開源軟體中 幾種經典的Hash演算法的實現

http://zh.wikipedia.org/wiki/%E5%93%88%E5%B8%8C%E8%A1%A8

http://en.wikipedia.org/wiki/Hash_table

 

 雜湊演算法將任意長度的二進位值對應為固定長度的較小二進位值，這個小的二進位值稱為雜湊值。雜湊值是一段資料唯一且極其緊湊的數值表示形式。如果散列一段明文而且哪怕只更改該段落的一個字母，隨後的雜湊都將產生不同的值。要找到散列為同一個值的兩個不同的輸入，在計算上是不可能的，所以資料的雜湊值可以檢驗資料的完整性。

 

 

鏈表尋找的時間效率為O(N)，二分法為log2N，B+ Tree為log2N，但Hash鏈表尋找的時間效率為O(1)。

設計高效演算法往往需要使用Hash鏈表，常數級的尋找速度是任何別的演算法無法比擬的，Hash鏈表的構造和衝突的不同實現方法對效率當然有一定的影響，然 而Hash函數是Hash鏈表最核心的部分，下面是幾款經典軟體中使用到的字串Hash函數實現，通過閱讀這些代碼，我們可以在Hash演算法的執行效率、離散性、空間利用率等方面有比較深刻的瞭解。

下面分別介紹幾個經典軟體中出現的字串Hash函數。

●PHP中出現的字串Hash函數

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中出現的字串Hash函數
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中出現的字串Hash函數
#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中對字串Hash函數還區分了大小寫
●另一個經典字串Hash函數
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;}
 
asterisk 伺服器
/*! * \brief Compute a hash value on a case-insensitive string * * Uses the same hash algorithm as ast_str_hash, but converts * all characters to lowercase prior to computing a hash. This * allows for easy case-insensitive lookups in a hash table. */static force_inline int attribute_pure ast_str_case_hash(const char *str){ int hash = 5381; while (*str) {  hash = hash * 33 ^ tolower(*str++); } return abs(hash);}