C program of Cyclic Redundancy Check

C program of Cyclic Redundancy Check

 

 Program

#include<stdio.h>

// shorthand for loops
#define forn(i,e) for(int i = 0; i < e; i++)
#define forsn(i,s,e) for(int i = s; i < e; i++)

int validate_data(int len_data, int data[], int len_divisor, int divisor[]){

    // Returns 1, if the data is correct
    // Returns 0, if the data is incorrect/corrupted.

    int current_bits[len_divisor];

    forn(i, len_divisor) { current_bits[i] = data[i]; }
    
    // the logic is same as generating the crc.
    forn(i, len_data) {

        if (!current_bits[0]) {

            forn(j, len_divisor-1) { current_bits[j] = current_bits[j+1]; }
            current_bits[len_divisor-1] = data[i+len_divisor];

        } else {

            forn(j, len_divisor) { current_bits[j] = current_bits[j]^divisor[j]; }
            forn(j, len_divisor-1) { current_bits[j] = current_bits[j+1]; }
            current_bits[len_divisor-1] = data[i+len_divisor];
        }
    }

    // if all the bits upto length len_divisor-1 is 0, return 1, else return 0;
    forn(i, len_divisor-1) { if(current_bits[i]) return 0; }
    return 1;
}

int main(){

    int len_divisor=0, len_data=0, len_data_and_pad=0;

    printf("Enter the length of divisor > ");
    scanf("%d",&len_divisor);

    printf("Enter the length of data > ");
    scanf("%d",&len_data);

    // total length of data with padded CRC bits
    len_data_and_pad = len_data+len_divisor-1;

    int divisor[len_divisor], data[len_data_and_pad];

    printf("\nEnter the divisor :- \n");
    forn(i,len_divisor) { printf("> "); scanf("%d",&divisor[i]); }

    printf("\nEnter the data :- \n");
    forn(i,len_data) { printf("> "); scanf("%d",&data[i]); }

    // initialize paddings with zero
    forsn(i, len_data,len_data+len_divisor) { data[i] = 0; }

    int current_bits[len_divisor];

    forn(i, len_divisor) { current_bits[i] = data[i]; }

    forn(i, len_data) {

        // if first bit is zero, bits shift left by one place
        // new bit is added at the end from the data bits.

        if (!current_bits[0]) {
            
            forn(j, len_divisor-1) { current_bits[j] = current_bits[j+1]; }
            current_bits[len_divisor-1] = data[i+len_divisor];

        } else {

            // XOR current bits with divisor.
            forn(j, len_divisor) { current_bits[j] = current_bits[j]^divisor[j]; }
            forn(j, len_divisor-1) { current_bits[j] = current_bits[j+1]; }
            current_bits[len_divisor-1] = data[i+len_divisor];
        }
    }

    // replace padded data bits with the CRC available in current_bits[0:-2]
    forsn(i, len_data, len_data_and_pad) { data[i] = current_bits[i-len_data]; }

    printf("DATA > ");
    forn(i, len_data_and_pad) { printf("%d ", data[i]); }

    printf("\nCRC > ");
    forn(i, len_divisor-1) { printf("%d ", current_bits[i]); }

    // Receiver side...
    int check = validate_data(len_data, data, len_divisor, divisor);

    if (check){
        printf("\n\nThe data is correct.\n");
    } else {
        printf("\n\nThe data is corrupted !!\n");
    }

    return 0;
}