What
A probabilistic data structure to count unique items.
Provides a very good approximation of the cardinality of a set even using a very small amount of memory. In the Redis implementation it only uses 12kbytes per key to count with a standard error of 0.81%, and there is no limit to the number of items you can count, unless you approach 2^64 items
How
b: Number of initial bits in a binary representation of a hash valuem: Number of registers / buckets =p: MSB position of 1 (leftmost position)