estimate

The estimated number of distinct values.

Exact while the sketch is sparse. Once dense, the harmonic-mean estimator with the standard bias constant, falling back to linear counting while empty registers remain — which is the range where the harmonic estimator is known to be badly biased. The large-range correction the original paper describes is not applied and must not be: it exists to undo the wrap-around of a 32-bit hash, and this one is 64 bits, so applying it would introduce the error it was written to remove.