原文
2 d( R: T Q6 @3 l* w' y( Rhttp://blog.csai.cn/user3/50125/archives/2009/35638.html _9 O, H2 d* X& c
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[pre]文章转摘自http://www.cmykrgb123.cn/blog/string-hash-compare/ : B C+ i0 U1 f% G, y s. _- _% j
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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 3 k0 @% m7 n( R! \/ @- U
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, 1 x0 x) _5 g8 }0 Z4 \
这些函数几乎不可能找到碰撞。 ) Q9 W. O, r/ |+ A6 j
常用字符串哈希函数有BKDRHash,APHash,DJBHash,JSHash,RSHash,SDBMHash, ( C% r' @1 d: d* j' S; i/ j
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。
: p! A+ m' ^& S8 c; Q6 jHash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 |
3 q, Y) f2 I2 r% @$ c# F- o1 U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 $ B+ b- U( Z$ {
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。 " H" Z5 G( ]8 O3 }; ]
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。 % @# R) ]' H( t5 h6 J# T
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
$ ?' O3 W. ?- ^ |$ o0 B& ~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
( X# }7 T9 n1 j7 T& I0 USDBMHash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。 / n6 z$ x9 K* d& y E X3 l7 ]
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。
; G: T1 e, g+ l' zCmYkRgB123原创,欢迎建议、交流、批评和指正。
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! o6 g; q( e& e4 ^* l% B* s附:各种哈希函数的C语言程序代码 ' n. p- o4 P: h* Q. s
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unsigned int SDBMHash(char *str) 0 `1 L# G9 l4 _7 y
{
, w* @# t, g0 m unsigned int hash = 0; * B1 z% E: q# U, {/ m
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while (*str)
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// equivalent to: hash = 65599*hash + (*str++); ( g+ j! v* F# o! f
hash = (*str++) + (hash << 6) + (hash << 16) - hash;
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return (hash & 0x7FFFFFFF);
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// RS Hash
) [' X) s( w) M/ Q. o$ E, Z. |unsigned int RSHash(char *str)
H* g* u. T. g" M{
2 l! o0 F( B4 N( ]$ Z unsigned int b = 378551; - y( ^' t. B: R- L' i3 y" c
unsigned int a = 63689;
: L4 Q& p, v/ r2 M- ? unsigned int hash = 0;
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{ % ^5 T B8 g4 O! P
hash = hash * a + (*str++);
+ g$ w) W5 l2 L a *= b; " g% H( w- T5 |! v3 Q/ G+ _
}
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return (hash & 0x7FFFFFFF);
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' f' o7 ]6 U, S K& U// JS Hash 1 m6 q$ O, x* {$ \
unsigned int JSHash(char *str) 5 o! j6 u+ F S" X/ `
{
. O& v1 D9 C6 p unsigned int hash = 1315423911; * u$ x2 i' w. E
! J$ t) F* e! _; N% e4 X while (*str)
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hash ^= ((hash << 5) + (*str++) + (hash >> 2));
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return (hash & 0x7FFFFFFF); 5 e8 `. m& V- \5 i& ~
}
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& N) q) j6 l2 Q! N# D: O! y// P. J. Weinberger Hash
3 r: e+ [9 c; m( |# }& iunsigned int PJWHash(char *str), _' O( | a; L5 ^ x% S* }
{
3 I* U! Q; M- b/ V& Q unsigned int BitsInUnignedInt = (unsigned int)(sizeof(unsigned int) * 8);& G8 Y7 T- f) @! X0 y3 {/ t% m
unsigned int ThreeQuarters = (unsigned int)((BitsInUnignedInt * 3) / 4);
1 O6 L1 z5 {% E' x2 m- z' L unsigned int OneEighth = (unsigned int)(BitsInUnignedInt / 8);
% _" \6 |$ J# R) A( R [: \ unsigned int HighBits = (unsigned int)(0xFFFFFFFF) << (BitsInUnignedInt [pre] - 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 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 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 unsigned int DJBHash(char *str){ unsigned int hash = 5381; while (*str) { hash += (hash << 5) + (*str++); } return (hash & 0x7FFFFFFF);} // AP Hash 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);}[/pre] |