各种字符串hash函数比较

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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BKDRHash20477448196.5510090.9582.0592.64APHash23475449396.5588.4610051.2886.28DJBHash22497547496.5592.31010083.43JSHash14476150610084.6296.8317.9581.94RSHash10486150510010051.5820.5175.96SDBMHash32484950493.192.3157.0123.0872.41PJWHash302648785130043.89021.95ELFHash302648785130043.89021.95

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。

BYVoid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 Functionunsigned 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 Functionunsigned int JSHash(char *str){    unsigned int hash = 1315423911;    while (*str)    {        hash ^= ((hash << 5) + (*str++) + (hash >> 2));    }    return (hash & 0x7FFFFFFF);}// P. J. Weinberger Hash Functionunsigned 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 Functionunsigned 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 Functionunsigned 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 Functionunsigned int DJBHash(char *str){    unsigned int hash = 5381;    while (*str)    {        hash += (hash << 5) + (*str++);    }    return (hash & 0x7FFFFFFF);}// AP Hash Functionunsigned 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);}

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