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use all 64 bits of the hash value instead of 63

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Otmar Ertl 2018-03-11 09:18:00 +01:00
parent 44698f45e7
commit 97bde9f623
1 changed files with 7 additions and 4 deletions
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11
src/hyperloglog.c
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@ -192,11 +192,12 @@ struct hllhdr {
#define HLL_VALID_CACHE(hdr) (((hdr)->card[7] & (1<<7)) == 0)
#define HLL_P 14 /* The greater is P, the smaller the error. */
#define HLL_Q (64-HLL_P) /* The number of bits of the hash value used for
determining the number of leading zeros. */
#define HLL_REGISTERS (1<<HLL_P) /* With P=14, 16384 registers. */
#define HLL_P_MASK (HLL_REGISTERS-1) /* Mask to index register. */
#define HLL_BITS 6 /* Enough to count up to 63 leading zeroes. */
#define HLL_REGISTER_MAX ((1<<HLL_BITS)-1)
#define HLL_Q (HLL_REGISTER_MAX-HLL_P)
#define HLL_HDR_SIZE sizeof(struct hllhdr)
#define HLL_DENSE_SIZE (HLL_HDR_SIZE+((HLL_REGISTERS*HLL_BITS+7)/8))
#define HLL_DENSE 0 /* Dense encoding. */
@ -452,7 +453,7 @@ int hllPatLen(unsigned char *ele, size_t elesize, long *regp) {
/* Count the number of zeroes starting from bit HLL_REGISTERS
* (that is a power of two corresponding to the first bit we don't use
* as index). The max run can be 64-P+1 bits.
* as index). The max run can be 64-P+1 = Q+1 bits.
*
* Note that the final "1" ending the sequence of zeroes must be
* included in the count, so if we find "001" the count is 3, and
@ -463,8 +464,10 @@ int hllPatLen(unsigned char *ele, size_t elesize, long *regp) {
* there are high probabilities to find a 1 after a few iterations. */
hash = MurmurHash64A(ele,elesize,0xadc83b19ULL);
index = hash & HLL_P_MASK; /* Register index. */
hash |= ((uint64_t)1<<63); /* Make sure the loop terminates. */
bit = HLL_REGISTERS; /* First bit not used to address the register. */
hash >>= HLL_P; /* Remove bits used to address the register. */
hash |= ((uint64_t)1<<HLL_Q); /* Make sure the loop terminates
and count will be <= Q+1. */
bit = 1;
count = 1; /* Initialized to 1 since we count the "00000...1" pattern. */
while((hash & bit) == 0) {
count++;