常用的Hash算法

1、RSHash

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;

}

return hash;

}

2、JSHash

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;

}

3、PJWHash

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;

}

4、ELFHash

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;

}

5、BKDRHash

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;

}

6、SDBMHash

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;

}

7、DJBHash(times33)-这个用得非常多，很多库都用它。

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;

}

8、DEKHash

unsigned int DEKHash(const std::string& str)

{

unsigned int hash = static_cast(str.length());

for(std::size_t i = 0; i < str.length(); i++)

{

hash = ((hash << 5) ^ (hash >> 27)) ^ str[i];

}

return hash;

}

9、BPHash

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;

}

10、FNVHash

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;

}

11、APHash

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;

}

12、MurmurHash - 非常新的一个哈希算法，应该是目前效率最高的一个哈希算法，使用率很高。(伪代码来自维基百科）

Murmur2(key, len, seed)

m <- 0x5bd1e995

r <- 24

seed 0x9747b28c

hash <- seed XOR len

for each fourByteChunk of key

k <- fourByteChunk

k <- k * m

k <- k XOR (k >> r)

k <- k * m

hash <- hash * m

hash <- hash XOR k

with any remainingBytesInKey

remainingBytes SwapEndianOrderOf(remainingBytesInKey)

hash <- hash XOR remainingBytes

hash <- hash * m

hash <- hash XOR (hash >> 13)

hash <- hash * m

hash <- hash XOR (hash >> 15)

MurmurHash算法：高运算性能，低碰撞率，由Austin Appleby创建于2008年，现已应用到Hadoop、libstdc++、nginx、libmemcached等开源系统。2011年Appleby被Google雇佣，随后Google推出其变种的CityHash算法。

官方网站：https://sites.google.com/site/murmurhash/

MurmurHash算法，自称超级快的hash算法，是FNV的4-5倍。官方数据如下：

OneAtATime – 354.163715 mb/sec
FNV – 443.668038 mb/sec
SuperFastHash – 985.335173 mb/sec
lookup3 – 988.080652 mb/sec
MurmurHash 1.0 – 1363.293480 mb/sec
MurmurHash 2.0 – 2056.885653 mb/sec

但也有文章声称，只有当key的长度大于10字节的时候，MurmurHash的运算速度才快于DJB。“从计算速度上来看，MurmurHash只适用于已知长度的、长度比较长的字符”。

巴特西

常用的Hash算法

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