I'm looking for a fast data structure for determining set membership. Suppose I have 10,000 numbers ranging from 1 to 10 billion or so and I want a quick test to see if a given number n is 'probably' one of those 10,000 values. A hashtable seems similar to what I want, but as I have no value to associate with these numbers it doesn't seem quite right -- and I'm pretty sure I've seen a data structure designed for just this problem. In particular, I'd like the structure, once initialized (it won't change thereafter), to return "true" if a given member is in the set with high probability, and "false" otherwise. 99.9% would quite suffice, and even lower probabilities may be acceptable.