[bluGill]

I did, found the clojure docs. Now I'm even more confused. What is so special about clojure vectors - as opposed to a python list, or C array? Some uses in the example seem like a better match to python dict, a C array of structs (linked list of structs?), perhaps a C++ std::map. Depending on what your goal is I know of dozens of different data structures and algorithms (they are part of the basic things a CS major covers) However clojure was called out specifically as if there is something more that is both important to this discussion and non-trival in some way with the way arrays/lists/vectors are implemented in other languages.

[sesm]

The key idea behind Clojure data structures is that they are immutable, but don't require a full copy on update, because an updated version partly reuses the initial version. The data structure that enables this is called HAMT (hash array mapped trie). Clojure's implementation modified the initial Phil Bagwell's implementation of HAMT in a subtle way in order to make performance characteristics not degrade over time.

[bluGill]

Interesting, it makes more sense now. Though I'm not clear if you want/need that in a video game. In general you want two data structures, one current state of the world and one next state - since they have a lot of similarity it at first makes sense to do what you say, except the you lose the advantage of vector: it is very cache friendly to iterate on a vector, or even two vectors, while it appears (without knowing internal details) that your HAMT is less cache friendly as the next element is not in the next memory location.

Of course I'd need benchmarks to make a real judgement. The above is my gut reaction.