将16进制unsigned char数组转换成整数

/** 
 * 将unsigned char数组转换成long long数值
 * {0x00 0x00 0x20 0x00}转换之后得到8192
 *  
 * @param str 数组 
 * @param len str数组长度
 * @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;
}