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Bias integral values so that small integers hash to themselves even when having a wide type

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This commit is contained in:
Peter Dimov
2021-10-17 19:03:15 +03:00
parent faea988310
commit 2424ada9f9
1 changed files with 15 additions and 2 deletions
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17
include/boost/container_hash/hash.hpp
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@@ -86,9 +86,17 @@ namespace boost
{ {
static std::size_t fn( T v ) static std::size_t fn( T v )
{ {
// The bias makes negative numbers that fit into a ssize_t hash to themselves
// E.g. hash_value( -4LL ) == (size_t)-4
std::size_t const bias = (std::numeric_limits<std::size_t>::max)() / 4;
// 4294967291 = 2^32-5, biggest prime under 2^32 // 4294967291 = 2^32-5, biggest prime under 2^32
// we use boost::uint32_t( -5 ), because g++ warns on 4294967291 // we use boost::uint32_t( -5 ), because g++ warns on 4294967291
return static_cast<std::size_t>( static_cast<typename boost::make_unsigned<T>::type>( v ) % static_cast<boost::uint32_t>( -5 ) );
return static_cast<std::size_t>(
( static_cast<typename boost::make_unsigned<T>::type>( v ) + bias )
% static_cast<boost::uint32_t>( -5 ) ) - bias;
} }
}; };
@@ -96,9 +104,14 @@ namespace boost
{ {
static std::size_t fn( T v ) static std::size_t fn( T v )
{ {
std::size_t const bias = (std::numeric_limits<std::size_t>::max)() / 4;
// 18446744073709551557ULL = 2^64-59, biggest prime under 2^64 // 18446744073709551557ULL = 2^64-59, biggest prime under 2^64
// we have to use boost::uint64_t( -59 ), because g++ warns in C++03 mode // we have to use boost::uint64_t( -59 ), because g++ warns in C++03 mode
return static_cast<std::size_t>( static_cast<typename boost::make_unsigned<T>::type>( v ) % static_cast<boost::uint64_t>( -59 ) );
return static_cast<std::size_t>(
( static_cast<typename boost::make_unsigned<T>::type>( v ) + bias )
% static_cast<boost::uint64_t>( -59 ) ) - bias;
} }
}; };