/*
 * Kevin Garcia
 * CIS 3360 CRC PROGRAM
 * Dr. Yuechun Chu
 */

package crcprototype;

import java.io.*;

class Checksum {

    /*

userHex is the message entered by user.

divisor is the polynomial entered which will be used for Xoring

crc is the CRC(Checksum) calculated

tempDVD is used to generate the Initial padded message which can be used to calculate CRC

dvdLen is the Length of Dividend

divLen is the length of the divisor

     */
    String userHex, crc, tempDVD;

    private static String divisor = "1100110110101";

    int dvdLen, divLen, i;

    int initialLength;

    BufferedReader br = new BufferedReader(new InputStreamReader(System.in));

    int padding = 0;//This holds the number of red zeros to be added. 0 represents the red zeros

    void printDivisor(String data) {

//Print the divisor
        if (data.length() >= divLen) {

            System.out.print("\t\t\t");

            for (i = 0; i < padding - 1; i++) {

                System.out.print(" ");

            }

            System.out.println("0" + divisor);

        }

    }

    void printXorResult(String data) {

//Print XOR result in the specified format

    System.out.print("result is \t\t0");

        for (i = 0; i < padding; i++) {

            System.out.print("0");

        }

        if (data.length() > divLen) {

            System.out.print(data.substring(0, divLen));

        } else {

            System.out.print(data);

        }

        for (i = (padding + divLen); i < initialLength; i++) {

            System.out.print("0");

        }

        System.out.println();

    }

    void calculateCRC(String data) {

        String procuserHex;//The string to be used for Xoring

        int temp;

        dvdLen = data.length();

        initialLength = dvdLen;//To keep a record of initial length of dividend since dvdLen changes again and again

        while (dvdLen >= divLen) {

            procuserHex = data.substring(0, divLen);//Get the bits to be used for Xoring

//Perform Xoring by converting them into integers
            temp = Integer.parseInt(procuserHex, 2) ^ Integer.parseInt(divisor, 2);

            padding += divLen - Integer.toString(temp, 2).length();//Check the number of Red 0's to be padded(*0 in this case)

            if (temp != 0) {

                data = Integer.toString(temp, 2) + data.substring(divLen, data.length());

//Bring down more bits if necessary
            } else {

                data = data.substring(divLen, data.length());

//The remainder is 0 so get more bits from dividend
            }

            printXorResult(data);

            printDivisor(data);

//Integer.toString(temp,2) or Integer.toBinaryString() will do the same thing
            dvdLen = data.length();

        }

//Append 0s to the Checksum to generate a valid checksum which is same in length as the divisor
        if (data.length() < (divLen - 1)) {

            while (data.length() != (divLen - 1)) {

                data = "0" + data;

            }

        }

        crc = data;
        System.out.println(">>> CRC is : 0000000000000" + data + "("
                + Integer.toHexString(Integer.parseInt(crc, 2)).toLowerCase()
                + ")");

    }

    void sender() throws Exception {
        System.out.println("=====CALCULATE CRC======");
        System.out.print("Enter Hexadecimal: ");

        userHex = br.readLine();

        System.out.print("\nMsg is: " + userHex);

//Convert the input hex to binary
        String hexPart = userHex.substring(2);//Get the Hex part since the input is padded with 0x

        userHex = Integer.toBinaryString(Integer.parseInt(hexPart, 16));

//userHex = "0" + userHex;
//System.out.print("Enter Divisor ");
//divisor = br.readLine();
        System.out.print(" (" + userHex + ")\n");
        System.out.println(">>>>>>");

        divLen = divisor.length();

        i = 0;

        tempDVD = userHex;

//Generate the padded message
        while (i < (divLen - 1)) {

            tempDVD = tempDVD.concat("0");
            i++;

        }

        System.out.println("padmsg is 0" + tempDVD);

        System.out.println("polnom is 0" + divisor + "0000000000");//X Represents XOR Operation

        dvdLen = tempDVD.length();

        calculateCRC(tempDVD);

        System.out.println("Final result is " + Integer.toHexString(Integer.parseInt(userHex + crc, 2)).toLowerCase());

    }

    void receiver() throws Exception {

        padding = 0;

        System.out.println("\n=====VERIFY CRC======");

        System.out.print("Enter the Message Received: ");

        String recData = br.readLine();

        String hexPart = recData.substring(2);//Get the Hex part since the input is padded with 0x

        recData = Integer.toBinaryString(Integer.parseInt(hexPart, 16));

        System.out.println("padmsg is 0" + tempDVD);

        //System.out.println("Received Data Is " + hexPart);
        calculateCRC(recData);

        if (Integer.parseInt(crc) == 0)//If the CRC is 0 then there is no error
        {

            System.out.println("Message has been received Without Error");

        } else {

            System.out.println("Message has been received With Error");

        }

    }

}

public class CRCPrototype {

    public static void main(String args[]) throws Exception {

        Checksum cc = new Checksum();

        cc.sender();

        cc.receiver();

    }

}