常见的字符串Hash

常用字符串哈希函数有BKDRHash，APHash，DJBHash，JSHash，RSHash，SDBMHash，PJWHash，ELFHash等等。

//SDBM Hash
unsigned int SDBMHash(char *str)
{
    unsigned int hash = 0;
    while (*str) {
        // equivalent to: hash = 65599*hash + (*str++); 
        hash = (*str++) + (hash << 6) + (hash << 16) - hash;
    }

    return (hash & 0x7FFFFFFF);
}

// RS Hash  
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  
unsigned int JSHash(char *str)
{
    unsigned int hash = 1315423911;

    while (*str) {
        hash ^= ((hash << 5) + (*str++) + (hash >> 2));
    }

    return (hash & 0x7FFFFFFF);
}

// P. J. Weinberger Hash   
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  
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   
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);
}

// DJB Hash  
unsigned int DJBHash(char *str)
{
    unsigned int hash = 5381;

    while (*str) {
        hash += (hash << 5) + (*str++);
    }

    return (hash & 0x7FFFFFFF);
}

// AP Hash  
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);
}