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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 |
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。
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
1 unsigned int SDBMHash(char *str) 2 { 3 unsigned int hash = 0; 4 5 while (*str) 6 { 7 // equivalent to: hash = 65599*hash + (*str++); 8 hash = (*str++) + (hash << 6) + (hash << 16) - hash; 9 } 10 11 return (hash & 0x7FFFFFFF); 12 } 13 14 // RS Hash Function 15 unsigned int RSHash(char *str) 16 { 17 unsigned int b = 378551; 18 unsigned int a = 63689; 19 unsigned int hash = 0; 20 21 while (*str) 22 { 23 hash = hash * a + (*str++); 24 a *= b; 25 } 26 27 return (hash & 0x7FFFFFFF); 28 } 29 30 // JS Hash Function 31 unsigned int JSHash(char *str) 32 { 33 unsigned int hash = 1315423911; 34 35 while (*str) 36 { 37 hash ^= ((hash << 5) + (*str++) + (hash >> 2)); 38 } 39 40 return (hash & 0x7FFFFFFF); 41 } 42 43 // P. J. Weinberger Hash Function 44 unsigned int PJWHash(char *str) 45 { 46 unsigned int BitsInUnignedInt = (unsigned int)(sizeof(unsigned int) * 8); 47 unsigned int ThreeQuarters = (unsigned int)((BitsInUnignedInt * 3) / 4); 48 unsigned int OneEighth = (unsigned int)(BitsInUnignedInt / 8); 49 unsigned int HighBits = (unsigned int)(0xFFFFFFFF) << (BitsInUnignedInt - OneEighth); 50 unsigned int hash = 0; 51 unsigned int test = 0; 52 53 while (*str) 54 { 55 hash = (hash << OneEighth) + (*str++); 56 if ((test = hash & HighBits) != 0) 57 { 58 hash = ((hash ^ (test >> ThreeQuarters)) & (~HighBits)); 59 } 60 } 61 62 return (hash & 0x7FFFFFFF); 63 } 64 65 // ELF Hash Function 66 unsigned int ELFHash(char *str) 67 { 68 unsigned int hash = 0; 69 unsigned int x = 0; 70 71 while (*str) 72 { 73 hash = (hash << 4) + (*str++); 74 if ((x = hash & 0xF0000000L) != 0) 75 { 76 hash ^= (x >> 24); 77 hash &= ~x; 78 } 79 } 80 81 return (hash & 0x7FFFFFFF); 82 } 83 84 // BKDR Hash Function 85 unsigned int BKDRHash(char *str) 86 { 87 unsigned int seed = 131; // 31 131 1313 13131 131313 etc.. 88 unsigned int hash = 0; 89 90 while (*str) 91 { 92 hash = hash * seed + (*str++); 93 } 94 95 return (hash & 0x7FFFFFFF); 96 } 97 98 // DJB Hash Function 99 unsigned int DJBHash(char *str) 100 { 101 unsigned int hash = 5381; 102 103 while (*str) 104 { 105 hash += (hash << 5) + (*str++); 106 } 107 108 return (hash & 0x7FFFFFFF); 109 } 110 111 // AP Hash Function 112 unsigned int APHash(char *str) 113 { 114 unsigned int hash = 0; 115 int i; 116 117 for (i=0; *str; i++) 118 { 119 if ((i & 1) == 0) 120 { 121 hash ^= ((hash << 7) ^ (*str++) ^ (hash >> 3)); 122 } 123 else 124 { 125 hash ^= (~((hash << 11) ^ (*str++) ^ (hash >> 5))); 126 } 127 } 128 129 return (hash & 0x7FFFFFFF); 130 }
转载自:http://www.byvoid.com/blog/string-hash-compare/
【转】各种字符串Hash函数比较,布布扣,bubuko.com
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原文地址:http://www.cnblogs.com/hoskey/p/3832102.html