字符串哈希算法

经典字符串Hash函数

工作中经常需要用大hash这个强有力的工具，hash表最核心的部分则在于怎么设计一个好的hash函数，以使数据更均匀地分布在若干个桶上。下面来介绍一下我现在用到的一个hash函数，我们来看代码：

unsigned chostcachehash::get_host_key(const string& host)

{

int result = 1;

unsigned i = 0;

for (i = 0; i

result = 31 * result + host[i];

return abs(result);

}

inline unsigned getkey(unsigned key)

{

return (key % m_capacity);

}

m_capacity为hash桶的个数

对于一个字符串，我们首先调用get_host_key()来得到一个key，然后再用这个key调用getkey来得到他在hash桶里的位置。这个是我们在工作一直使用的hash函数，效果也还可以。今天忽然心血来潮，在网上搜了一下看还有没有更好的hash函数，被我发现了这篇文章，于是转过来看看。

php中出现的字符串hash函数

static unsigned long hashpjw(char *arkey，unsigned intnkeylength)

{

unsigned long h = 0，g;

char *arend=arkey+nkeylength;

while (arkey

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 %有点看不懂

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 == "{post.abstract}"))

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

#ifndefnew_hash_function

/* calchashvalue for a key */

static uintcalc_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);

}

/* calchashvalue for a key，case indepenently */

static uintcalc_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 * ieeeposix p1003.2 mailing list from phongvo (kpv@) and * glenn fowler (gsf@). landon curt noll (chongo@) * 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.

*/

uintcalc_hashnr(const byte *key，uintlen)