將16進制unsigned char數組轉換成整數

/** 
 * 將unsigned char數組轉換成long long數值
 * {0x00 0x00 0x20 0x00}轉換之后得到8192
 *  
 * @param str 數組 
 * @param len str數組長度
 * @returns 0 on error
 */
LONGLONG PNGProcesser::readCharToLL(unsigned char* str, int len)
{
    if (len <= 0)
    {
        return 0;
    }

    int i = 0;
    int value = 0;
    for (i=0;i<len;i++)
    {
        value = value*16*16 + (str[i]/16)*16 + str[i]%16;
    }

    return value;
}