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Fix PFADD infinite loop.

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We need to guarantee that the last bit is 1, otherwise an element may
hash to just zeroes with probability 1/(2^64) and trigger an infinite
loop.

See issue #1657.
This commit is contained in:
antirez 2014-04-03 19:31:26 +02:00
parent b612affba3
commit 349c978189
1 changed files with 3 additions and 6 deletions
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9
src/hyperloglog.c
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@ -317,14 +317,11 @@ int hllAdd(uint8_t *registers, unsigned char *ele, size_t elesize) {
* This may sound like inefficient, but actually in the average case * This may sound like inefficient, but actually in the average case
* there are high probabilities to find a 1 after a few iterations. */ * there are high probabilities to find a 1 after a few iterations. */
hash = MurmurHash64A(ele,elesize,0); hash = MurmurHash64A(ele,elesize,0);
bit = REDIS_HLL_REGISTERS; hash |= ((uint64_t)1<<63); /* Make sure the loop terminates. */
count = 1; bit = REDIS_HLL_REGISTERS; /* First bit not used to address the register. */
count = 1; /* Initialized to 1 since we count the "00000...1" pattern. */
while((hash & bit) == 0) { while((hash & bit) == 0) {
count++; count++;
/* Test the next bit. Note that if we run out of bits in the 64
* bit integer, bit will be set to 0, and the while test will fail,
* so we can save the explicit check and yet the algorithm will
* terminate. */
bit <<= 1; bit <<= 1;
} }