mirrors/valkey
1
0
Fork 0
mirror of http://github.com/valkey-io/valkey synced 2024-11-23 20:00:00 +00:00
Code Issues Packages Projects Releases Wiki Activity

Changed HyperLogLog hash seed to a non-zero value.

Browse Source 
Using a seed of zero has the side effect of having the empty string
hashing to what is a very special case in the context of HyperLogLog: a
very long run of zeroes.

This did not influenced the correctness of the result with 16k registers
because of the harmonic mean, but still it is inconvenient that a so
obvious value maps to a so special hash.

The seed 0xadc83b19 is used instead, which is the first 64 bits of the
SHA1 of the empty string.

Reference: issue #1657.
This commit is contained in:
antirez 2014-04-04 09:33:35 +02:00
parent 352208ff68
commit 433ce7f85c
1 changed files with 1 additions and 1 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

2
src/hyperloglog.c
View File

@ -316,7 +316,7 @@ int hllAdd(uint8_t *registers, unsigned char *ele, size_t elesize) {
*
* This may sound like inefficient, but actually in the average case
* there are high probabilities to find a 1 after a few iterations. */
hash = MurmurHash64A(ele,elesize,0);
hash = MurmurHash64A(ele,elesize,0xadc83b19ULL);
hash |= ((uint64_t)1<<63); /* Make sure the loop terminates. */
bit = REDIS_HLL_REGISTERS; /* First bit not used to address the register. */
count = 1; /* Initialized to 1 since we count the "00000...1" pattern. */