[Question] HyperLogLog++

[felipepessoto]

Hi, I couldn't find any reference to HyperLogLog++, https://en.wikipedia.org/wiki/HyperLogLog#HLL++. Idk if that name HyperLogLog++ is widely used, so I'd like to confirm if the algorithm used by Apache Data Sketches is the HLL++ or the original version.

Thanks.

[AlexanderSaydakov]

No, the implementation in this library is not HLL++. We believe it is better than HLL++, at least compared to an HLL++ implementation we found.
From a big picture perspective the changes to the original HLL are similar: 64-bit hash, better estimator and some sort of a sparse (exact or nearly exact) mode before going into (lossy) HLL. We implemented two different flavors of sparse mode (list and set). We also added Historical Inverse Probability (HIP) estimator on top of the improved HLL estimator, which is more accurate, but (simply speaking) does not survive merging.
You may want to take a look at this:
https://datasketches.apache.org/docs/HLL/Hll_vs_CS_Hllpp.html