astar_search_no_init that references a presentation that suggests someone has done the work to make this possible, but it looks like the actual code is not available.

Second, I thought I might be able to exit the algorithm when I get to a place where I need to page in more data, and save the state of the distance_map, predecessor_map, and color_map so that I could use them to “resume” the search using astar_search_no_init. I am not sure whether this would work, because once I switch over to using astar_search_no_init, I see that while the visitor appears to do the work of pathfinding, the predecessor map is empty – since I am using the predecessor for building the path after the visitor is done, I need to know how the visitor is then building a path.

Here is the definition of my graph and how I am calling astar_search, if that helps at all.

typedef adjacency_list<
     vecS,         
     vecS,        
     undirectedS, 
     VertexInfo,        //contains geographic location     
     EdgeInfo,          //contains weight information             
     no_property,     
     setS>            
        BoostGraph;
...
ColorMap cmap = get(vertex_color_t, myGraph);           
astar_search(
     myGraph, 
     source,
     distance_heuristic(myGraph, destination), //geometric distance heuristic
     predecessor_map(&srcPredmap[0]).
     distance_map(&distMap[0]).
     color_map(cmap).
     visitor(astar_goal_visitor<vertex_descriptor>(destination, this)). //throws an exception when it finds the goal
     weight_map(make_function_property_map<edge_descriptor>(  //copied make_function_property_map functionality from boost 1.51 since I can't upgrade just yet
        EdgeWeightAdjuster(&myGraph, weightFactors))));       //modifies edge weights based on weightFactors construct

Answer 1

You write: “however, since the graph is const, this is not possible…” And what about a simple CAST (event an old C-cast)? You should give this option at least a try. Modifying the graph may invalidate iterators, etc. This is true. Nevertheless you should still try.

There is a difference between “technical const-ness” and “conceptual const-ness”. In your case you will break only the technical, not the conceptual one.