bloom filter + murmurhash

是一种hash方法，其实核心思想就是，将一个字符串通过多个普通hash函数映射到hash表上，然后再进行检索的时候同样计算hash函数，如果全都都hash表上出现过，那么说明有极大的可能出现过，如果没有出现那么一定没有出现过。

可以通过公式计算采取多少个普通hash函数和多大映射空间使正确率变得最低。

有一个称为万能hash函数的，看了下简介看起来很屌，所以纪录下来。

//-----------------------------------------------------------------------------
// MurmurHash2, 64-bit versions, by Austin Appleby

// The same caveats as 32-bit MurmurHash2 apply here - beware of alignment 
// and endian-ness issues if used across multiple platforms.

typedef unsigned long int uint64_t;

// 64-bit hash for 64-bit platforms
uint64_t MurmurHash64A ( const void * key, int len, unsigned int seed )
{
        const uint64_t m = 0xc6a4a7935bd1e995;
        const int r = 47;

        uint64_t h = seed ^ (len * m);

        const uint64_t * data = (const uint64_t *)key;
        const uint64_t * end = data + (len/8);

        while(data != end)
        {
                uint64_t k = *data++;

                k *= m; 
                k ^= k >> r; 
                k *= m; 

                h ^= k;
                h *= m; 
        }

        const unsigned char * data2 = (const unsigned char*)data;

        switch(len & 7)
        {
        case 7: h ^= uint64_t(data2[6]) << 48;
        case 6: h ^= uint64_t(data2[5]) << 40;
        case 5: h ^= uint64_t(data2[4]) << 32;
        case 4: h ^= uint64_t(data2[3]) << 24;
        case 3: h ^= uint64_t(data2[2]) << 16;
        case 2: h ^= uint64_t(data2[1]) << 8;
        case 1: h ^= uint64_t(data2[0]);
                h *= m;
        };
 
        h ^= h >> r;
        h *= m;
        h ^= h >> r;

        return h;
} 


// 64-bit hash for 32-bit platforms
uint64_t MurmurHash64B ( const void * key, int len, unsigned int seed )
{
        const unsigned int m = 0x5bd1e995;
        const int r = 24;

        unsigned int h1 = seed ^ len;
        unsigned int h2 = 0;

        const unsigned int * data = (const unsigned int *)key;

        while(len >= 8)
        {
                unsigned int k1 = *data++;
                k1 *= m; k1 ^= k1 >> r; k1 *= m;
                h1 *= m; h1 ^= k1;
                len -= 4;

                unsigned int k2 = *data++;
                k2 *= m; k2 ^= k2 >> r; k2 *= m;
                h2 *= m; h2 ^= k2;
                len -= 4;
        }

        if(len >= 4)
        {
                unsigned int k1 = *data++;
                k1 *= m; k1 ^= k1 >> r; k1 *= m;
                h1 *= m; h1 ^= k1;
                len -= 4;
        }

        switch(len)
        {
        case 3: h2 ^= ((unsigned char*)data)[2] << 16;
        case 2: h2 ^= ((unsigned char*)data)[1] << 8;
        case 1: h2 ^= ((unsigned char*)data)[0];
                        h2 *= m;
        };

        h1 ^= h2 >> 18; h1 *= m;
        h2 ^= h1 >> 22; h2 *= m;
        h1 ^= h2 >> 17; h1 *= m;
        h2 ^= h1 >> 19; h2 *= m;

        uint64_t h = h1;

        h = (h << 32) | h2;

        return h;
}