Hash算法不只是表面上的样子，它是有着很多种的，这次我们就来看看几种经典的hash算法。
例：
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 & 0xF0000000 L) != 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，使用率极高
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)以上就是本篇文章的所有内容，如若还需了解更多相关java架构师知识，欢迎关注本站了解详情。
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