/* Author: Greg Koumis
 * Class: CS 1150
 * Assignment: Lab 3 5.31
 * Teacher: Terry Spence
 * Due Date: 10/13/2011
 * Summary: user inputs a credit card number and prgram outputs whether or not the number is valid or invalid
 */

import java.util.Scanner;

public class Lab3 {
    /** Return true if the card number is valid */
    public static boolean isValid(long number){
        int sum = sumOfDoubleEvenPlace(number) + sumOfOddPlace(number);
        if((sum%10) == 0){
            if (prefixMatched(number)){
                if(getSize(number) > 12 && getSize(number) < 17){
                    return true;
                } else {
                    return false;
                }
            } else {
                return false;
            }
        }else{
            return false;
        }
    }

    /** Get the result from Step 2*/
    public static int sumOfDoubleEvenPlace(long number){
        int size = getSize(number);
        int num = 0;
        number = number / 10;

        for (int i = 0; i < size; i +=2){
            long d = number % 10;
            int digit = (int)d * 2;
            number = number / 100;
            num = num + getDigit(digit);
        }
        return num;
    }

    /** Return this number if it is a single digit, otherwise , return
     * the sum of the two digits */
    public static int getDigit(int number){
        if ((number / 10) > 0){
            int d = number / 10;
            int n = number % 10;
            return d + n;
        }else{
            return number;
        }
    }

    /** Return sum of odd place digits in number */
    public static int sumOfOddPlace(long number){
        int size = getSize(number);
        int num = 0;

        for (int i = 0; i < size; i +=2){
            long d = number % 10;
            number = number / 100;
            num = num + (int)d;
        }
        return num;
    }

    /** Return true if the digit d is a prefix for number */
    public static boolean prefixMatched(long number){
        int prefix = getPrefix(number, 2);

        if (prefix == 4 || prefix == 37 || prefix == 6 || prefix ==5){
            return true;
        } else {
            return false;
        }
    }

    /**Return the number of digits in d */
    public static int getSize(long d){
        int size = 0;

        while (d > 0){
            size++;
            d = d / 10;
        }
        return size;
    }

    /** Return the first k number of digits from number. If the
     * number of digits in is less than k, return number*/
    public static int getPrefix(long number, int k){
        int size = getSize(number);

        for(int i = 0; i < (size - 2); i++){
            number = number / 10;
        }

        if (number == 37){
            return (int)number;
        } else {
            number = number / 10;
        }
        return (int)number;
    }

     //Asks the user for inmput of a credit card number and outpust whether it is valid or invalid
     public static void main(String[] args){
        Scanner input = new Scanner(System.in);
        System.out.println("enter a credit card number");
        long cardNo = input.nextLong();

        if (isValid(cardNo)){
            System.out.println("The number is valid!");
        } else {
            System.out.println("The number is invalid...");
        }
    }

}