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Judy arrays might be good for this. For the 64 bit int example you could use Judy1: http://judy.sourceforge.net/doc/index.html http://judy.sourceforge.net/doc/

• hoytech 9y ago
  ... and I wrote a blog post testing this :) https://logperiodic.com/blog/2017/05/counting-distinct-elements-judy-arrays https://logperiodic.com/blog/2017/05/counting-distinct-eleme...