//Java applet programmed by ... on the 11 June 2002, At ...
//This applet is to check whether an isbn number is valid.

import java.awt.*;
import java.applet.*;
import java.util.*;

public class isbn extends Applet
{
	public String ValidationVerdict;
	public String HelpfulComment;

	public void init()
	{
		//Initial Displays
		ValidationVerdict = "Plase type in the ISBN number you wish to check in the box below.";
	}

	public void paint(Graphics g)
	{
		//Output the string to the screen
		g.drawString(ValidationVerdict,10,10);
		g.drawString(HelpfulComment,10,20);
	}

	public void CheckIsbn(String ISBN_Number)
	{
		//This counter counts through the string backward.
		int digit_number;
		//This counter stores the weight value of the string number
		int weight_of_digit;
		//th stores the integer value of the digit to be analysed
		int digit_value;
		//Total is where the total of all weights of digits end up
		int Total = 0;
		//Remainder is to determine whether the ISBN is correct remainder = Total mod 11
		int Remainder = 0;
		//First digit is where the first numeric value of the ISBN number will be stored.
		int first_digit = 0;

		//Initialise weight_of_digit
		weight_of_digit=1;

		//Step through the string backwards.
		for(digit_number = (ISBN_Number.length()-1);digit_number >=0;digit_number--)
		{
			//Make sure the current character isn't a -, these are separaters, dont pay attention to them.
			if(ISBN_Number.charAt(digit_number)!='-')
			{
				//is there a Roman character X in the ISBN
				if((ISBN_Number.charAt(digit_number) == 'X') 
				|| (ISBN_Number.charAt(digit_number) == 'x'))
					//X = 10
					digit_value = 10;
				else
					//Get the integer value of the character.
					digit_value = (int)ISBN_Number.charAt(digit_number) - 48;

				//make sure we store the first numeric value.
				if(weight_of_digit == 1)
					first_digit = digit_value;

				//Add digit_value*weight_of_digit (number * wieght) to the cumulative total.
				Total += digit_value * weight_of_digit;
				
				//increase the wieght.
				weight_of_digit++;
			}
		}

		//find out whether the ISBN is corrent.
		Remainder = Total % 11;

		//Display outputs.
		if(Remainder == 0)
		{
			ValidationVerdict = "Yes, This is a valid ISBN Number.";
			HelpfulComment = "";
		}
		else
		{
			ValidationVerdict = "No, This is an invalid ISBN number.";
			if((first_digit - Remainder) > 0)
				HelpfulComment = "The correct ISBN number would end with the digit " + (first_digit - Remainder);
			if((first_digit - Remainder) < 0)
			{
				int i=0;
				for (i = (first_digit*1+1); i < 11; i++)
				{
					Remainder = (Total - first_digit + i) % 11;
					if (Remainder == 0) break;
				}
			HelpfulComment = "The correct ISBN number would end with the digit " + i;
			}
		}

		repaint();
	}
}