public strictfp long estimateCardinality()

in spark-load/spark-load-common/src/main/java/org/apache/doris/common/io/Hll.java [318:365]

• 40 lines of code
• 12 McCabe index (conditional complexity)

    public strictfp long estimateCardinality() {
        if (type == HLL_DATA_EMPTY) {
            return 0;
        }
        if (type == HLL_DATA_EXPLICIT) {
            return hashSet.size();
        }

        int numStreams = HLL_REGISTERS_COUNT;
        float alpha = 0;

        if (numStreams == 16) {
            alpha = 0.673f;
        } else if (numStreams == 32) {
            alpha = 0.697f;
        } else if (numStreams == 64) {
            alpha = 0.709f;
        } else {
            alpha = 0.7213f / (1 + 1.079f / numStreams);
        }

        float harmonicMean = 0;
        int numZeroRegisters = 0;

        for (int i = 0; i < HLL_REGISTERS_COUNT; i++) {
            harmonicMean += Math.pow(2.0f, -registers[i]);

            if (registers[i] == 0) {
                numZeroRegisters++;
            }
        }

        harmonicMean = 1.0f / harmonicMean;
        double estimate = alpha * numStreams * numStreams * harmonicMean;

        if (estimate <= numStreams * 2.5 && numZeroRegisters != 0) {
            estimate = numStreams * Math.log(((float) numStreams) / ((float) numZeroRegisters));
        } else if (numStreams == 16384 && estimate < 72000) {
            double bias = 5.9119 * 1.0e-18 * (estimate * estimate * estimate * estimate)
                    - 1.4253 * 1.0e-12 * (estimate * estimate * estimate)
                    + 1.2940 * 1.0e-7 * (estimate * estimate)
                    - 5.2921 * 1.0e-3 * estimate
                    + 83.3216;
            estimate -= estimate * (bias / 100);
        }

        return (long) (estimate + 0.5);
    }