some hash function

hash_function

// 一种简单快捷的hash算法
unsigned int BKDRHash(char *str) { 
    register unsigned int hash = 0; 
    while (unsigned int ch = (unsigned int)*str++) 
        hash = hash * 131 + ch;   // 也可以乘以31、131、1313、13131、131313..  
    return hash; 
}   

// 开源项目SDBM使用
unsigned int SDBMHash(char*str) {
    register unsigned int hash = 0;
    while (unsigned int ch = (unsigned int)*str++)
        hash = 65599 * hash + ch;  
    return hash;
}

// Unix system系统中使用的一种著名hash算法，后来微软也在其hash_map中实现。
unsigned int FNVHash(char* str) {
    register unsigned int hash = 2166136261;
    while (unsigned int ch = (unsigned int)*str++) {
        hash *= 16777619;
        hash ^= ch;
    }
    return hash;
}

unsigned int APHash(char *str) {  
    register 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);  
}

// RS Hash Function  
unsigned int RSHash(char *str) {  
    register unsigned int b = 378551;  
    register unsigned int a = 63689;  
    register unsigned int hash = 0;  
    while (*str) {  
        hash = hash * a + (*str++);  
        a *= b;  
    }  
   return (hash & 0x7FFFFFFF);  
}