How can you track exactly which of 100 million users were active each day, in little memory?

Answer

A bitmap per day, with bit n set when user n is active: 100 million bits is 12.5 MB. "Active on both days" is a bitwise AND, "either day" an OR, and counts are exact, unlike HyperLogLog. It needs small, dense integer ids; sparse ids call for compressed bitmaps such as Roaring.

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