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HyperLogLog: use precomputed table for 2^(-M[i]).

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antirez 2014-03-28 22:49:24 +01:00
parent fdf81b2d35
commit ac8fbe8829
1 changed files with 14 additions and 1 deletions
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15
src/hyperloglog.c
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@ -277,6 +277,19 @@ uint64_t hllCount(uint8_t *registers) {
int ez = 0; /* Number of registers equal to 0. */
int j;
/* We precompute 2^(-reg[j]) in a small table in order to
* speedup the computation of SUM(2^-register[0..i]). */
static int initialized = 0;
static double PE[64];
if (!initialized) {
PE[0] = 1; /* 2^(-reg[j]) is 1 when m is 0. */
for (j = 1; j < 64; j++) {
/* 2^(-reg[j]) is the same as 1/2^reg[j]. */
PE[j] = 1.0/(1ULL << j);
}
initialized = 1;
}
for (j = 0; j < REDIS_HLL_REGISTERS; j++) {
uint8_t reg;
@ -286,7 +299,7 @@ uint64_t hllCount(uint8_t *registers) {
ez++;
E += 1; /* 2^(-reg[j]) is 1 when m is 0. */
} else {
E += 1.0/(1ULL << reg); /* 2^(-reg[j]) is the same as 1/2^reg[j]. */
E += PE[reg]; /* Precomputed 2^(-reg[j]). */
}
}
/* Muliply the inverse of E for alpha_m * m^2 to have the raw estimate. */