【转】字符串哈希
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。
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1 //SDBMHash 2 unsigned int SDBMHash(char *str){ 3 unsigned int hash = 0; 4 while (*str){ 5 // equivalent to: hash = 65599*hash + (*str++); 6 hash = (*str++) + (hash << 6) + (hash << 16) - hash; 7 } 8 return (hash & 0x7FFFFFFF); 9 } 10 // RS Hash 11 unsigned int RSHash(char *str){ 12 unsigned int b = 378551; 13 unsigned int a = 63689; 14 unsigned int hash = 0; 15 while (*str){ 16 hash = hash * a + (*str++); 17 a *= b; 18 } 19 return (hash & 0x7FFFFFFF); 20 } 21 // JS Hash 22 unsigned int JSHash(char *str){ 23 unsigned int hash = 1315423911; 24 while (*str){ 25 hash ^= ((hash << 5) + (*str++) + (hash >> 2)); 26 } 27 return (hash & 0x7FFFFFFF); 28 } 29 // P. J. Weinberger Hash 30 unsigned int PJWHash(char *str){ 31 unsigned int BitsInUnignedInt = (unsigned int)(sizeof(unsigned int) * 8); 32 unsigned int ThreeQuarters = (unsigned int)((BitsInUnignedInt * 3) / 4); 33 unsigned int OneEighth = (unsigned int)(BitsInUnignedInt / 8); 34 unsigned int HighBits = (unsigned int)(0xFFFFFFFF) << (BitsInUnignedInt 35 - OneEighth); 36 unsigned int hash = 0; 37 unsigned int test = 0; 38 while (*str){ 39 hash = (hash << OneEighth) + (*str++); 40 if ((test = hash & HighBits) != 0){ 41 hash = ((hash ^ (test >> ThreeQuarters)) & (~HighBits)); 42 } 43 } 44 return (hash & 0x7FFFFFFF); 45 } 46 // ELF Hash 47 unsigned int ELFHash(char *str){ 48 unsigned int hash = 0; 49 unsigned int x = 0; 50 while (*str){ 51 hash = (hash << 4) + (*str++); 52 if ((x = hash & 0xF0000000L) != 0){ 53 hash ^= (x >> 24); 54 hash &= ~x; 55 } 56 } 57 return (hash & 0x7FFFFFFF); 58 } 59 // BKDR Hash 60 unsigned int BKDRHash(char *str){ 61 unsigned int seed = 131; // 31 131 1313 13131 131313 etc.. 62 unsigned int hash = 0; 63 while (*str){ 64 hash = hash * seed + (*str++); 65 } 66 return (hash & 0x7FFFFFFF); 67 } 68 // DJB Hash 69 unsigned int DJBHash(char *str){ 70 unsigned int hash = 5381; 71 while (*str){ 72 hash += (hash << 5) + (*str++); 73 } 74 return (hash & 0x7FFFFFFF); 75 } 76 // AP Hash 77 unsigned int APHash(char *str){ 78 unsigned int hash = 0; 79 int i; 80 for (i=0; *str; i++){ 81 if ((i & 1) == 0){ 82 hash ^= ((hash << 7) ^ (*str++) ^ (hash >> 3)); 83 } 84 else{ 85 hash ^= (~((hash << 11) ^ (*str++) ^ (hash >> 5))); 86 } 87 } 88 return (hash & 0x7FFFFFFF); 89 }