On 11/21/05, Doug Gregor <dgregor@cs.indiana.edu> wrote: <snip>
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?
2) For the auto_index_property_map that allows duplicate keys, was there any particular reason to use a map of vectors instead of a multimap?
No, you're right - multimap is better here. I'll change this and put a new version in the vault.
I also have some higher-level, non-code comments. The primary concern I have is that we're adding more functionality to the BGL to make it easier. It's the same thing we did with bundled properties: add a new, easier-to-use mechanism on top of what we already have. Unfortunately, features interact and I'm not entirely sure that we've managed to make life easier overall.
Auto-indexing maps are a great feature, but overall will they make it easier to learn and use the BGL? They will make some things easier: when users ask "why can't I call this algorithm with my adjacency_list?" we'll have the simple answer of "add edge_index(auto_edge_index_map(g))", followed by the obligatory "if you find that it's too slow, do this other thing" comment.
It seems that the way to make the BGL easier to use would be to make auto-indexing property maps automatic. When a BGL algorithm tries to pull out a vertex_index map, it checks the parameter list, then the graph itself, then falls back to generating an auto-indexing map. This would be convenient, but it also means that there are hidden performance penalties, which we've tried to avoid in the BGL.
What to do?
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.
It's becoming more and more important to make the BGL easier to use out-of-the-box. I even think that most users will understand if at a later point in time they need to do a little work to get their code to give maximum performance, but we need to give them the tools to do so. For instance, I can imagine a macro BOOST_GRAPH_PERFORMANCE_WARNINGS that produces run-time warnings when the library is secretly building an auto-indexing map behind-the-scenes and a macro BOOST_GRAPH_PERFORMANCE_WARNINGS_ARE_ERRORS that causes compile-time errors instead.
Interesting. I'll think some more about this...
But for now, I think once I understand (1) and (2), the auto-indexing property maps should go into CVS HEAD and we can discuss just how automatic we want to make them.
Thanks for taking the time to look at this and give some feedback, Doug. I'll update with the multimap fix, probably after the holidays. Aaron