几种经典的Hash算法的实现(源代码)

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; i

ret^=(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 ([e-mail]) and

* Glenn Fowler ([e-mail]). Landon Curt Noll ([e-mail])

* 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;

}