From f90a4af3d75a3a97c6572429ad7bdb9a207c8bce Mon Sep 17 00:00:00 2001
From: antirez <antirez@gmail.com>
Date: Fri, 28 Mar 2014 18:24:05 +0100
Subject: [PATCH] HyperLogLog algorithm fixed in two ways.
There was an error in the computation of 2^register, and the sequence of
zeroes computed after the hashing did not included the "1".
---
src/hyperloglog.c | 11 ++++++++---
1 file changed, 8 insertions(+), 3 deletions(-)
diff --git a/src/hyperloglog.c b/src/hyperloglog.c
index e095c6cf..b8404862 100644
--- a/src/hyperloglog.c
+++ b/src/hyperloglog.c
@@ -236,13 +236,18 @@ int hllAdd(uint8_t *registers, unsigned char *ele, size_t elesize) {
/* Count the number of zeroes starting from bit REDIS_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 bits.
+ * as index). The max run can be 64-P+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
+ * the smallest count possible is no zeroes at all, just a 1 bit
+ * at the first position, that is a count of 1.
*
* 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);
bit = REDIS_HLL_REGISTERS;
- count = 0;
+ count = 1;
while((hash & bit) == 0) {
count++;
/* Test the next bit. Note that if we run out of bits in the 64
@@ -281,7 +286,7 @@ uint64_t hllCount(uint8_t *registers) {
ez++;
E += 1; /* 2^(-reg[j]) is 1 when m is 0. */
} else {
- E += 1.0/(2ULL << reg); /* 2^(-reg[j]) is the same as 1/2^reg[j]. */
+ E += 1.0/(1ULL << reg); /* 2^(-reg[j]) is the same as 1/2^reg[j]. */
}
}
/* Muliply the inverse of E for alpha_m * m^2 to have the raw estimate. */