C语言实现CRC算法实例分析

/*  本程序实现CRC算法，编译环境为turbo C 2.0 */

#include
int main()
{
void crc(unsigned long M,unsigned long G);
unsigned long M,G;

printf("Please put 2 sequences:/n");
scanf("%lu%lu",&M,&G);
printf("/n/nThe data is =%lu,the generator polynomial is =%lu/n/n/n",M,G);
crc(M,G);
return 0;
}

void crc(unsigned long M,unsigned long G)
{
/* M是数据，G是生成多项式，皆为无符长整型*/
void trans2(unsigned long s,unsigned *t,int *n);
void result_printf(unsigned s,int n);
unsigned mD,gD,g;                       /*将输入的序列(10进制形式)用2进制mD,gD存储*/
int mN=0,gN=0;
int MASK=1,i;

trans2(M,&mD,&mN);
trans2(G,&gD,&gN);
g=gD;
mD<<=gN-1;
gD<<=mN-1;
MASK<<=gN+mN-2;
for(mD=mD^gD;mD!=0;mD=mD^gD)      /*for语句，模2除法的实现*/
{
  for(i=0;(mD&MASK)!=MASK;MASK>>=1) i++;  /*MASK的作用：确认生成多项式G该向右移多少位*/
  gD>>=i;
  if(g>gD) break;
}
printf("The check is:");
result_printf(mD,gN-1);
}

void trans2(unsigned long s,unsigned *t,int *n)  
/*10进制格式转换成2进制存储，
  s为待转换数，t为转换结果，n为转换后的2进制位数*/
{                  
    if(s<10) *t=s;
else {
  trans2(s/10,t,n);
  *t=(*t)*2+s%10;
  }
(*n)++;
}

void result_printf(unsigned s,int n)
{
/*输出结果，即余数*/
 if(n!=1) result_printf(s/2,n-1);
 printf("%d",s%2);
}