[contravariant]

All excellent points but no matter how impressive the savings it's not going to be interesting if the total time spent checking for membership is low to begin with. You'd need to be optimizing a very tight loop to even notice the difference between brute force and something more sophisticated like a hash map. And you'd need to be pretty damn desperate for more performance to even consider implementing a algorithm described in a paper in the hopes of beating the standard solutions.

It's pretty good fun though, so don't let pragmatism stop you. Just beware of premature optimization and all that.