循环冗余校验_循环冗余校验码计算_循环冗余校验 java实现(3)

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;

}

以上的讨论中，消息的每个字节都是先传输MSB，CRC16-CCITT标准却是按照先传输LSB，消息右移进寄存器来计算的。只需将代码改成判断寄存器的LSB，将0x1021按位颠倒后(0x8408)与寄存器异或即可，如下所示：

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];

for (j = 0; j < 8; j)

{

if ((crc_reg ^ current) & 0x0001)

crc_reg = (crc_reg >> 1) ^ 0x8408;

else

crc_reg >>= 1;

current >>= 1;

}

}

return crc_reg;

}

该算法使用了两层循环，对消息逐位进行处理，这样效率是很低的。为了提高时间效率，通常的思想是以空间换时间。考虑到内循环只与当前的消息字节和crc_reg的低字节有关，对该算法做以下等效转换：

unsigned short do_crc(unsigned char *message, unsigned int len)

{

int i, j;

unsigned short crc_reg = 0;

unsigned char index;

unsigned short to_xor;

for (i = 0; i < len; i)

{

index = (crc_reg ^ message[i]) & 0xff;

to_xor = index;

for (j = 0; j < 8; j)

{

if (to_xor & 0x0001)

to_xor = (to_xor >> 1) ^ 0x8408;

else

to_xor >>= 1;

}

crc_reg = (crc_reg >> 8) ^ to_xor;

}

return crc_reg;

}

现在内循环只与index相关了，可以事先以数组形式生成一个表crc16_ccitt_table，使得to_xor = crc16_ccitt_table[index]，于是可以简化为：

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