error_detection.java

// Write a program for error detecting code using CRC-CCITT (16- bits)


import java.util.Scanner;
import java.io.*;
public class CRC1 {

    public static void main(String args[]) {

    Scanner sc = new Scanner(System.in);

    //Input Data Stream
    System.out.print("Enter message bits: ");
    String message = sc.nextLine();
    System.out.print("Enter generator: ");
    String generator = sc.nextLine();
int data[] = new int[message.length() + generator.length() - 1];
int divisor[] = new int[generator.length()];
for(int i=0;i<message.length();i++)
    data[i] = Integer.parseInt(message.charAt(i)+"");
for(int i=0;i<generator.length();i++)
    divisor[i] = Integer.parseInt(generator.charAt(i)+"");

//Calculation of CRC
for(int i=0;i<message.length();i++)
{
    if(data[i]==1)
        for(int j=0;j<divisor.length;j++)
            data[i+j] ^= divisor[j];
}

//Display CRC
System.out.print("The checksum code is: ");
for(int i=0;i<message.length();i++)
    data[i] = Integer.parseInt(message.charAt(i)+"");
for(int i=0;i<data.length;i++) 
    System.out.print(data[i]);
System.out.println();

//Check for input CRC code
System.out.print("Enter checksum code: ");
    message = sc.nextLine();
System.out.print("Enter generator: ");
    generator = sc.nextLine();
data = new int[message.length() + generator.length() - 1];
divisor = new int[generator.length()];
for(int i=0;i<message.length();i++)
    data[i] = Integer.parseInt(message.charAt(i)+"");
for(int i=0;i<generator.length();i++)
    divisor[i] = Integer.parseInt(generator.charAt(i)+"");

//Calculation of remainder
for(int i=0;i<message.length();i++) {
    if(data[i]==1)
        for(int j=0;j<divisor.length;j++)
            data[i+j] ^= divisor[j];
}

//Display validity of data
boolean valid = true;
for(int i=0;i<data.length;i++)
    if(data[i]==1){
        valid = false;
        break;
}

if(valid==true) 
    System.out.println("Data stream is valid");
else 
    System.out.println("Data stream is invalid. CRC error occurred.");
}

}