//Cyclic Redundancy Check (CRC) - Program:
import java.io.*;
class crc_gen
{
    public static void main(String args[]) throws IOException
    {
        BufferedReader br=new BufferedReader(new InputStreamReader(System.in));
        int[] data;
        int[] div;
        int[] divisor;
        int[] rem;
        int[] crc;
        int data_bits, divisor_bits, tot_length;
        
        System.out.println("Enter number of data bits : ");
        data_bits=Integer.parseInt(br.readLine());
        data=new int[data_bits];

        System.out.println("Enter data bits : ");
        for(int i=0; i<data_bits; i++)
            data[i]=Integer.parseInt(br.readLine());

        System.out.println("Enter number of bits in divisor : ");
        divisor_bits=Integer.parseInt(br.readLine());
        divisor=new int[divisor_bits];
        
        System.out.println("Enter Divisor bits : ");
        for(int i=0; i<divisor_bits; i++)
            divisor[i]=Integer.parseInt(br.readLine());

       System.out.print("Data bits are : ");
        for(int i=0; i< data_bits; i++)
            System.out.print(data[i]);        
        System.out.println();
        
        System.out.print("divisor bits are : ");
        for(int i=0; i< divisor_bits; i++)
            System.out.print(divisor[i]);        
        System.out.println();
        
      tot_length=data_bits+divisor_bits-1;
        
        div=new int[tot_length];
        rem=new int[tot_length];
        crc=new int[tot_length];
    /*------------------ CRC GENERATION-----------------------*/    
        for(int i=0;i<data.length;i++)
            div[i]=data[i];
        
        System.out.print("Dividend (after appending 0's) are : ");
        for(int i=0; i< div.length; i++)
            System.out.print(div[i]);        
        System.out.println();
        
        for(int j=0; j<div.length; j++){
              rem[j] = div[j];
        }
    
        rem=divide(div, divisor, rem);
        
        for(int i=0;i<div.length;i++)           //append dividend and remainder
        {
            crc[i]=(div[i]^rem[i]);
        }
        
        System.out.println();
        System.out.println("CRC code : ");    
        for(int i=0;i<crc.length;i++)
            System.out.print(crc[i]);
            
    /*-------------------ERROR DETECTION---------------------*/    
        System.out.println();
        System.out.println("Enter CRC code of "+tot_length+" bits : ");
        for(int i=0; i<crc.length; i++)
            crc[i]=Integer.parseInt(br.readLine());
        
               System.out.print("crc bits are : ");
        for(int i=0; i< crc.length; i++)
            System.out.print(crc[i]);        
        System.out.println();      
        for(int j=0; j<crc.length; j++){
              rem[j] = crc[j];
        }
    
        rem=divide(crc, divisor, rem);
        
        for(int i=0; i< rem.length; i++)
        {
            if(rem[i]!=0)
            {
                System.out.println("Error");
                break;
            }
            if(i==rem.length-1)
                System.out.println("No Error");
        }
        
        System.out.println("THANK YOU.... :)");
    }
    
    static int[] divide(int div[],int divisor[], int rem[])
    {
        int cur=0;
        while(true)
        {
            for(int i=0;i<divisor.length;i++)
                rem[cur+i]=(rem[cur+i]^divisor[i]);
            
            while(rem[cur]==0 && cur!=rem.length-1)
                cur++;
    
            if((rem.length-cur)<divisor.length)
                break;
        }
        return rem;
    }
}

/**
 * Enter number of data bits : 
4
Enter data bits : 
1
0
1
0
Enter number of bits in divisor : 
2
Enter Divisor bits : 
1
0
Data bits are : 1010
divisor bits are : 10
Dividend (after appending 0's) are : 10100

CRC code : 
10100
Enter CRC code of 5 bits : 
1
0
1
0
0
crc bits are : 10100
No Error
THANK YOU.... :)
 */