一些常用的字符串hash函数

unsigned int RSHash(const std::string& str)
{
unsigned int b    = 378551;
unsigned int a    = 63689;
unsigned int hash = 0;

for(std::size_t i = 0; i < str.length(); i++)
{
hash = hash * a + str[i];
a    = a * b;  // 这保证对于每个字符串  处理出的值  相同的概率很小  a 可能会自然爆 long long 没关系  爆就让它爆

} return hash; } /* End Of RS Hash Function */

unsigned int JSHash(const std::string& str)
{
unsigned int hash = 1315423911;

for(std::size_t i = 0; i < str.length(); i++)
{
hash ^= ((hash << 5) + str[i] + (hash >> 2));
}

return hash;
}
/* End Of JS Hash Function */

unsigned int PJWHash(const std::string& str)
{
unsigned int BitsInUnsignedInt = (unsigned int)(sizeof(unsigned int) * 8);
unsigned int ThreeQuarters     = (unsigned int)((BitsInUnsignedInt  * 3) / 4);
unsigned int OneEighth         = (unsigned int)(BitsInUnsignedInt / 8);
unsigned int HighBits          = (unsigned int)(0xFFFFFFFF) << (BitsInUnsignedInt - OneEighth);
unsigned int hash              = 0;
unsigned int test              = 0;

for(std::size_t i = 0; i < str.length(); i++)
{
hash = (hash << OneEighth) + str[i];

if((test = hash & HighBits)  != 0)
{
hash = (( hash ^ (test >> ThreeQuarters)) & (~HighBits));
}
}

return hash;
}
/* End Of  P. J. Weinberger Hash Function */

unsigned int ELFHash(const std::string& str)
{
unsigned int hash = 0;
unsigned int x    = 0;

for(std::size_t i = 0; i < str.length(); i++)
{
hash = (hash << 4) + str[i];
if((x = hash & 0xF0000000L) != 0)
{
hash ^= (x >> 24);
}
hash &= ~x;
}

return hash;
}
/* End Of ELF Hash Function */

unsigned int BKDRHash(const std::string& str)
{
unsigned int seed = 131; // 31 131 1313 13131 131313 etc..
unsigned int hash = 0;

for(std::size_t i = 0; i < str.length(); i++)
{
hash = (hash * seed) + str[i];
}

return hash;
}
/* End Of BKDR Hash Function */

unsigned int SDBMHash(const std::string& str)
{
unsigned int hash = 0;

for(std::size_t i = 0; i < str.length(); i++)
{
hash = str[i] + (hash << 6) + (hash << 16) - hash;
}

return hash;
}
/* End Of SDBM Hash Function */

unsigned int DJBHash(const std::string& str)
{
unsigned int hash = 5381;

for(std::size_t i = 0; i < str.length(); i++)
{
hash = ((hash << 5) + hash) + str[i];
}

return hash;
}
/* End Of DJB Hash Function */

unsigned int DEKHash(const std::string& str)
{
unsigned int hash = static_cast<unsigned int>(str.length());

for(std::size_t i = 0; i < str.length(); i++)
{
hash = ((hash << 5) ^ (hash >> 27)) ^ str[i];
}

return hash;
}
/* End Of DEK Hash Function */

unsigned int BPHash(const std::string& str)
{
unsigned int hash = 0;
for(std::size_t i = 0; i < str.length(); i++)
{
hash = hash << 7 ^ str[i];
}

return hash;
}
/* End Of BP Hash Function */

unsigned int FNVHash(const std::string& str)
{
const unsigned int fnv_prime = 0x811C9DC5;
unsigned int hash = 0;
for(std::size_t i = 0; i < str.length(); i++)
{
hash *= fnv_prime;
hash ^= str[i];
}

return hash;
}
/* End Of FNV Hash Function */

unsigned int APHash(const std::string& str)
{
unsigned int hash = 0xAAAAAAAA;

for(std::size_t i = 0; i < str.length(); i++)
{
hash ^= ((i & 1) == 0) ? (  (hash <<  7) ^ str[i] * (hash >> 3)) :
(~((hash << 11) + (str[i] ^ (hash >> 5))));
}

return hash;
}
/* End Of AP Hash Function */
处哥曰：Hash 精髓所在 是 搞出来的数据别让他重复就好~~~