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Speedup SUM(2^-reg[m]) in HyperLogLog computation.

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When the register is set to zero, we need to add 2^-0 to E, which is 1,
but it is faster to just add 'ez' at the end, which is the number of
registers set to zero, a value we need to compute anyway.
This commit is contained in:
antirez 2014-04-17 17:53:20 +02:00
parent 0feb2aabca
commit 192a213274
1 changed files with 8 additions and 4 deletions
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12
src/hyperloglog.c
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@ -546,11 +546,12 @@ double hllDenseSum(uint8_t *registers, double *PE, int *ezp) {
HLL_DENSE_GET_REGISTER(reg,registers,j);
if (reg == 0) {
ez++;
E += 1; /* 2^(-reg[j]) is 1 when m is 0. */
/* Increment E at the end of the loop. */
} else {
E += PE[reg]; /* Precomputed 2^(-reg[j]). */
}
}
E += ez; /* Add 2^0 'ez' times. */
}
*ezp = ez;
return E;
@ -894,13 +895,13 @@ double hllSparseSum(uint8_t *sparse, int sparselen, double *PE, int *ezp, int *i
runlen = HLL_SPARSE_ZERO_LEN(p);
idx += runlen;
ez += runlen;
E += 1*runlen; /* 2^(-reg[j]) is 1 when m is 0. */
/* Increment E at the end of the loop. */
p++;
} else if (HLL_SPARSE_IS_XZERO(p)) {
runlen = HLL_SPARSE_XZERO_LEN(p);
idx += runlen;
ez += runlen;
E += 1*runlen; /* 2^(-reg[j]) is 1 when m is 0. */
/* Increment E at the end of the loop. */
p += 2;
} else {
runlen = HLL_SPARSE_VAL_LEN(p);
@ -911,6 +912,7 @@ double hllSparseSum(uint8_t *sparse, int sparselen, double *PE, int *ezp, int *i
}
}
if (idx != HLL_REGISTERS && invalid) *invalid = 1;
E += ez; /* Add 2^0 'ez' times. */
*ezp = ez;
return E;
}
@ -932,11 +934,13 @@ double hllRawSum(uint8_t *registers, double *PE, int *ezp) {
reg = registers[j];
if (reg == 0) {
ez++;
E += 1; /* 2^(-reg[j]) is 1 when m is 0. */
/* Increment E at the end of the loop. */
} else {
E += PE[reg]; /* Precomputed 2^(-reg[j]). */
}
}
E += ez; /* 2^(-reg[j]) is 1 when m is 0, add it 'ez' times for every
zero register in the HLL. */
*ezp = ez;
return E;
}