private static int countZeroBuckets(NormalSketch sketch) {
        int nZeroBuckets = 0;
        for (byte prefixLength : sketch.data()) {
            if (prefixLength == 0) {
                ++nZeroBuckets;
            }
        }
        return nZeroBuckets;
    }
Estimates the number of unique elements.

@param sketch A sketch populated with values from the aggregation phase of HLL.
@return The estimated number of unique elements.
searchlib/src/main/java/com/yahoo/searchlib/aggregation/hll/HyperLogLogEstimator.java