การ Implement Checksum ด้วย Java

ตัวอย่างโค้ด

import java.util.*;

public class Demo {
    public static void main(String args[]) {
        Scanner my_scan = new Scanner(System.in);
        System.out.println("Enter the input string ");
        String my_in = my_scan.next();
        int my_checksum = generate_checksum(my_in);
        System.out.println("The checksum that has been generated is " + Integer.toHexString(my_checksum));
        System.out.println("Enter the data that needs to be sent to the receiver ");
        my_in = my_scan.next();
        System.out.println("Enter the checksum that needs to be sent to the receiver ");
        my_checksum = Integer.parseInt((my_scan.next()), 16);
        receive(my_in, my_checksum);
        my_scan.close();
    }

    static int generate_checksum(String s) {
        String my_hex_val = new String();
        int x, i, my_checksum = 0;
        for (i = 0; i < s.length() - 2; i = i + 2) {
            x = (int) (s.charAt(i));
            my_hex_val = Integer.toHexString(x);
            x = (int) (s.charAt(i + 1));
            my_hex_val = my_hex_val + Integer.toHexString(x);
            System.out.println(s.charAt(i) + "" + s.charAt(i + 1) + " : " + my_hex_val);
            x = Integer.parseInt(my_hex_val, 16);
            my_checksum += x;
        }
        if (s.length() % 2 == 0) {
            x = (int) (s.charAt(i));
            my_hex_val = Integer.toHexString(x);
            x = (int) (s.charAt(i + 1));
            my_hex_val = my_hex_val + Integer.toHexString(x);
            System.out.println(s.charAt(i) + "" + s.charAt(i + 1) + " : " + my_hex_val);
            x = Integer.parseInt(my_hex_val, 16);
        } else {
            x = (int) (s.charAt(i));
            my_hex_val = "00" + Integer.toHexString(x);
            x = Integer.parseInt(my_hex_val, 16);
            System.out.println(s.charAt(i) + " : " + my_hex_val);
        }
        my_checksum += x;
        my_hex_val = Integer.toHexString(my_checksum);
        if (my_hex_val.length() > 4) {
            int carry = Integer.parseInt(("" + my_hex_val.charAt(0)), 16);
            my_hex_val = my_hex_val.substring(1, 5);
            my_checksum = Integer.parseInt(my_hex_val, 16);
            my_checksum += carry;
        }
        my_checksum = generate_complement(my_checksum);
        return my_checksum;
    }

    static void receive(String s, int my_checksum) {
        int gen_checksum = generate_checksum(s);
        gen_checksum = generate_complement(gen_checksum);
        int syndrome = gen_checksum + my_checksum;
        syndrome = generate_complement(syndrome);
        System.out.println("The value of syndrome is " + Integer.toHexString(syndrome));
        if (syndrome == 0) {
            System.out.println("Data has been received without any errors");
        } else {
            System.out.println("An error was encountered in the received data");
        }
    }

    static int generate_complement(int my_checksum) {
        my_checksum = Integer.parseInt("FFFF", 16) - my_checksum;
        return my_checksum;
    }
}

Inputs

sample
sample
b2c8

Output

Enter the input string
sa : 7361
mp : 6d70
le : 6c65
The checksum that has been generated is b2c8
Enter the data that needs to be sent to the receiver
Enter the checksum that needs to be sent to the receiver
sa : 7361
mp : 6d70
le : 6c65
The value of syndrome is 0
Data has been received without any errors

โค้ดสาธิตการสร้าง checksum (Internet checksum style) แปลง string เป็น hex pairs รวมกัน handle carry ทำ 1's complement สำหรับ receiver คำนวณ checksum ใหม่จาก data ที่รับมา complement อีกรอบ บวกกับ checksum ที่ส่งมา complement อีกรอบ ได้ syndrome หากเป็น 0 = ไม่มี error