Dutor » Hash http://www.dutor.net 熟读而精思，循序而渐进，厚积而薄发。 Tue, 17 Jan 2012 14:44:19 +0000 en hourly 1 http://wordpress.org/?v=3.3.1 常用的几个字符串Hash函数 http://www.dutor.net/index.php/2009/11/string-hash/ http://www.dutor.net/index.php/2009/11/string-hash/#comments Tue, 17 Nov 2009 07:47:11 +0000 dutor http://www.dutor.net/?p=1670 // SDBM Hash Function unsigned int SDBMHash (char *str) { unsigned int hash = 0; while (*str) { hash = (*str++) + (hash << 6) + (hash << 16) - hash; } return (hash & 0x7FFFFFFF); } ]]> 
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// 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);
}
 
// SDBM Hash Function
unsigned int
SDBMHash (char *str)
{
    unsigned int hash = 0;
    while (*str)
      {
	  hash = (*str++) + (hash << 6) + (hash << 16) - hash;
      }
    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);
}

　　对最简单的SDBMHash(char*)做了一个简单的测试。随机生成200个长度为31的字符串，利用SDBM得到一个Hash值，然后用直接取余法插入长度为113的HashSet，为简单起见，只是将HashSet对应的位置进行加一。

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const int N = 32;
char str[N];
int HashSet[113] = {0};
void RandStr(char* str, const int n)
{
	int i = 0;
	for( ; i < n - 1; ++i)
	{
		str[i] = 'a' + rand() % 26;
	}
	str[i] = '\0';
}
 
int main()
{
	srand(time(NULL));
	const int count = 200;
	for(int i = 0; i < count; ++i)
	{
		RandStr(str, N);
		HashSet[SDBMHash(str)%113]++;
	}
	FILE* out = fopen("./result.txt", "w");
	//~ FILE* out = stdout;
	for(int i = 0; i < 113; ++i)
	{
		fprintf(out, "%d\n", HashSet[i]);
	}
	return 0;
}

　　测试效果还是很不错的，相当均匀了已经。

HashTest Diagram

HashTest Diagram

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