import java.util.ArrayList;
import java.util.HashMap;
import java.util.Map.Entry;
import java.util.Scanner;

/**
 * Firstly, read an positive integer from user. Secondly, read a list of positive
 * integers one by one from user. Finally, find all pairs of numbers in the list
 * that their sum is equal to the first number.
 *
 * When user input numbers, the program will keep asking user to input until get a
 * valid integer.
 *
 * Instruction to run the code:
 * 1. Compile the code use following command: javac PairFinder.java
 * 2. Run complied use following command: java PairFinder
 *
 * Java version: 1.8
 *
 * @author tzha0024@student.monash.edu
 *
 */
public class PairFinder {
	private static int firstNumber;					// User input first number
	private static ArrayList<Integer> theList;		// User input a list of numbers
	private static HashMap<Integer, Integer> pairs;	// Pairs found
	private static Scanner sc;						// User input reader

	/**
	 * Main method.
	 *
	 * @param args
	 */
	public static void main(String[] args) {
		sc = new Scanner(System.in);

		readFirstNumber();

		readTheList();

		findPairs();

		sc.close();
	}

	/**
	 * Read an positive integer number from user. The program will keep asking
	 * user to input until get a valid integer.
	 */
	private static void readFirstNumber() {
		boolean flag = true;
		while (flag)
		{
			System.out.println("Please input a positive integer: ");
			String numberStr = sc.nextLine();

			if (isValidNumber(numberStr))
			{
				int tempNum = Integer.parseInt(numberStr);
				if (tempNum > 0) {
					firstNumber = tempNum;
					flag = false;
				}
				else
				{
					System.out.println("[ERROR] Your input is not a positive integer!");
				}
			}
		}
	}

	/**
	 * Read a list of positive integer numbers one by one from user. The program will
	 * keep asking user to add numbers to the list. When user input '0', the program
	 * will stop asking and the list is complete.
	 */
	private static void readTheList() {
		theList = new ArrayList<Integer>();
		System.out.println("Please input add numbers to the list one by one. Input '0' to finish the list.");
		boolean flag = true;
		while (flag) {
			// Show user the current numbers of the list
			printList();

			System.out.println("Please add your next positive integer to the list. Input '0' to finish the list. ");
			String numberStr = sc.nextLine();

			if (isValidNumber(numberStr))
			{
				int tempNum = Integer.parseInt(numberStr);
				if (tempNum > 0) {
					theList.add(tempNum);
				}
				else
				{
					// User input 0, stop the loop
					flag = false;
				}
			}
		}
	}

	/**
	 * Find all pairs of numbers in the list that their sum is equal to
	 * the first number.
	 * 1. Duplicates such as [4,4] are not considered.
	 * 2. Same numbers in different order such as [4,8] and [8,4] will
	 * 	  be consider as a single pair.
	 *
	 * The algorithm is nested loop. Outer loop find a number1 which is
	 * less than the firstNumber and not equals to the half of firstNumber.
	 * Then, inner loop iterate from the next index of outer loop to the end of
	 * the list to find number2 which satisfy: number1 + number2 = firstNumber.
	 */
	private static void findPairs() {
		pairs = new HashMap<Integer, Integer>();
		int listSize = theList.size();

		// iterate the list
		for (int i = 0; i < listSize - 1; i++)
		{
			int number1 = theList.get(i);

			// check the number less than firstNumber and not duplicates
			if (number1 < firstNumber && number1 != (firstNumber - number1))
			{
				// iterate the rest of the list
				for (int j = i; j < listSize; j++)
				{
					int number2 = theList.get(j);

					// Check if number1 + number2 = firstNumber
					if (number2 == (firstNumber - number1))
					{
						// Check duplicates
						if (!pairs.containsKey(number2) && !pairs.containsValue(number2))
						{
							pairs.put(number1, number2);
						}
					}
				}
			}
		}

		// print the pairs
		System.out.println("=============================");
		System.out.println("First number: " + firstNumber);
		printList();
		System.out.println("All pairs of numbers are: ");
		for (Entry<Integer, Integer> e : pairs.entrySet())
		{
			System.out.println("[" + e.getKey() + "," + e.getValue() + "]");
		}
	}

	/**
	 * Show the current numbers of the list.
	 */
	private static void printList() {
		System.out.print("Current list: [ ");
		if (theList.size() > 0) {
			for(Integer i : theList)
			{
				System.out.print(i + " ");
			}
		}
		System.out.print("]");
		System.out.println();
	}

	/**
	 * Check whether user's input string is a valid integer or not.
	 * @param numberStr	User's input string
	 * @return True if user's input string is a valid integer
	 */
	private static boolean isValidNumber(String numberStr) {
		boolean flag = false;
		if (numberStr.trim().length() > 0)
		{
			if (numberStr.length() < 10)
			{
				flag = true;
				for (int i = 0; i < numberStr.length(); i++)
				{
					char c = numberStr.charAt(i);
					if (c < '0' || c > '9')
					{
						flag = false;
						System.out.println("[ERROR] Your input is not a valid integer!");
						break;
					}
				}
			}
			else
			{
				System.out.println("[ERROR] Please only input within 10 digits.");
			}
		}
		else
		{
			System.out.println("[ERROR] Please do not input empty space.");
		}
		return flag;
	}
}