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Utilities - Hash combining helper

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This commit is contained in:
Emmanuel BENOîT 2020-09-26 10:37:13 +02:00
parent 90cf63c592
commit 3889efde67
3 changed files with 35 additions and 0 deletions
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5
include/ebcl/Utilities.hh
View file

@ -466,6 +466,11 @@ struct T_HashFunction
template< typename T >
uint32_t ComputeHash( T const& item );
// CombineHash( &hv , T ) - Helper that consolidates multiple hashes into a
// single value.
template< typename T >
void CombineHash( uint32_t& hashValue , T const& item );
// Helper macros to declare and define specialised hash functions.
#define M_DECLARE_HASH( Type ) \
template< > \

18
include/ebcl/inline/Utilities.hh
View file

@ -315,6 +315,12 @@ inline uint32_t ComputeHash( T const& item )
return T_HashFunction< T >::hash( item );
}
template< typename T >
inline void CombineHash( uint32_t& hashValue , T const& item )
{
hashValue = ComputeHash( item ) + 0x9e3779b9 + ( hashValue << 6 ) + ( hashValue >> 2 );
}
/*----------------------------------------------------------------------------*/
// Hash function specializations for data sizes 1, 2, 4 and 8.
@ -358,6 +364,18 @@ struct T_HashFunction< T , 8 >
}
};
/*----------------------------------------------------------------------------*/
// Hash functions for pairs and tuples
template< typename T1 , typename T2 >
inline uint32_t ComputeHash( std::pair< T1 , T2 > const& item )
{
uint32_t hv{ 0 };
CombineHash( hv , item.first );
CombineHash( hv , item.second );
return hv;
}
/*= PRIVATE IMPLEMENTATION BASE ==============================================*/

12
tests/hash-functions.cc
View file

@ -13,6 +13,7 @@ class ComputeHashTest : public CppUnit::TestFixture
CPPUNIT_TEST( testInt32 );
CPPUNIT_TEST( testInt64 );
CPPUNIT_TEST( testStructDefault );
CPPUNIT_TEST( testPair );
CPPUNIT_TEST_SUITE_END( );
public:
@ -21,6 +22,7 @@ public:
void testInt32( );
void testInt64( );
void testStructDefault( );
void testPair( );
};
@ -74,3 +76,13 @@ void ComputeHashTest::testStructDefault( )
CPPUNIT_ASSERT_EQUAL( uint32_t( HashData( test.meh , 16 ) ) ,
ComputeHash( test ) );
}
void ComputeHashTest::testPair( )
{
auto p{ std::make_pair< uint8_t , uint32_t >( 12 , 128 ) };
uint32_t hv = 0x9e3779b9 + 12;
hv = 128 + 0x9e3779b9 + ( hv << 6 ) + ( hv >> 2 );
CPPUNIT_ASSERT_EQUAL( hv , ComputeHash( p ) );
}