private static double calculateIndicator(NormalSketch sketch) {
        double sum = 0;
        for (byte prefixLength : sketch.data()) {
            sum += Math.pow(2, -prefixLength);
        }
        return 1 / sum;
    }
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