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Improving hash performance for pairs of integral types

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* spot/misc/hash.hh: Here
This commit is contained in:
philipp 2021-07-09 23:05:50 +02:00
parent f4776efb63
commit 3da41aa9a1
1 changed files with 20 additions and 4 deletions
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24
spot/misc/hash.hh
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@ -22,6 +22,7 @@
#pragma once
#include <climits>
#include <string>
#include <functional>
#include <spot/misc/hashfunc.hh>
@ -80,11 +81,26 @@ namespace spot
template<typename T, typename U>
std::size_t operator()(const std::pair<T, U> &p) const noexcept
{
std::hash<T> th;
std::hash<U> uh;
return wang32_hash(static_cast<size_t>(th(p.first)) ^
static_cast<size_t>(uh(p.second)));
if constexpr (std::is_integral<T>::value
&& sizeof(T) <= sizeof(std::size_t)/2
&& std::is_integral<U>::value
&& sizeof(U) <= sizeof(std::size_t)/2)
{
constexpr unsigned shift = (sizeof(std::size_t)/2)*CHAR_BIT;
std::size_t h = p.first;
h <<= shift;
h += p.second;
return h;
}
else
{
std::hash<T> th;
std::hash<U> uh;
return wang32_hash(static_cast<size_t>(th(p.first)) ^
static_cast<size_t>(uh(p.second)));
}
}
};