CRC算法问题，请高手们帮我看看吧！！！！谢谢了！

Question

http://blog.chinaunix.net/uid-20416869-id-173134.html 随便找的博客地址，别人的

unsigned short do_crc(unsigned char *message, unsigned int len)
{
int i, j;
unsigned short crc_reg;

crc_reg = (message[0] << 8) + message[1];
for (i = 0; i < len; i++) 
{
    if (i < len - 2)
        for (j = 0; j <= 7; j++) 
        { 
            if ((short)crc_reg < 0)
                crc_reg = ((crc_reg << 1) + (message[i + 2] >> (7 - i))) ^ 0x1021;
            else 
                crc_reg = (crc_reg << 1) + (message[i + 2] >> (7 - i));      
        }
     else
        for (j = 0; j <= 7; j++) 
        { 
            if ((short)crc_reg < 0)
                crc_reg = (crc_reg << 1) ^ 0x1021;
            else 
                crc_reg <<= 1;             
        }         
}
return crc_reg;

}

显然，每次内循环的行为取决于寄存器首位。由于异或运算满足交换率和结合律，以及与0异或无影响，消息可以不移入寄存器，而在每次内循环的时候，寄存器首位再与对应的消息位异或。改进的代码如下：
unsigned short do_crc(unsigned char *message, unsigned int len)
{
int i, j;
unsigned short crc_reg = 0;
unsigned short current;

for (i = 0; i < len; i++) 
{
    current = message[i] << 8;
    for (j = 0; j < 8; j++) 
    { 
        if ((short)(crc_reg ^ current) < 0)
            crc_reg = (crc_reg << 1) ^ 0x1021;
        else 
            crc_reg <<= 1; 
        current <<= 1;            
    }
}
return crc_reg;

}

上面那个程序搞懂了，下面那个看不懂啊，怎么变成那样的！？帮我看看吧谢谢了！

Answer 1

https://zhidao.baidu.com/question/808804937123643132.html