HyperLogLog.add() have a return value.

[pocorall]

HyperLogLog.add() returns true when the buckets are updated. It has nice use cases for caching or persistence.

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[pocorall]

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[pocorall]

Protected access of the class members enables some useful subclass extensions like custom bucket format. I use base64 encoded String for buckets, and directly access to byte[] buckets will be more efficient than using int[] buckets().

[jesboat]

If previous == 0 and lowestBitPosition == 0, then the code does nonZeroBuckets++ but would still return false. I don't know HLL well enough to say whether previous = lowestBitPosition = 0 can happen or whether the behavior of nonZeroBuckets++ modifies the HLL state in a way that add 'should' return true.

[pocorall]

We use HyperLogLog to store view count of user article. buckets represents internal representation of the count, which is persisted in DB, and estimate() converts buckets to a number. nonZeroBuckets used for estimate() to speedup count non zero buckets, in other words, it totally depends on buckets. So we only care for change of buckets.

[pocorall]

After I reviewed the HLL paper, I found that the updating of nonZeroBuckets in add() function should be moved into if (lowestBitPosition > previous) { ... } part, otherwise nonZeroBuckets quickly grow larger than buckets.length without changing buckets itself.