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| author | antirez <antirez@gmail.com> | 2014-04-04 09:33:35 +0200 |
|---|---|---|
| committer | antirez <antirez@gmail.com> | 2014-04-04 09:36:32 +0200 |
| commit | 433ce7f85c3fb81b2ac9becb3cb5a94c798a148d (patch) | |
| tree | 87e670fe1cf3bc1d50d99b2f00f22a6f1b8f72ca /src/hyperloglog.c | |
| parent | 352208ff68f213a59729809ae3eaa2f0b0d0a2ad (diff) | |
| download | redis-433ce7f85c3fb81b2ac9becb3cb5a94c798a148d.tar.gz | |
Changed HyperLogLog hash seed to a non-zero value.
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.
Diffstat (limited to 'src/hyperloglog.c')
| -rw-r--r-- | src/hyperloglog.c | 2 |
1 files changed, 1 insertions, 1 deletions
diff --git a/src/hyperloglog.c b/src/hyperloglog.c index 02fd2728a..0ae66a0fc 100644 --- a/src/hyperloglog.c +++ b/src/hyperloglog.c @@ -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. */ |