原文
: x/ S% D0 J2 O/ }" ahttp://blog.csai.cn/user3/50125/archives/2009/35638.html
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[pre]文章转摘自http://www.cmykrgb123.cn/blog/string-hash-compare/ ( h8 k& _6 z9 w* S
9 A( @3 z, R+ t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 4 E! U: H, H* N% O1 M1 S
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,
9 l9 m" Y, A9 m7 |: p5 R这些函数几乎不可能找到碰撞。
3 B( C( U6 |& L2 a常用字符串哈希函数有BKDRHash,APHash,DJBHash,JSHash,RSHash,SDBMHash,
A# J$ j2 t3 ^2 ?& F2 E( l- L5 UPJWHash,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。 + l* a( W8 W( y# d5 J* @: J: d5 P
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 | ! s* S) l; l E& {$ T; X" d* q
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
7 I2 q3 H0 ~" s4 m% E% _9 X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。 ( G8 g% W2 [; l8 U3 a3 H
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。
; y* r5 @" x: }, J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
! V5 A' v7 @# I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
3 ~* F: ~1 Z/ A* R; ]2 RSDBMHash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。
- }# A7 U2 t+ g+ S& Q& H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。
! t G6 \2 H9 O' K# ACmYkRgB123原创,欢迎建议、交流、批评和指正。 + N' P4 r0 a$ P; s+ _
: ]& J- M5 A7 k' I' p, \- A3 F附:各种哈希函数的C语言程序代码
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" i8 X7 I8 S0 d: O3 z* q, \unsigned int SDBMHash(char *str) % D) s/ z+ h4 w# z; m' G1 Z+ d
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unsigned int hash = 0;
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L" ~" s" y6 L* o+ V4 } while (*str) 7 T. E0 D' S% w' r4 U# O( S, }
{
" M; X% \, I5 w+ Q* J7 h4 i // equivalent to: hash = 65599*hash + (*str++);
8 g9 y+ {' h. c7 ~% J1 Y hash = (*str++) + (hash << 6) + (hash << 16) - hash; $ s4 ?" ^9 j3 { g* O# L4 L' u
}
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return (hash & 0x7FFFFFFF);
+ [& \. E1 S: h" P3 g}
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) k& `7 J: }# E. [// RS Hash - I( o) H6 ~6 x. B% O' {( N
unsigned int RSHash(char *str)
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unsigned int b = 378551;
3 k7 x" F/ d3 p7 O8 u7 {- \ unsigned int a = 63689; / X: H' `) q. \2 N9 n6 x7 g
unsigned int hash = 0;
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while (*str)
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2 N. A; j' p0 G2 x hash = hash * a + (*str++); ' R L2 Z7 m, }' W& k# ~* W
a *= b; ' m/ L9 z9 y2 E' S6 g& b ^6 R
} ; Z$ Y! I7 l2 j7 ]
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return (hash & 0x7FFFFFFF);
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1 L7 `% g+ J4 b: z3 X. ^/ S// JS Hash
i% Z% U5 e9 S' uunsigned int JSHash(char *str)
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- T+ L! K. j! v$ y7 [ unsigned int hash = 1315423911;
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while (*str)
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hash ^= ((hash << 5) + (*str++) + (hash >> 2));
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$ y! X- C0 r+ o, y+ z return (hash & 0x7FFFFFFF); / w7 d. O V- {$ F3 ?
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- y) _) Y0 O" b+ }* d4 x3 i// P. J. Weinberger Hash
) ]) }1 E/ d( A( Q# Gunsigned int PJWHash(char *str)" T5 ~* Y! }1 }+ O+ h9 ]; C
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unsigned int BitsInUnignedInt = (unsigned int)(sizeof(unsigned int) * 8);
: ]3 r: | z8 z# q unsigned int ThreeQuarters = (unsigned int)((BitsInUnignedInt * 3) / 4);+ x# a; F k/ D. h' X3 J# z6 J
unsigned int OneEighth = (unsigned int)(BitsInUnignedInt / 8);( K W1 F6 e; n' d g) @
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] |