main.java

CRC via table lookup

/* Programmed By: Dhaval Tamboli
                  2493194
         Program: Program to compute the CRC of a message using CRC-16
*/
package crcmain;
import java.io.*;

public class Main {

     int crc16(char[] message)
    {
        int i,j,bytes,crc,mask;
        int[] table = new int[256];

        if(table[1]==0)
        {
            for(bytes=0;bytes<=255;bytes++)
            {
                crc = bytes;
                for(j=7;j>=0;j--)
                {
                    mask = -(crc & 1);
                    crc = (crc >> 1) ^ (0xA001 & mask);

                }
                table[bytes] = crc;
            }
        }

        i=0;
        crc = 0xFFFF;
        while (message[i] != 0)
        {
            bytes = message[i];
            crc = (crc >> 8) ^ table[(crc ^ bytes) & 0xFF];
            i = i+1;
        }
        return crc;
    }

    //Function Which calculates CRC16.
    int crc(char[] msg,int len)
    {

        int index;
        int crc=0xFFFF,genpoly=0xA001;
        int i=0,bytes;
        while(msg[i]!=0)
        {
            bytes = msg[i];
            crc = crc ^ bytes;
            for(index=0;index<8;index++)
            {
               if((crc & 0x0001)==1) crc = (crc >> 1) ^ genpoly;
               else crc =crc >> 1;
            }
            i++;
        }
        return crc;
}

    public static void main(String[] args) {

        String record = "";
        char[] array = new char[512];
        int j=0;
        int result;

        if(args.length != 1)
        {
            System.out.println("Please Enter File name");
            System.exit(1);
        }

        try
        {
            FileInputStream fis = new FileInputStream(args[0]);
            BufferedInputStream bis = new BufferedInputStream(fis);
            DataInputStream dis = new DataInputStream(bis);

            //Reads Data from file and stores in to Character Array
            while ( (record=dis.readLine()) != null )
            {
                int length = record.length();
                for(int i=0;i<length;i++)
                {
                    array[j]=record.charAt(i);
                    j++;
                }
           }

           //Copy Character Array into another array for future purpose
           char newArray[] = new char[j];
           for(int i=0;i<j;i++)
           {
                newArray[i] = array[i];
            }

           //Calls the crc method and stores result in integer variable named 'result'
           Main main = new Main();
           result = main.crc(array, j);

           //convert crc into Binary string
           String res1 = Integer.toBinaryString(result);
           int result1 = main.crc16(array);
           String str1 = Integer.toHexString(result1);

           //convert result into Hexadecimal string
           String finalResult = Integer.toHexString(result);
           System.out.println("CRC of Data (In HexaDecimal):"+finalResult+"\t via table:"+str1);

           //Takes crc in reverse binary order and stores in temporary string
           if(res1.length()!=16) res1=res1+'0';
           String crc1 = res1.substring(8, 16);
           String temp = res1.substring(0, 8);
           String crc2="";
           
           for(int k=temp.length()-1;k>=0;k--)
           {
               crc2 = crc2 + temp.charAt(k);
           }
         
           // convert binary string into ASCII character
           byte crcByte1 = (byte) Integer.parseInt(crc1,2);
           byte crcByte2 = (byte) Integer.parseInt(crc2,2);

           //Make a new character array with the same file data
           String str="";
           for(int i=0;i<newArray.length;i++)
           str = str + newArray[i];
           finalResult = str;

           //Appending CRC characters at the last of character array
           finalResult = finalResult + (char)crcByte1;
           finalResult = finalResult + (char)crcByte2;
           
           System.out.println("String after appending CRC: "+finalResult);

           //Converting string into character array
           char[] crcArray = new char[512];
           for(int i=0;i<finalResult.length();i++)
           {
               crcArray[i]=finalResult.charAt(i);
           }

           //calling crc method again
           result = main.crc(crcArray, j);
           finalResult = Integer.toHexString(result);
           System.out.println("CRC after appending CRC16 characters:"+finalResult+"  \t via table:"+str1);
           
        }catch(Exception e){System.out.println("Exception Cause: "+e.toString());}
    }
}