各种字符串Hash函数比较

装载自：http://www.byvoid.com/blog/string-hash-compare/

常用的字符串Hash函数还有ELFHash，APHash等等，都是十分简单有效的方法。这些函数使用位运算使得每一个字符都对最后的函数值产生影响。另外还有以MD5和SHA1为代表的杂凑函数，这些函数几乎不可能找到碰撞。

常用字符串哈希函数有BKDRHash，APHash，DJBHash，JSHash，RSHash，SDBMHash，PJWHash，ELFHash等等。对于以上几种哈希函数，我对其进行了一个小小的评测。

其中数据1为100000个字母和数字组成的随机串哈希冲突个数。数据2为100000个有意义的英文句子哈希冲突个数。数据3为数据1的哈希值与1000003(大素数)求模后存储到线性表中冲突的个数。数据4为数据1的哈希值与10000019(更大素数)求模后存储到线性表中冲突的个数。

经过比较，得出以上平均得分。平均数为平方平均数。可以发现，BKDRHash无论是在实际效果还是编码实现中，效果都是最突出的。APHash也是较为优秀的算法。DJBHash,JSHash,RSHash与SDBMHash各有千秋。PJWHash与ELFHash效果最差，但得分相似，其算法本质是相似的。

在信息修竞赛中，要本着易于编码调试的原则，个人认为BKDRHash是最适合记忆和使用的。

BYVoid原创，欢迎建议、交流、批评和指正。

附：各种哈希函数的C语言程序代码

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);
}
 
// RS Hash Function
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 Function
unsigned int JSHash(char *str)
{
	unsigned int hash = 1315423911;
 
	while (*str)
	{
		hash ^= ((hash << 5) + (*str++) + (hash >> 2));
	}
 
	return (hash & 0x7FFFFFFF);
}
 
// P. J. Weinberger Hash Function
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);
}
 
// ELF Hash Function
unsigned int ELFHash(char *str)
{
	unsigned int hash = 0;
	unsigned int x	= 0;
 
	while (*str)
	{
		hash = (hash << 4) + (*str++);
		if ((x = hash & 0xF0000000L) != 0)
		{
			hash ^= (x >> 24);
			hash &= ~x;
		}
	}
 
	return (hash & 0x7FFFFFFF);
}
 
// BKDR Hash Function
unsigned int BKDRHash(char *str)
{
	unsigned int seed = 131; // 31 131 1313 13131 131313 etc..
	unsigned int hash = 0;
 
	while (*str)
	{
		hash = hash * seed + (*str++);
	}
 
	return (hash & 0x7FFFFFFF);
}
 
// DJB Hash Function
unsigned int DJBHash(char *str)
{
	unsigned int hash = 5381;
 
	while (*str)
	{
		hash += (hash << 5) + (*str++);
	}
 
	return (hash & 0x7FFFFFFF);
}
 
// AP Hash Function
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);
}