Hash算法的经典实现

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原文出处：http://blog.minidx.com/2008/01/27/446.html

原文作者：Minidxer

哈希算法将任意长度的二进制值映射为固定长度的较小二进制值，这个小的二进制值称为哈希值。哈希值是一段数据唯一且极其紧凑的数值表示形式。如果散列一段明文而且哪怕只更改该段落的一个字母，随后的哈希都将产生不同的值。要找到散列为同一个值的两个不同的输入，在计算上是不可能的，所以数据的哈希值可以检验数据的完整性。

链表查找的时间效率为O(N)，二分法为log2N，B+ Tree为log2N，但Hash链表查找的时间效率为O(1)。

设计高效算法往往需要使用Hash链表，常数级的查找速度是任何别的算法无法比拟的，Hash链表的构造和冲突的不同实现方法对效率当然有一定的影响，然而Hash函数是Hash链表最核心的部分，下面是几款经典软件中使用到的字符串Hash函数实现，通过阅读这些代码，我们可以在Hash算法的执行效率、离散性、空间利用率等方面有比较深刻的了解。

下面分别介绍几个经典软件中出现的字符串Hash函数。

●PHP中出现的字符串Hash函数

static unsigned long hashpjw(char *arKey, unsigned int nKeyLength){unsigned long h = 0, g;char *arEnd=arKey+nKeyLength; while (arKey < arEnd) {h = (h << 4) + *arKey++;if ((g = (h & 0xF0000000))) {h = h ^ (g >> 24);h = h ^ g;}}return h;}

●OpenSSL中出现的字符串Hash函数

unsigned long lh_strhash(char *str){int i,l;unsigned long ret=0;unsigned short *s; if (str == NULL) return(0);l=(strlen(str)+1)/2;s=(unsigned short *)str; for (i=0; iret^=(s[i]<<(i&0x0f));return(ret);} /* The following hash seems to work very well on normal text strings * no collisions on /usr/dict/words and it distributes on %2^n quite * well, not as good as MD5, but still good. */unsigned long lh_strhash(const char *c){unsigned long ret=0;long n;unsigned long v;int r; if ((c == NULL) || (*c == '/0'))return(ret);/*unsigned char b[16]; MD5(c,strlen(c),b); return(b[0]|(b[1]<<8)|(b[2]<<16)|(b[3]<<24)); */ n=0x100;while (*c){v=n|(*c);n+=0x100;r= (int)((v>>2)^v)&0x0f;ret=(ret(32-r));ret&=0xFFFFFFFFL;ret^=v*v;c++;} return((ret>>16)^ret);}

●MySql中出现的字符串Hash函数

#ifndef NEW_HASH_FUNCTION /* Calc hashvalue for a key */static uint calc_hashnr(const byte *key,uint length){register uint nr=1, nr2=4; while (length--){nr^= (((nr & 63)+nr2)*((uint) (uchar) *key++))+ (nr << 8);nr2+=3;} return((uint) nr);} /* Calc hashvalue for a key, case indepenently */static uint calc_hashnr_caseup(const byte *key,uint length){register uint nr=1, nr2=4; while (length--){nr^= (((nr & 63)+nr2)*((uint) (uchar) toupper(*key++)))+ (nr << 8);nr2+=3;} return((uint) nr);}#else/* * Fowler/Noll/Vo hash * * The basis of the hash algorithm was taken from an idea sent by email to the * IEEE Posix P1003.2 mailing list from Phong Vo (kpv@research.att.com) and * Glenn Fowler (gsf@research.att.com). Landon Curt Noll (chongo@toad.com) * later improved on their algorithm. * * The magic is in the interesting relationship between the special prime * 16777619 (2^24 + 403) and 2^32 and 2^8. * * This hash produces the fewest collisions of any function that we've seen so * far, and works well on both numbers and strings. */uint calc_hashnr(const byte *key, uint len){const byte *end=key+len;uint hash; for (hash = 0; key < end; key++){hash *= 16777619;hash ^= (uint) *(uchar*) key;} return (hash);} uint calc_hashnr_caseup(const byte *key, uint len){const byte *end=key+len;uint hash; for (hash = 0; key < end; key++){hash *= 16777619;hash ^= (uint) (uchar) toupper(*key);} return (hash);}#endif

Mysql中对字符串Hash函数还区分了大小写

●另一个经典字符串Hash函数

unsigned int hash(char *str){register unsigned int h;register unsigned char *p; for(h=0, p = (unsigned char *)str; *p ; p++)h = 31 * h + *p; return h;}