crc-ccitt is based on the polynomial x^16+x^12+x^5+1

crc-ccitt is based on the polynomial x^16+x^12+x^5+1

How to write a C Program crc-ccitt which based on the polynomial x^16+x^12+x^5+1 in C Programming Language ?

The prescription for determining the mask to use for a given polynomial is as follows:

                 1.  Represent the polynomial by a 17-bit number
                2.  Assume that the most and least significant bits are 1
                3.  Place the right 16 bits into an integer

Solution:

  1. /* compute crc's */
  2. /* crc-ccitt is based on the polynomial x^16+x^12+x^5+1 */
  3. /* The prescription for determining the mask to use for a given polynomial
  4.         is as follows:
  5.                 1.  Represent the polynomial by a 17-bit number
  6.                 2.  Assume that the most and least significant bits are 1
  7.                 3.  Place the right 16 bits into an integer
  8. */
  10. #include        <stdio.h>
  11. #include        <stdlib.h>
  12. #include        <string.h>
  13. #include        <time.h>
  15. #define         MTT     0x1021          /* crc-ccitt mask */
  16. #define     CLK_TCK CLOCKS_PER_SEC
  18. /* function declarations */
  19. unsigned short int updcrc(unsigned short int,int,unsigned short int);
  20. void perr(char *);
  22. /* variables */
  23. char filename[100];
  24. unsigned short int crctt;
  25. int ch;
  26. unsigned long num;
  27. FILE *fp;
  29. /* driver */
  30. int main(argc,argv)
  31. int argc; char **argv;
  32. {
  33.  clock_t start,end;
  34.         if(argc>2) perr("Usage:  ccitt [filename]");
  35.         if(argc==2) strcpy(filename,argv[1]);
  36.         else
  37.         {
  38.                 printf("\nEnter filename:  ");
  39.                 gets(filename);
  40.         }
  41.         if((fp=fopen(filename,"rb"))==NULL) perr("Can't open file");
  42.         num=0L; crctt=0;
  43.         start=clock();
  44.        
  45.         while((ch=fgetc(fp))!=EOF)
  46.         {
  47.                 //if (ch == '\n')
  48.                 //      continue;
  49.                 num++;
  50.                 //int i; for (i=0;i<10000;i++){
  51.                 crctt=updcrc(crctt,ch,MTT);
  52.                 //}
  53.         }
  54.         end=clock();
  55.         fclose(fp);
  56.         printf("\nNumber of bytes = %lu\n CRCTT = %04X\n",      num,crctt);
  57.         printf("Czas trwania: %f\n",(double)(end-start)/CLK_TCK);
  58.         return 0;
  59. }
  61. /* update crc */
  62. unsigned short int updcrc(crc,c,mask)
  63. unsigned short int crc,mask; int c;
  64. {
  65.         int i;
  66.         c<<=8;
  67.         for(i=0;i<8;i++)
  68.         {
  69.                 if((crc ^ c) & 0x8000) crc=(crc<<1)^mask;
  70.                 else crc<<=1;
  71.                 c<<=1;
  72.         }
  73.         return crc;
  74. }
  76. /* error abort */
  77. void perr(s)
  78. char *s;
  79. {
  80.         printf("\n%s",s); exit(1);
  81. }


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