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You could get away with much less memory using Redis hashes with a few more lines of code: http://redis.io/topics/memory-optimization.

I mean, a bloom filter isn't super difficult, but it's also probabilistic instead of deterministic.

Exactly, so you'd need the array anyway...

Also, typically bloom filters don't come out of the box with the language you're using, so it's just more potential for bugs.

A lookup on a sorted array should take 8.6 comparisons anyway, I bet the hashing takes longer...

That's true, good call.