private static double getAlphaCoefficient(int nBuckets) {
        switch (nBuckets) {
            case 16:
                return 0.673;
            case 32:
                return 0.697;
            case 64:
                return 0.709;
            default: /* nBuckets >= 128 */
                return 0.7213 / (1 + 1.079 / nBuckets);
        }
    }
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