/**
 * Assignment 2, COMP268 Class: WeekDay.java
 *
 * @description A class used to determine the week day for any given date.
 * @author: mo khan Student ID: 3431709
 * @date Jul 13, 2019
 * @version 1.0
 */
package Q6;

import java.util.*;

public class WeekDay {
  private static final String[] DAYS =
      new String[] {"Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday"};
  private static final int[] MONTHS = new int[] {31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};

  /**
   * Returns the day of the week that a certain date falls on.
   *
   * @param day the day of the month
   * @param month the month of the year
   * @param year the year
   * @return the name of the day of the week. (Sunday to Saturday)
   */
  public String getWeekDay(int day, int month, int year) {
    this.ensureValidDate(year, month, day);
    return DAYS[daysSinceEpoch(day, month, year) % 7];
  }

  private int daysSinceEpoch(int day, int month, int year) {
    int days = ((year - 1900) * 365) + ((year - 1900) / 4);
    if (isLeapYear(year) && (month == 1 || month == 2)) days -= 1;
    days += daysThisYearUpTo(day, month);
    return days;
  }

  private boolean isLeapYear(int year) {
    return year % 4 == 0;
  }

  private int daysThisYearUpTo(int day, int month) {
    int days = 0;
    for (int i = 1; i < month; i++) days += daysInMonth(i);
    return days + day;
  }

  private int daysInMonth(int month) {
    return MONTHS[month - 1];
  }

  private void ensureValidDate(int year, int month, int day) {
    if (month < 1 || month > 12 || day < 1 || day > MONTHS[month - 1])
      throw new IllegalArgumentException();
  }

  /**
   * The entry point to the console application.
   *
   * @param args the argument vector given to the program.
   */
  public static void main(String[] args) {
    Scanner in = new Scanner(System.in);

    System.out.println("--- Question 6 ---");
    System.out.println("Enter year:");
    int year = in.nextInt();
    if (year < 1900 || year > 2100) {
      System.out.println("Invalid year.");
      System.exit(-1);
    }

    System.out.println("Enter month:");
    int month = in.nextInt();
    if (month < 1 || month > 12 || (year == 1900 && month < 3) || (year == 2100 && month > 2)) {
      System.out.println("Invalid month.");
      System.exit(-1);
    }

    System.out.println("Enter day:");
    int day = in.nextInt();

    System.out.println();
    System.out.println("Today is:");

    try {
      System.out.println(new WeekDay().getWeekDay(day, month, year));
    } catch (IllegalArgumentException error) {
      System.out.println("Invalid date.");
      System.exit(-1);
    }
    System.out.println();
  }
}