C语言实现CRC算法实例改进

/* 本程序实现CRC算法，编译环境为turbo C 2.0 */

/*
改进1:增加可输入序列长度,数据+多项式序列总长增至32位(unsigned long位长)；
改进2:增加了一些检验措施；
改进3:直接得出校验码(和余数)。
参考测试数据：
1.M=10101010,G=111
2.M=10101010,G=1
3.M=10101010,G=011
4.M=12345678,G=111
5.M=10101010,G=ABC
*/

#include<stdio.h>
int main()
{
void seq_scanf(unsigned long *s,int *n); /*序列输入函数申明*/
void crc(unsigned long M,unsigned long G,int mN,int gN);
unsigned long M=0,G=0;
int mN=0,gN=0;

printf("/n/n/nPlease input the data sequence:/n"); /*输入数据序列*/
seq_scanf(&M,&mN);
if(mN<1)
{printf("/n/nThe data sequence's length is too short!");exit(1);}
printf("/nPlease input the generator polynomial sequence:/n"); /*输入多项式序列*/
seq_scanf(&G,&gN);
if(gN<=1)
{printf("/n/nThe generator polynomial sequence's length is too short!");exit(1);}
crc(M,G,mN,gN); /*求crc校验码*/
return 0;
}

void seq_scanf(unsigned long *s,int *n)
{
/*s存储输入序列(按位存储),n存储序列长度*/
char c;
while((c=getchar())!='/n') /*检测到回车时退出*/
{
if(c<48||c>49)
{printf("/nNot valid sequence,you must input the sequence with 0 and 1./n/n");
exit(1);} /*序列必须由0和1组成*/
if(*s<1&&c==48) continue; /*当输入序列开头为0时，忽略*/
if(*s<1) *s=c-48; /*-48：由字符转位存储*/
else *s=*s*2+c-48;
(*n)++;
}
}

void crc(unsigned long M,unsigned long G,int mN,int gN)
{
/* M是数据项，G是生成多项式，皆为无符长整型，mN，gN分别为M,G的有效位长度*/
void result_printf(unsigned long s,int n);
unsigned long g,MASK=1; /*MASK的作用：确认生成多项式G该向右移多少位*/
int i;

g=G;
M<<=gN-1;
G<<=mN-1;
MASK<<=gN+mN-2;
for(M=M^G;M!=0;M=M^G) /*for语句，模2除法的实现*/
{
for(i=0;(M&MASK)!=MASK;MASK>>=1) i++;
G>>=i;
if(g>G) break; /*检验G是否已移至末尾，是则退出*/
}
printf("/nThe remainder is:");
result_printf(M,gN-1); /* 求余数,即校验码*/
}

void result_printf(unsigned long s,int n)
{
/*输出数据s的存储单元中的后n位*/
if(n!=1) result_printf(s/2,n-1);
printf("%d",s%2);
}