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Change Hashing from H3 to Siphash.

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This commit mostly changes the hash function that is used for Internal
hashing of data < 36 bytes from H3 to Siphash. This change is motivated
by the fact that it turns out that H3 apparently does not deliver a very
good source of data uniqueness; running HLL with H3 as a hashing
function results in quite poor results (up to of 75% off in my tests).
In difference, running HLL with Siphash (or HMAC-MD5) changes this
factor to ~2%.

This also fixes a long-standing bug in Hash.h which truncated our hash
values to 32 bit on most machines.

Furthermore, it once again fixes a problem with the Rank function in
HLL.
This commit is contained in:
Johanna Amann 2016-07-13 06:35:32 -07:00
parent c15f48661d
commit e1218cc7fa
10 changed files with 257 additions and 25 deletions
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2
src/main.cc
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@ -667,7 +667,7 @@ int main(int argc, char** argv)
case 'K':
MD5((const u_char*) optarg, strlen(optarg), shared_hmac_md5_key);
hmac_key_set = 1;
hmac_key_set = true;
break;
case 'N':