crc.c

#include <stdio.h>
#include <string.h>
#define N strlen(g)
// declare the header libraries
char t[50], cs[50], g[50];
int a, e, c;
void xor () {
  for (c = 1; c < N; c++)
    //
    cs[c] = ((cs[c] == g[c]) ? '0' : '1');
  // Checking the XOR operation. If both operands are same, then output will b
  // "0" otherwise its "1".
} void crc() {
  for (e = 0; e < N; e++)
    // Consider only first FIVE bits from the modified data
    cs[e] = t[e];
  // Copy those first FIVE bits to CHECKSUM cs[e] from t[e]
  do {
    if (cs[0] == '1')
      // If first leftmost bit is 1 then perform XOR operation
      xor();
    // Calling XOR function
    for (c = 0; c < N - 1; c++)
      // Performing XOR operation at the first iteration for FIVE bits (0 to
      // N-1)
      cs[c] = cs[c + 1];
    // Peform the same for all the data by right shift by 1
    cs[c] = t[e++];
  } while (e <= a + N - 1);
  // Continue the operation for the entire data.
}
int main() {
  printf("\n Enter the data: ");
  // Enter the data as 1101011011
  scanf("%s", t);
  // Data stored in a string t
  printf("\n Enter the generator polynomial: ");
  scanf("%s", g);
  // Enter the generator polynomial: Since we have hard coded the GP as 10011
  a = strlen(t);
  // "a" defines the total length of the data
  for (e = a; e < a + N - 1; e++)
    // Appending N-1 zeros to the data where N is the length of the GP
    t[e] = '0';
  // t[e] defines appending zeros from e=a;e<a+N-1;e++
  printf("\n Modified data is: %s", t);
  // MOdified data is 11010110110000
  crc();
  // Call CRC function
  printf("\n Checksum is: %s", cs);
  // Print the checksum after XOR operation
  for (e = a; e < a + N - 1; e++)
    // To append the checksum value instead of N-1 zeros in total length of the
    // data
    t[e] = cs[e - a];
  // The remodified data with checksum (FINAL CODEWORD)
  printf("\n The final codeword is: %s", t);
  // Print the final codeword
  printf("\n Test error detection: 0 for YES and 1 for NO: ");
  // To check for error detection
  scanf("%d", &e);
  if (e == 0)
  // If the value of "e" is 0
  {
    do {
      printf("\n Enter the position where error is to be inserted: ");
      // Specify the position
      scanf("%d", &e);
      // Say for example, e=6
    } while (e == 0 || e > a + N - 1);
    // WHILE states the boundary, means ranging for 0 to a+N-1
    t[e - 1] = (t[e - 1] == '0') ? '1' : '0';
    // Changing the bit from 0 to 1 and vice versa for error detection
    printf("\n Erroneous data: %s\n", t);
  }
  crc();
  for (e = 0; (e < N - 1) && (cs[e] != '1'); e++);
  // If CHECKSUM is not equal to 1 then error is detected else no error
  if (e < N - 1)
    printf("\n Error detected \n \n");
  else
    printf("\n No error detected \n \n");
  return 0;
}