On Nov 22, 2005, at 7:23 AM, Aaron Windsor wrote:
On 11/21/05, Doug Gregor <dgregor@cs.indiana.edu> wrote:
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I've only had a few minutes to look over this, so I only have two questions on the code itself:
1) edge_less_than seems more complicated than it needs to be. Instead of creating an integer inside edge_to_index_dispatch, then comparing the integers for two edges to order them, why not just have edge_less_than produce an ordering itself? That would avoid having to store the number of vertices in the edge_less_than predicate.
Do you mean that edge_less_than should be a stateful predicate, creating an ordering as it goes along (first edge it sees is ranked 1, second edge is ranked 2, etc.?) Because this scheme would make lookup in a map an log^2 n operation - at each node in the tree, a rank lookup costing log n needs to be made in order to figure out where to go next. Maybe I'm misunderstanding you?
I believe I was actually thinking of a requirement for operator< on the key type of the auto-index property map. Users have been asking for the ability to compare vertex and edge descriptors with <for a long time, and I believe we can provide it for all of the graph types in the BGL. It would simplify the auto-index property maps a bit, and help users overall. Granted, the problem remains that we essentially need to know whether the less-than operator for the key type is a total order (so we can use std::map) or only a partial order (we need to use std::multimap): perhaps assume it's a partial order, but have a trait is_totally_ordered<Key> that will be specialized to tell us when we can use the std::map.
In the boost-users thread that I linked to in the original post, I suggested that get(vertex_index, g) return an auto index if there was no interior index. You thought this was a bad idea at the time, too misleading for the user, and I tend to agree with your earlier self. I came to like the idea of saying "make_auto_vertex_index" and "make_auto_edge_index" as an acknowledgement of the fact that you, as a user, realize that the algorithm needs an index on the vertices or edges, and realize that you don't have one, but still want the algorithm to work.
My earlier self hadn't received quite as many complaints about the usability of the BGL :( My concern is that we lose people when they try an algorithm and get some big error message. If they ask on the list, we'll just tell them to us make_auto_vertex_index or make_auto_edge_index and everything will work perfectly; but if they don't ask, we'll probably have lost them completely. Granted, I think adjacency_list is a bigger initial problem: if we had an easy-to-use graph type (or two) that had built- in vertex and edge index property maps, users wouldn't see these problems. Then, when they decide to tweak a bit more, they could move to adjacency_list<> and use auto-index property maps where necessary. In any case: as soon as we figure out whether we should use/require operator< on keys to the auto-index property map, please go ahead and integrate your code and documentation into CVS HEAD. Doug