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This commit is contained in:
Nikolai Kochetov 2020-03-23 23:03:52 +03:00
parent 4cfdef4403
commit ad56da8961
1 changed files with 7 additions and 0 deletions
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7
dbms/src/Common/HashTable/Hash.h
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@ -109,10 +109,17 @@ inline UInt32 updateWeakHash32(const DB::UInt8 * pos, size_t size, DB::UInt32 up
if (pos < end)
{
/// If string size is not divisible by 8.
/// Lets' assume the string was 'abcdefghXYZ', so it's tail is 'XYZ'.
DB::UInt8 tail_size = end - pos;
/// Load tailing 8 bytes. Word is 'defghXYZ'.
auto word = unalignedLoad<UInt64>(end - 8);
/// Prepare mask which will set other 5 bytes to 0. It is 0xFFFFFFFFFFFFFFFF << 5 = 0xFFFFFF0000000000.
/// word & mask = '\0\0\0\0\0XYZ' (bytes are reversed because of little ending)
word &= (~UInt64(0)) << DB::UInt8(8 * (8 - tail_size));
/// Use least byte to store tail length.
word |= tail_size;
/// Now word is '\3\0\0\0\0XYZ'
updated_value = intHashCRC32(word, updated_value);
}