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Improved(?) hash function.

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Given the existing interface, it's quite tricky to use most popular hash
functions without a change, so I'm using a modified version of FNV1a.
The current function always starts with a seed of 0 (and will in user
functions), so I'm adding the offset each time instead. I'm not sure if
that will work as well.
This commit is contained in:
Daniel James
2014-01-28 23:33:25 +00:00
parent e961a8a2f8
commit c965760ea1
1 changed files with 40 additions and 1 deletions
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41
include/boost/functional/hash/hash.hpp
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@@ -192,6 +192,44 @@ namespace boost
return seed;
}
template <int Size>
struct hash_combine_impl
{
inline static void combine(std::size_t& seed,
std::size_t value)
{
seed ^= value + 0x9e3779b9 + (seed<<6) + (seed>>2);
}
};
template <>
struct hash_combine_impl<4>
{
template <typename T>
inline static void combine(T& seed, std::size_t value)
{
const T offset = 2166136261UL;
const T prime = 16777619UL;
seed ^= (value + offset);
seed *= prime;
}
};
template <>
struct hash_combine_impl<8>
{
template <typename T>
inline static void combine(T& seed, std::size_t value)
{
const T offset = 14695981039346656037ULL;
const T prime = 1099511628211ULL;
seed ^= (value + offset);
seed *= prime;
}
};
}
template <typename T>
@@ -252,7 +290,8 @@ namespace boost
inline void hash_combine(std::size_t& seed, T const& v)
{
boost::hash<T> hasher;
seed ^= hasher(v) + 0x9e3779b9 + (seed<<6) + (seed>>2);
return boost::hash_detail::hash_combine_impl<
sizeof(std::size_t)>::combine(seed, hasher(v));
}
#if defined(BOOST_MSVC)