Question

Can anyone explain to me the Lossy Counting algorithm? It is a streaming algorithm on finding frequency of items in a stream. Thanks.

Answer 1

Say you’re looking at the traffic for facebook profiles. You have billions of hits. You want to find which profiles are accessed the most often. You could keep a count for each profile, but then you’d have a very large number of counts to keep track of, the vast majority of which would be meaningless.

With lossy counting, you periodically remove very low count elements from the table. The most-frequently accessed profiles would almost never have low counts anyway, and if they did, they wouldn’t be likely to stay there for long.

The algorithm basically involves grouping the inputs into blocks or chunks and counting within each chunk. Then you reduce the count for each element by one, dropping any elements whose counts drop to zero.

The most-frequently hit profiles will get on your count and stay there. Any profiles that aren’t hit very often will drop to zero in a few blocks and you won’t have to track them any more.

Note that the final results are order-dependent, giving heavier weight to the counts processed last. In some cases, this makes perfect sense and is an upside rather than a downside. (If you want to know basically which profiles are the most popular now, you want to weigh accesses today more than accesses last month.)

There are a large number of refinements to the algorithm. But the basic idea is this — to find the heavy hitters without having to track every element, periodically purge your counts of any elements that don’t seem likely to be heavy hitters based on the data so far.