Function addHyperLogLog

src/backend/lib/hyperloglog.c:166–180

* Adds element to the estimator, from caller-supplied hash. * * It is critical that the hash value passed be an actual hash value, typically * generated using hash_any(). The algorithm relies on a specific bit-pattern * observable in conjunction with stochastic averaging. There must be a * uniform distribution of bits in hash values for each distinct original value * observed. */

 * observed.
 */
void
addHyperLogLog(hyperLogLogState *cState, uint32 hash)
{
	uint8		count;
	uint32		index;

	/* Use the first "k" (registerWidth) bits as a zero based index */
	index = hash >> (BITS_PER_BYTE * sizeof(uint32) - cState->registerWidth);

	/* Compute the rank of the remaining 32 - "k" (registerWidth) bits */
	count = rho(hash << cState->registerWidth,
				BITS_PER_BYTE * sizeof(uint32) - cState->registerWidth);

	cState->hashesArr[index] = Max(count, cState->hashesArr[index]);
}

/*
 * Estimates cardinality, based on elements added so far