Jenkins hash算法

最新推荐文章于 2023-12-20 08:01:11 发布

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最新推荐文章于 2023-12-20 08:01:11 发布

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本文介绍了Jenkins hash算法，一种高效的散列算法，被广泛应用于memcached等系统中。该算法能够提供良好的散列分布特性，适用于哈希表的开放寻址实现。文章详细展示了Jenkins hash的实现代码。

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Jenkins hash算法， memcached使用了此算法
Jenkins hash，可能是目前能看到的最好的hash算法之一，可以产生很好的分布，缺点是相比其他常见的hash算法更耗时。可以考虑用于hash表的open addressing实现上。如果想了解细节的话，可以去Bob Jenkins的站点看一看。
#define hashsize(n) ( 1U << (n) )
#define hashmask(n) ( hashsize ( n ) – 1 )
#define mix(a,b,c)
{
a -= b; a -= c; a ^= ( c >> 13 );
b -= c; b -= a; b ^= ( a << 8 );
c -= a; c -= b; c ^= ( b >> 13 );
a -= b; a -= c; a ^= ( c >> 12 );
b -= c; b -= a; b ^= ( a << 16 );
c -= a; c -= b; c ^= ( b >> 5 );
a -= b; a -= c; a ^= ( c >> 3 );
b -= c; b -= a; b ^= ( a << 10 );
c -= a; c -= b; c ^= ( b >> 15 );
}
unsigned jen_hash ( unsigned char *k,
unsigned length, unsigned initval )
{
unsigned a, b;
unsigned c = initval;
unsigned len = length;
a = b = 0x9e3779b9;
while ( len >= 12 ) {
a += ( k[0] + ( (unsigned)k[1] << 8 )
+ ( (unsigned)k[2] << 16 )
+ ( (unsigned)k[3] << 24 ) );
b += ( k[4] + ( (unsigned)k[5] << 8 )
+ ( (unsigned)k[6] << 16 )
+ ( (unsigned)k[7] << 24 ) );
c += ( k[8] + ( (unsigned)k[9] << 8 )
+ ( (unsigned)k[10] << 16 )
+ ( (unsigned)k[11] << 24 ) );
mix ( a, b, c );
k += 12;
len -= 12;
}
c += length;
switch ( len ) {
case 11: c += ( (unsigned)k[10] << 24 );
case 10: c += ( (unsigned)k[9] << 16 );
case 9 : c += ( (unsigned)k[8] << 8 );
/* First byte of c reserved for length */
case 8 : b += ( (unsigned)k[7] << 24 );
case 7 : b += ( (unsigned)k[6] << 16 );
case 6 : b += ( (unsigned)k[5] << 8 );
case 5 : b += k[4];
case 4 : a += ( (unsigned)k[3] << 24 );
case 3 : a += ( (unsigned)k[2] << 16 );
case 2 : a += ( (unsigned)k[1] << 8 );
case 1 : a += k[0];
}
mix ( a, b, c );
return c;
}