[andreareina]

The algorithm's accuracy is known. From the wiki[1]:

    The HyperLogLog algorithm is able to estimate 
    cardinalities of > 10^9 with a typical error rate of 2%

[1] https://en.wikipedia.org/wiki/HyperLogLog

[federicoponzi]

But what about the implementation accuracy? :)

[zeroxfe]

Tests against both historical and synthetic datasets.