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Change [#6689]: Tweak HashTable hash calculation to reduce collisions. (kernigh2) (#6786)

pull/6789/merge
PeterN 2018-05-19 22:04:25 +01:00 committed by GitHub
parent 7bd2fa3516
commit 8a7de36463
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GPG Key ID: 4AEE18F83AFDEB23
1 changed files with 4 additions and 6 deletions
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10
src/misc/hashtable.hpp
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@ -161,12 +161,10 @@ protected:
/** static helper - return hash for the given key modulo number of slots */
inline static int CalcHash(const Tkey &key)
{
int32 hash = key.CalcHash();
if ((8 * Thash_bits) < 32) hash ^= hash >> (min(8 * Thash_bits, 31));
if ((4 * Thash_bits) < 32) hash ^= hash >> (min(4 * Thash_bits, 31));
if ((2 * Thash_bits) < 32) hash ^= hash >> (min(2 * Thash_bits, 31));
if ((1 * Thash_bits) < 32) hash ^= hash >> (min(1 * Thash_bits, 31));
hash &= (1 << Thash_bits) - 1;
uint32 hash = key.CalcHash();
hash -= (hash >> 17); // hash * 131071 / 131072
hash -= (hash >> 5); // * 31 / 32
hash &= (1 << Thash_bits) - 1; // modulo slots
return hash;
}