Bloom filters and count-min sketch are awesome. http://en.wikipedia.org/wiki/Bloom_filter http://en.wikipedia.org/wiki/Bloom_filter http://en.wikipedia.org/wik

Bloom filters and count-min sketch are awesome. http://en.wikipedia.org/wiki/Bloom_filter http://en.wikipedia.org/wiki/Bloom_filter http://en.wikipedia.org/wiki/Count%E2%80%93min_sketch http://en.wikipedia.org/wiki/Count%E2%80%93min_sketch