C语言CRC-16 X25格式校验函数
CRC-16校验产生2个字节长度的数据校验码，通过计算得到的校验码和获得的校验码比较，用于验证获得的数据的正确性。基本的CRC-16校验算法实现，参考： C语言标准CRC-16校验函数。
不同应用规范通过对输入数据前处理和输出数据后处理的方式不同，又产生了不同的应用规范校验函数，这里介绍X25格式的CRC-16校验函数。X25格式对输入数据，预置值为0XFFFF（当做最初的余数），输入数据按照单个字节进行位反序。对于输出的校验码，进行整体位反序, 然后异或0xFFFF。
生成多项式为x^16 + x^12 + x^5 + 1
正向算法
正向算法是符合标准CRC-16的计算理论，从左向右计算，也即计算过程中移位时，向左移出。几种正向算法的实现如下：
CRC-16 X25格式校验函数一(8位输入数据格式，64位装载计算)：
#include <stdio.h>
#include <stdlib.h>
uint16_t PY_CRC_16_X25(uint8_t *di, uint32_t len)
{
uint32_t crc_poly = 0x00011021; //X^16+X^12+X^5+1 total 17 effective bits. Computed total data shall be compensated 16-bit '0' before CRC computing.
uint8_t *datain;
uint64_t cdata = 0; //Computed total data
uint32_t data_t = 0; //Process data of CRC computing
uint16_t index_t = 63; ///bit shifting index for initial '1' searching
uint16_t index = 63; //bit shifting index for CRC computing
uint8_t rec = 0; //bit number needed to be compensated for next CRC computing
uint32_t cn=(len+2)/6;
uint32_t cr=(len+2)%6;
uint32_t j;
datain = malloc(len+2);
for(j=0;j<len;j++)
{
datain[j] = 0;
for(uint8_t m=0; m<=7; m++)
{
datain[j] |= ( ( di[j]>>(7-m) ) & 1 ) << m;
}
}
datain[len] = 0; datain[len+1] = 0;//Compensate 16-bit '0' for input data
datain[0] ^= 0xff; datain[1] ^= 0xff;
if(len<=6) //Mount data for only one segment
{
for(j=0;j<=(len+1);j++)
{
cdata = (cdata<<8);
cdata = cdata|datain[j];
}
cn = 1;
}
else
{
if(cr==0)
{
cr = 6;
}
else if(cr==1)
{
cr = 7;
}
else if(cr==2)
{
cr = 8;
}
else
{
cn++;
}
for(j=0;j<cr;j++)
{
cdata = (cdata<<8);
cdata = cdata|datain[j];
}
}
do
{
cn--;
while(index_t>0)
{
if( (cdata>>index_t)&1 )
{
index = index_t;
index_t = 0;
data_t |= (cdata>>(index-16));
{
data_t = data_t ^ crc_poly;
}
while((index!=0x5555)&&(index!=0xaaaa))
{
for(uint8_t n=1;n<17;n++)
{
if ((data_t>>(16-n))&1) {rec = n;break;}
if (n==16) rec=17;
}
if((index-16)<rec)
{
data_t = data_t<<(index-16);
data_t |= (uint32_t)((cdata<<(64-(index-16)))>>(64-(index-16)));
index = 0x5555;
}
else
{
for(uint8_t i=1;i<=rec;i++)
{
data_t = (data_t<<1)|((cdata>>(index-16-i))&1) ;
}
if(rec!= 17)
{
data_t = data_t ^ crc_poly;
index -= rec;
}
else
{
data_t = 0;
index_t = index-16-1;
index = 0xaaaa;
}
}
}
if(index==0x5555) break;
}
else
{
index_t--;
if(index_t<16) break;
}
}
if(cn>0) //next segment
{
cdata = data_t&0x00ffff;
for(uint8_t k=0;k<6;k++)
{
cdata = (cdata<<8);
cdata = cdata|datain[j++];
}
data_t = 0;
index_t = 63; ///bit shifting index for initial '1' searching
index = 63; //bit shifting index for CRC computing
rec = 0; //bit number needed to be compensated for next CRC computing
}
}
while(cn>0);
free(datain);
uint16_t i_data_t = 0;
for(uint8_t n=0; n<=15; n++)
{
i_data_t |= ( ( data_t>>(15-n) ) & 1 ) << n;
}
return i_data_t ^ 0xFFFF;
}
CRC-16 X25格式校验函数二（8位输入数据格式）：
uint16_t PY_CRC_16_S_X25(uint8_t *di, uint32_t len)
{
uint16_t crc_poly = 0x1021; //X^16+X^12+X^5+1 total 16 effective bits without X^16. Computed total data shall be compensated 16-bit '0' before CRC computing.
uint32_t clen = len+2;
uint8_t cdata[clen] ;
for(uint32_t j=0;j<len;j++)
{
cdata[j] = 0;
for(uint8_t m=0; m<=7; m++)
{
cdata[j] |= ( ( di[j]>>(7-m) ) & 1 ) << m;
}
}
cdata[len]=0; cdata[len+1]=0;
cdata[0] ^= 0xff; cdata[1] ^= 0xff;
uint16_t data_t = (((uint16_t)cdata[0]) << 8) + cdata[1]; //CRC register
for (uint32_t i = 2; i < clen; i++)
{
for (uint8_t j = 0; j <= 7; j++)
{
if(data_t&0x8000)
data_t = ( (data_t<<1) | ( (cdata[i]>>(7-j))&0x01) ) ^ crc_poly;
else
data_t = ( (data_t<<1) | ( (cdata[i]>>(7-j))&0x01) ) ;
}
}
uint16_t i_data_t = 0;
for(uint8_t n=0; n<=15; n++)
{
i_data_t |= ( ( data_t>>(15-n) ) & 1 ) << n;
}
return i_data_t ^ 0xFFFF;
}
CRC-16 X25格式校验函数三（16位输入数据格式）：
uint16_t PY_CRC_16_T16_X25(uint16_t *di, uint32_t len)
{
uint16_t crc_poly = 0x1021; //X^16+X^12+X^5+1 total 16 effective bits without X^16.
uint16_t data_t = 0; //CRC register
uint16_t cdata[len];
for(uint32_t j=0;j<len;j++)
{
cdata[j] = 0;
for(uint8_t m=0; m<=7; m++)
{
cdata[j] |= ( ( ( (di[j]>>8)>>(7-m) ) & 1 ) << m ) | ( ( ( ( (di[j]&0x00ff)>>(7-m) ) & 1 ) << m ) <<8 );
}
}
cdata[0] ^= 0xffff;
for(uint32_t i = 0; i < len; i++)
{
data_t ^= cdata[i]; //16-bit data
for (uint8_t j = 0; j < 16; j++)
{
if (data_t & 0x8000)
data_t = (data_t << 1) ^ crc_poly;
else
data_t <<= 1;
}
}
uint16_t i_data_t = 0;
for(uint8_t n=0; n<=15; n++)
{
i_data_t |= ( ( data_t>>(15-n) ) & 1 ) << n;
}
return i_data_t ^ 0xFFFF;
}
CRC-16 X25格式校验函数四（8位输入数据格式）：
uint16_t PY_CRC_16_T8_X25(uint8_t *di, uint32_t len)
{
uint16_t crc_poly = 0x1021; //X^16+X^12+X^5+1 total 16 effective bits without X^16.
uint16_t data_t = 0; //CRC register
uint8_t cdata[len];
for(uint32_t j=0;j<len;j++)
{
cdata[j] = 0;
for(uint8_t m=0; m<=7; m++)
{
cdata[j] |= ( ( di[j]>>(7-m) ) & 1 ) << m;
}
}
cdata[0] ^= 0xff; cdata[1] ^= 0xff;
for(uint32_t i = 0; i < len; i++)
{
data_t ^= cdata[i]<<8; //8-bit data
for (uint8_t j = 0; j < 8; j++)
{
if (data_t & 0x8000)
data_t = (data_t << 1) ^ crc_poly;
else
data_t <<= 1;
}
}
uint16_t i_data_t = 0;
for(uint8_t n=0; n<=15; n++)
{
i_data_t |= ( ( data_t>>(15-n) ) & 1 ) << n;
}
return i_data_t ^ 0xFFFF;
}
反向算法
反向算法是从由右向左计算，也即计算过程中移位时，向右移出。而计算过程中的输入数据高优先计算位和校验参数的对齐关系不变。因此把一个字节放在CRC计算寄存器的最低字节时，对于X25格式，最右侧最低位实际上是高优先计算位，而校验参数要相应倒序，从而计算位置对照关系不变。
CRC-16 X25格式校验函数五（反向算法，8位输入数据格式）：
uint16_t PY_CRC_16_T8_X25_i(uint8_t *di, uint32_t len)
{
uint16_t crc_poly = 0x8408; //Bit sequence inversion of 0x1021
uint16_t data_t = 0xFFFF; //CRC register
for(uint32_t i = 0; i < len; i++)
{
data_t ^= di[i]; //8-bit data
for (uint8_t j = 0; j < 8; j++)
{
if (data_t & 0x0001)
data_t = (data_t >> 1) ^ crc_poly;
else
data_t >>= 1;
}
}
return data_t ^ 0xFFFF;
}
算法验证
5种算法结果相同：
通过在线CRC工具对照验证成功：
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