几种hash函数的汇总

翻帖子的时候无意间发现了几种hash函数，决定对其汇总一下。

ELFHASH，BKDRHash，SDBMHash，RSHash ，JSHash，PJWHash，DJBHash，APHash；

首先ELFhash。

ELFhash函数是对字符串的散列。它对于长字符串和短字符串都很有效，字符串中每个字符都有同样的作用，它巧妙地对字符的ASCII编码值进行计算，ELFhash函数对于能够比较均匀地把字符串分布在散列表中。具体做法是：将一个字符串的数组中的每个元素依次按前四位与上一个元素的低四位相与，组成一个长整形，如果长整的高四位大于零，那么就将它折回再与长整的低四位相异或，这样最后得到的长整对HASH表长取余，得到在HASH中的位置。   

unsigned long elf_hash(const unsigned char *name)

  {

      unsigned long       h = 0, g;

      while (*name) {

          h = (h << 4) + *name++;

          if (g = h & 0xf0000000)

              h ^= g >> 24;

          h &= ~g;

      }

      return h;

  }

BKDRHash。

unsigned int BKDRHash(char *str) { // BKDR Hash Function 

     unsigned int seed = 131; // 31 131 1313 13131 131313 etc.. 

     unsigned int hash = 0; 

     while (*str) { 

         hash = hash * seed + (*str++); 

     } 

     return (hash & 0x7FFFFFFF); 

}       

SDBMHash。

unsigned int SDBMHash(char *str)
{
	unsigned int hash = 0;
 
	while (*str)
	{
		// equivalent to: hash = 65599*hash + (*str++);
		hash = (*str++) + (hash << 6) + (hash << 16) - hash;
	}
 
	return (hash & 0x7FFFFFFF);
}
           

RShash。
           

unsigned int RSHash(char *str)
{
	unsigned int b = 378551;
	unsigned int a = 63689;
	unsigned int hash = 0;
 
	while (*str)
	{
		hash = hash * a + (*str++);
		a *= b;
	}
 
	return (hash & 0x7FFFFFFF);
}
           
    
JS Hash。
           
    
unsigned int JSHash(char *str)
{
	unsigned int hash = 1315423911;
 
	while (*str)
	{
		hash ^= ((hash << 5) + (*str++) + (hash >> 2));
	}
 
	return (hash & 0x7FFFFFFF);
}
           
    
PJhash。
           
    
unsigned int PJWHash(char *str)
{
	unsigned int BitsInUnignedInt = (unsigned int)(sizeof(unsigned int) * 8);
	unsigned int ThreeQuarters	= (unsigned int)((BitsInUnignedInt  * 3) / 4);
	unsigned int OneEighth		= (unsigned int)(BitsInUnignedInt / 8);
	unsigned int HighBits		 = (unsigned int)(0xFFFFFFFF) << (BitsInUnignedInt - OneEighth);
	unsigned int hash			 = 0;
	unsigned int test			 = 0;
 
	while (*str)
	{
		hash = (hash << OneEighth) + (*str++);
		if ((test = hash & HighBits) != 0)
		{
			hash = ((hash ^ (test >> ThreeQuarters)) & (~HighBits));
		}
	}
 
	return (hash & 0x7FFFFFFF);
}
           
    
DJhash。
           
    
unsigned int DJBHash(char *str)
{
	unsigned int hash = 5381;
 
	while (*str)
	{
		hash += (hash << 5) + (*str++);
	}
 
	return (hash & 0x7FFFFFFF);
}
           
    
APhash。
           
    
unsigned int APHash(char *str)
{
	unsigned int hash = 0;
	int i;
 
	for (i=0; *str; i++)
	{
		if ((i & 1) == 0)
		{
			hash ^= ((hash << 7) ^ (*str++) ^ (hash >> 3));
		}
		else
		{
			hash ^= (~((hash << 11) ^ (*str++) ^ (hash >> 5)));
		}
	}
 
	return (hash & 0x7FFFFFFF);
}
           
    
呵呵 水平有限欢迎大家批评，需要使用hash函数时应该根据相应的领域进行测试吧。好像没有所有都非常适用的吧。