Apache foundation has a fantastic DataSketches library that includes HLL and many other powerful data analytics algorithms: https://datasketches.apache.org/
Lee Rhodes has done an excellent introduction to this library - explaining some of the use cases, advantages, and things to be aware of when using these techniques: https://www.youtube.com/watch?v=nO9pauS-mGQ
On sketches, there is a genre of structure for estimating histogram-like statistics (median, 99th centile, etc) in fixed space, which i really like. Two examples:
Apache foundation has a fantastic DataSketches library that includes HLL and many other powerful data analytics algorithms: https://datasketches.apache.org/
Lee Rhodes has done an excellent introduction to this library - explaining some of the use cases, advantages, and things to be aware of when using these techniques: https://www.youtube.com/watch?v=nO9pauS-mGQ