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Update mix64_policy to use Fibonacci hashing

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Peter Dimov
2022-01-19 04:11:08 +02:00
parent d96d5335b4
commit 2f331b7a8b
1 changed files with 10 additions and 16 deletions
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26
include/boost/unordered/detail/implementation.hpp
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@ -2000,27 +2000,21 @@ namespace boost {
template <typename Hash, typename T>
static inline SizeT apply_hash(Hash const& hf, T const& x)
{
SizeT key = hf(x);
// "Integer Hash Function", Thomas Wang, 1997
// https://gist.github.com/badboy/6267743
// http://web.archive.org/web/20071223173210/http://www.concentric.net/~Ttwang/tech/inthash.htm
key = (~key) + (key << 21); // key = (key << 21) - key - 1;
key = key ^ (key >> 24);
key = (key + (key << 3)) + (key << 8); // key * 265
key = key ^ (key >> 14);
key = (key + (key << 2)) + (key << 4); // key * 21
key = key ^ (key >> 28);
key = key + (key << 31);
return key;
// https://en.wikipedia.org/wiki/Hash_function#Fibonacci_hashing
SizeT const m = 11400714819323198485ull; // 2^64 / phi
return hf(x) * m;
}
static inline SizeT to_bucket(SizeT bucket_count, SizeT hash, int bcount_log2)
{
BOOST_ASSERT( boost::core::has_single_bit( bucket_count ) );
BOOST_ASSERT( bucket_count == ( SizeT(1) << bcount_log2 ) );
return hash & (bucket_count - 1);
(void)bucket_count;
SizeT r = hash >> (64 - bcount_log2);
BOOST_ASSERT( r < bucket_count );
return r;
}
static inline SizeT new_bucket_count(SizeT min)