哈希函数

希实现原理和常用算法
所有的字符串哈希算法都是基于对字符编码的迭代运算，只是运算规则不同而已。
1）BKDRHash算法
// BKDR Hash Function
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);
}
关于BKDRHash算法的较为详细的解析可以参考BKDRHash详解
2）APHash算法
// AP Hash Function
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);
}
3）DJBHash算法
// DJB Hash Function
unsigned int DJBHash(char *str)
{
    unsigned int hash = 5381;
 
    while (*str)
    {
        hash += (hash << 5) + (*str++);
    }
 
    return (hash & 0x7FFFFFFF);
}

4）JSHash算法
// JS Hash Function
unsigned int JSHash(char *str)
{
    unsigned int hash = 1315423911;
 
    while (*str)
    {
        hash ^= ((hash << 5) + (*str++) + (hash >> 2));
    }
 
    return (hash & 0x7FFFFFFF);
}

5）RSHash算法
// RS Hash Function
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);
}

6）SDBMHash算法
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);
}

7）PJWHash算法
// P. J. Weinberger Hash Function
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);
}

8）ELFHash算法
// ELF Hash Function
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);

转载：https://blog.csdn.net/mylinchi/article/details/79508112