几种经典的Hash算法的实现

几种经典的Hash算法的实现(源代码)
By Minidxer  January 27, 2008

 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 usrdictwords 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)((v2)^v)&0x0f;
ret=(ret(32-r));
ret&=0xFFFFFFFFL;
ret^=vv;
c++;
}

return((ret16)^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
 
 FowlerNollVo 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);
}
#endifMysql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;
}