各种字符串Hash函数比较

转自 beyond the void

常用的字符串Hash函数还有ELFHash，APHash等等，都是十分简单有效的方法。这些函数使用位运算使得每一个字符都对最后的函数值产生影响。另外还有以MD5和SHA1为代表的杂凑函数，这些函数几乎不可能找到碰撞。

常用字符串哈希函数有BKDRHash，APHash，DJBHash，JSHash，RSHash，SDBMHash，PJWHash，ELFHash等等。对于以上几种哈希函数，我对其进行了一个小小的评测。

Hash函数	数据1	数据2	数据3	数据4	数据1得分	数据2得分	数据3得分	数据4得分	平均分
BKDRHash	2	0	4774	481	96.55	100	90.95	82.05	92.64
APHash	2	3	4754	493	96.55	88.46	100	51.28	86.28
DJBHash	2	2	4975	474	96.55	92.31	0	100	83.43
JSHash	1	4	4761	506	100	84.62	96.83	17.95	81.94
RSHash	1	0	4861	505	100	100	51.58	20.51	75.96
SDBMHash	3	2	4849	504	93.1	92.31	57.01	23.08	72.41
PJWHash	30	26	4878	513	0	0	43.89	0	21.95
ELFHash	30	26	4878	513	0	0	43.89	0	21.95

其中数据1为100000个字母和数字组成的随机串哈希冲突个数。数据2为100000个有意义的英文句子哈希冲突个数。数据3为数据1的哈希值与1000003(大素数)求模后存储到线性表中冲突的个数。数据4为数据1的哈希值与10000019(更大素数)求模后存储到线性表中冲突的个数。

经过比较，得出以上平均得分。平均数为平方平均数。可以发现，BKDRHash无论是在实际效果还是编码实现中，效果都是最突出的。APHash也是较为优秀的算法。DJBHash,JSHash,RSHash与SDBMHash各有千秋。PJWHash与ELFHash效果最差，但得分相似，其算法本质是相似的。

附：各种哈希函数的C语言

unsigned int SDBMHash( char * str)
{
unsigned int hash = 0 ;

while ( * str)
{
// equivalent to: hash = 65599*hash + (*str++);
hash = ( * str ++ ) + (hash << 6 ) + (hash << 16 ) - hash;
}

return (hash & 0x7FFFFFFF );
}

// RS Hash Function
unsigned int RSHash( char * str)
{
unsigned int b = 378551 ;
unsigned int a = 63689 ;
unsigned int hash = 0 ;

while ( * str)
{
hash = hash * a + ( * str ++ );
a *= b;
}

return (hash & 0x7FFFFFFF );
}

// JS Hash Function
unsigned int JSHash( char * str)
{
unsigned int hash = 1315423911 ;

while ( * str)
{
hash ^= ((hash << 5 ) + ( * str ++ ) + (hash >> 2 ));
}

return (hash & 0x7FFFFFFF );
}

// P. J. Weinberger Hash Function
unsigned int PJWHash( char * str)
{
unsigned int BitsInUnignedInt = (unsigned int )( sizeof (unsigned int ) * 8 );
unsigned int ThreeQuarters = (unsigned int )((BitsInUnignedInt * 3 ) / 4 );
unsigned int OneEighth = (unsigned int )(BitsInUnignedInt / 8 );
unsigned int HighBits = (unsigned int )( 0xFFFFFFFF ) << (BitsInUnignedInt - OneEighth);
unsigned int hash = 0 ;
unsigned int test = 0 ;

while ( * str)
{
hash = (hash << OneEighth) + ( * str ++ );
if ((test = hash & HighBits) != 0 )
{
hash = ((hash ^ (test >> ThreeQuarters)) & ( ~ HighBits));
}
}

return (hash & 0x7FFFFFFF );
}

// ELF Hash Function
unsigned int ELFHash( char * str)
{
unsigned int hash = 0 ;
unsigned int x = 0 ;

while ( * str)
{
hash = (hash << 4 ) + ( * str ++ );
if ((x = hash & 0xF0000000L ) != 0 )
{
hash ^= (x >> 24 );
hash &= ~ x;
}
}

return (hash & 0x7FFFFFFF );
}

// BKDR Hash Function
unsigned int BKDRHash( char * str)
{
unsigned int seed = 131 ; // 31 131 1313 13131 131313 etc..
unsigned int hash = 0 ;

while ( * str)
{
hash = hash * seed + ( * str ++ );
}

return (hash & 0x7FFFFFFF );
}

// DJB Hash Function
unsigned int DJBHash( char * str)
{
unsigned int hash = 5381 ;

while ( * str)
{
hash += (hash << 5 ) + ( * str ++ );
}

return (hash & 0x7FFFFFFF );
}

// AP Hash Function
unsigned int APHash( char * str)
{
unsigned int hash = 0 ;
int i;

for (i = 0 ; * str; i ++ )
{
if ((i & 1 ) == 0 )
{
hash ^= ((hash << 7 ) ^ ( * str ++ ) ^ (hash >> 3 ));
}
else
{
hash ^= ( ~ ((hash << 11 ) ^ ( * str ++ ) ^ (hash >> 5 )));
}
}

return (hash & 0x7FFFFFFF );
}