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Add comment describing the origin of mix64_policy::apply_hash

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Peter Dimov
2021-11-29 06:14:20 +02:00
parent bbd0eedb5f
commit 5e8b6a9e55
1 changed files with 5 additions and 0 deletions
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5
include/boost/unordered/detail/implementation.hpp
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@ -2632,6 +2632,11 @@ namespace boost {
static inline SizeT apply_hash(Hash const& hf, T const& x) static inline SizeT apply_hash(Hash const& hf, T const& x)
{ {
SizeT key = hf(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 << 21); // key = (key << 21) - key - 1;
key = key ^ (key >> 24); key = key ^ (key >> 24);
key = (key + (key << 3)) + (key << 8); // key * 265 key = (key + (key << 3)) + (key << 8); // key * 265