//ProjectEuler/Java/Problem19.java
//Matthew Ellison
// Created: 03-13-19
//Modified: 03-28-19
//How many Sundays fell on the first of the month during the twentieth century (1 Jan 1901 to 31 Dec 2000)?
/*
You are given the following information, but you may prefer to do some research for yourself.
1 Jan 1900 was a Monday.
Thirty days has September,
April, June and November.
All the rest have thirty-one,
Saving February alone,
Which has twenty-eight, rain or shine.
And on leap years, twenty-nine.
A leap year occurs on any year evenly divisible by 4, but not on a century unless it is divisible by 400.
*/
//Unless otherwise listed all non-standard includes are my own creation and available from https://bibucket.org/Mattrixwv/JavaClasses
/*
	Copyright (C) 2019  Matthew Ellison

	This program is free software: you can redistribute it and/or modify
	it under the terms of the GNU Lesser General Public License as published by
	the Free Software Foundation, either version 3 of the License, or
	(at your option) any later version.

	This program is distributed in the hope that it will be useful,
	but WITHOUT ANY WARRANTY; without even the implied warranty of
	MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
	GNU Lesser General Public License for more details.

	You should have received a copy of the GNU Lesser General Public License
	along with this program.  If not, see <https://www.gnu.org/licenses/>.
*/


import mattrixwv.Stopwatch;


public class Problem19{
	private static enum DAYS{SUNDAY, MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY, NUMBER_OF_DAYS, ERROR};
	private static final Integer START_YEAR = 1901;	//The first year we are going to test
	private static final Integer END_YEAR = 2000;	//The last year we are going to test
	public static void main(String[] argv){
		Stopwatch timer = new Stopwatch();
		Long totalSundays = 0L;

		//Start the timer
		timer.start();

		//Run for all years 1901-2000
		for(Integer year = START_YEAR;year <= END_YEAR;++year){
			//Run for all months in the year
			for(Integer month = 1;month <= 12;++month){
				DAYS day = getDay(month, 1, year);
				if(day == DAYS.ERROR){
					System.out.println("There was an error with the day");
				}
				else if(day == DAYS.SUNDAY){
					++totalSundays;
				}
			}
		}

		//Stop the timer
		timer.stop();

		//Print the results
		System.out.printf("There are %d Sundays that landed on the first of the months from %d to %d\n", totalSundays, START_YEAR, END_YEAR);
		System.out.println("It took " + timer.getStr() + " to run this algorithms");
	}
	//Return the day of the week that the date you pass into it is on
	private static DAYS getDay(Integer month, Integer day, Integer year){
		//Make sure the numbers are within propper bounds
		if((month < 1) || (month > 12) || (day < 1) || (day > 31) || (year < 1)){
			return DAYS.ERROR;
		}
		Integer numDays = 0;
		Integer currentYear = 1;
		Integer currentMonth = 1;
		Integer currentDay = DAYS.SATURDAY.ordinal();
		--day;

		//Add teh currect number of days for every year
		while(currentYear < year){
			if(isLeapYear(currentYear)){
				numDays += 366;
			}
			else{
				numDays += 365;
			}
			++currentYear;
		}
		//Add the correct number of days for every month
		while(currentMonth < month){
			//February
			if(currentMonth == 2){
				if(isLeapYear(currentYear)){
					numDays += 29;
				}
				else{
					numDays += 28;
				}
			}
			//31 day months
			else if((currentMonth == 1) || (currentMonth == 3) || (currentMonth == 5) || (currentMonth == 7) || (currentMonth == 8) || (currentMonth == 10) || (currentMonth == 12)){
				numDays += 31;
			}
			else{
				numDays += 30;
			}
			++currentMonth;
		}
		//Account for the weird year of 1752
		if(year > 1752){
			numDays -= 11;
		}
		else if(year == 1752){
			if(month > 9){
				numDays -= 11;
			}
			else if(month == 9){
				if(day >= 14){
					numDays -= 11;
				}
				//Days 3-13 were skipped that year
				else if((day > 2) && (day < 14)){
					System.out.println("Hit 1752 error");
					return DAYS.ERROR;
				}
			}
		}
		//Add the correct number of days for every day
		numDays += day;

		currentDay += numDays;
		currentDay = currentDay % DAYS.NUMBER_OF_DAYS.ordinal();
		DAYS typeDay = DAYS.values()[currentDay];
		switch(typeDay){
			case SUNDAY: return DAYS.SUNDAY;
			case MONDAY:	return DAYS.MONDAY;
			case TUESDAY:	return DAYS.TUESDAY;
			case WEDNESDAY:	return DAYS.WEDNESDAY;
			case THURSDAY:	return DAYS.THURSDAY;
			case FRIDAY:	return DAYS.FRIDAY;
			case SATURDAY:	return DAYS.SATURDAY;
			default:		System.out.println("Hit default error"); return DAYS.ERROR;
		}
	}
	//Returns true if the year passed to it is a leap year
	private static Boolean isLeapYear(Integer year){
		if(year < 1){
			return false;
		}
		else if((year % 100) == 0){
			//This rule only applies at and after 1800
			if(year <= 1700){
				return true;
			}
			else if((year % 400) == 0){
				return true;
			}
		}
		else if((year % 4) == 0){
			return true;
		}
		return false;
	}
}

/* Results:
There are 171 Sundays that landed on the first of the months from 1901 to 2000
It took 67.638 milliseconds to run this algorithms
*/