字符串hash函数

转自http://blog.csdn.net/oanqoanq/article/details/7003626

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。

在信息修竞赛中，要本着易于编码调试的原则，个人认为BKDRHash是最适合记忆和使用的。

CmYkRgB123原创，欢迎建议、交流、批评和指正。

 

附：各种哈希函数的C语言程序代码

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 intThreeQuarters        = (unsigned int)((BitsInUnignedInt  * 3) / 4);
        unsigned intOneEighth                = (unsigned int)(BitsInUnignedInt / 8);
        unsigned intHighBits                = (unsigned int)(0xFFFFFFFF) << (BitsInUnignedInt - OneEighth);
        unsigned inthash                        = 0;
        unsigned inttest                        = 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);
}