Hello Janek, Janek Kozicki <janek_listy <at> wp.pl> writes:
Since I can't stop thinking about that, I'll try to describe the most difficult data structure that I have here. Maybe you will have some suggestions. And maybe it will help me to sort the thing out.
In my program I am calculating bodies collisions and other kind of interactions. This is a physical simualtion stuff.
I have a container (an easier one) with bodies where each body has an integer ID number. And the bodies can interact together, there are various kinds of interaction. And I need to store those interactions in a container. The keys are:
- interaction type: each polymorphic interaction type stored in this container is a different kind of interaction. Sometimes I need to interate over all interactions of selected type, thus it becomes one of the keys. This is an integer.
- a set of interacting bodies: any number of bodies can interact. Two or more bodies, but very rarely as much as four. - Sometimes I need to iterate over all interactions which involve a selected body or, - check if an interaction of given type between bodies 1, 2 and 3 already exists, then get it, or add it, if it didn't exist. This is a list of integer values (IDs of bodies). [...] One solution (a bit of pseudocode) could be:
multi_index_container< interaction*, indexed_by< ordered_non_unique<member<interaction*,int,&interaction::type> >
ordered_unique <member<interaction*,std::set<int>,&interaction::bodies> > // non_unique in fact. But uniqe with respect to &interaction::type. // That's why it's pseudocode
;
But how to make this and get a reasonable performance?
Well, your proposed structure has reasonable performance and complies with all your requirements except the ability to iterate on interactions involving a given body. In order to get this extra feature, you might need to set up an extra container like std::[unordered_]multimap<int /* body ID */, interaction*> which is kept in sync with the multi-index container: when an interaction is inserted, the multimap must be fed as many entries as bodies the given interaction involves. I can't think of a more elegant solution than this, which is a pity since one of the key goals of B.MI is to elminate the need to manually maintain this kind of composite structures :( [...]
Performance priorities (from most important, to least important): A)iterating over all elements with the same &interaction::type must be constant access time.
B)finding if there is an interaction between (1,2,3) - great if it was constant time (N-dimensional matrix gives that), I guess that log(O) is the best we can have. C)deleting and adding an interaction, again I guess that constant time is not possible. D)iterating over all interactions involving a body with given ID is used less often so can be a bit slower.
If you use hashed indices instead of ordered ones and also use an unordered_multimap for the additional data structure, you can get the performance you aim for, constant time for A), B), C) and D). HTH, Joaquín M López Muñoz Telefónica, Investigación y Desarrollo