/*

Program to implement CRC (Cyclic Redundancy Check) error detection algorithm in Java

Author: Manav Sanghavi Author Link: https://www.facebook.com/manav.sanghavi

www.pracspedia.com

*/

import java.util.*;

class CRC {

public static void main(String args[]) {

Scanner scan = new Scanner(System.in);

int n;

// Accept the input

System.out.println("Enter the size of the data:");

n = scan.nextInt();

int data[] = new int[n];

System.out.println("Enter the data, bit by bit:");

for(int i=0 ; i < n ; i++) {

System.out.println("Enter bit number " + (n-i) + ":");

data[i] = scan.nextInt();

}

// Accept the divisor

System.out.println("Enter the size of the divisor:");

n = scan.nextInt();

int divisor[] = new int[n];

System.out.println("Enter the divisor, bit by bit:");

for(int i=0 ; i < n ; i++) {

System.out.println("Enter bit number " + (n-i) + ":");

divisor[i] = scan.nextInt();

}

// Divide the inputted data by the inputted divisor

// Store the remainder that is returned by the method

int remainder[] = divide(data, divisor);

for(int i=0 ; i < remainder.length-1 ; i++) {

System.out.print(remainder[i]);

}

System.out.println("\nThe CRC code generated is:");

for(int i=0 ; i < data.length ; i++) {

System.out.print(data[i]);

}

for(int i=0 ; i < remainder.length-1 ; i++) {

System.out.print(remainder[i]);

}

System.out.println();

// Create a new array

// It will have the remainder generated by the above method appended

// to the inputted data

int sent_data[] = new int[data.length + remainder.length - 1];

System.out.println("Enter the data to be sent:");

for(int i=0 ; i < sent_data.length ; i++) {

System.out.println("Enter bit number " + (sent_data.length-i)

+ ":");

sent_data[i] = scan.nextInt();

}

receive(sent_data, divisor);

}

static int[] divide(int old_data[], int divisor[]) {

int remainder[] , i;

int data[] = new int[old_data.length + divisor.length];

System.arraycopy(old_data, 0, data, 0, old_data.length);

// Remainder array stores the remainder

remainder = new int[divisor.length];

// Initially, remainder's bits will be set to the data bits

System.arraycopy(data, 0, remainder, 0, divisor.length);

// Loop runs for same number of times as number of bits of data

// This loop will continuously exor the bits of the remainder and

// divisor

for(i=0 ; i < old_data.length ; i++) {

System.out.println((i+1) + ".) First data bit is : "

+ remainder[0]);

System.out.print("Remainder : ");

if(remainder[0] == 1) {

// We have to exor the remainder bits with divisor bits

for(int j=1 ; j < divisor.length ; j++) {

remainder[j-1] = exor(remainder[j], divisor[j]);

System.out.print(remainder[j-1]);

}

}

else {

// We have to exor the remainder bits with 0

for(int j=1 ; j < divisor.length ; j++) {

remainder[j-1] = exor(remainder[j], 0);

System.out.print(remainder[j-1]);

}

}

// The last bit of the remainder will be taken from the data

// This is the 'carry' taken from the dividend after every step

// of division

remainder[divisor.length-1] = data[i+divisor.length];

System.out.println(remainder[divisor.length-1]);

}

return remainder;

}

static int exor(int a, int b) {

// This simple function returns the exor of two bits

if(a == b) {

return 0;

}

return 1;

}

static void receive(int data[], int divisor[]) {

// This is the receiver method

// It accepts the data and divisor (although the receiver already has

// the divisor value stored, with no need for the sender to resend it)

int remainder[] = divide(data, divisor);

// Division is done

for(int i=0 ; i < remainder.length ; i++) {

if(remainder[i] != 0) {

// If remainder is not zero then there is an error

System.out.println("There is an error in received data...");

return;

}

}

//Otherwise there is no error in the received data

System.out.println("Data was received without any error.");

}

}