#include <iostream>
#include <iomanip>
using namespace std;

//Function Prototypes
int ReadMonth();
int ReadYear();
int IsLeap(int);
int DaysInMonth();
int GetMonthDays(int, int);
int GetMonthName(int, int);
int DaysInYears(int, int);
void DisplayCal(int, int, int);

int main()
{
	int month, year, leap, days, startDay;

	month = ReadMonth(); 
	year = ReadYear();
	leap = IsLeap(year); //determines if it is a leap year
	GetMonthName(month, year); //displays the month name
	days = GetMonthDays(month, year); //how many days the month has
	startDay = DaysInYears(year, month); //what day of the week the month starts on
	DisplayCal(year, days, startDay); //prints out the calender

	return 0;
}

int ReadMonth()
{
	int month;
	cout << "Enter month number: ";
	cin >> month;
	return month;
}

int ReadYear()
{
	int year;
	cout << "Enter year (1900 to 9999): ";
	cin >> year;
	return year;
}

int IsLeap(int year)
{
	int leap;
	if ((year % 4 == 0) && (year % 100 != 0) || (year % 400 == 0))
		leap = 1; //is a leap year
	else
		leap = 0; //is not a leap year
	return leap;
}

int GetMonthName(int month, int year)
{
	int days = 0;
	switch(month)
	{
		case 1: cout << "January";
				days = 31;
				break;
		case 3: cout << "March";
				days = 31;
				break;
		case 5: cout << "May";
				days = 31;
				break;
		case 7: cout << "July";
				days = 31;
				break;
		case 8: cout << "August";
				days = 31;
				break;
		case 10: cout << "October";
				days = 31;
				break;
		case 12: cout << "December";
				days = 31;
				break;
		case 4: cout << "April";
				days = 30;
				break;
		case 6: cout << "June";
				days = 30;
				break;
		case 9: cout << "September";
				days = 30;
				break;
		case 11: cout << "November";
				days = 30;
				break;
		case 2: cout << "February";
				days = 28 + IsLeap(year);
	}
	return days;
}

int GetMonthDays(int month, int year)
{
	int days = 0;
	switch(month)
	{
		case 1: 
				days = 31;
				break;
		case 3: 
				days = 31;
				break;
		case 5: 
				days = 31;
				break;
		case 7: 
				days = 31;
				break;
		case 8: 
				days = 31;
				break;
		case 10: 
				days = 31;
				break;
		case 12: 
				days = 31;
				break;
		case 4: 
				days = 30;
				break;
		case 6: 
				days = 30;
				break;
		case 9: 
				days = 30;
				break;
		case 11:
				days = 30;
				break;
		case 2: 
				days = 28 + IsLeap(year);
	}
	return days;
}

void DisplayCal(int year, int days, int startDay)
{
	cout << "\nS" << setw(4) << "M" << setw(4) << "T" << setw(4) << "W" << setw(4) << "T"
		 << setw(4) << "F" << setw(4) << "S\n";
	cout << setw(startDay);
	for (int i = 1; i <= days; i++)
	{
		if (startDay == 0 && i <= 7)
			cout << i << "\n";
	}

}

int DaysInYears(int year, int month) //calculates what day of the week the calendar starts on
{
	int totalLeap = 0, totalDays = 0, day_year = 0, startDay;
	for (int i = 1900; i < year; i++)
		totalLeap += IsLeap(i);
	totalDays = (year - 1900) * 365 + totalLeap;
	for (int i = 0; i < month; i++)
		day_year += GetMonthDays(i, year);
	totalDays += day_year;
	if (totalDays % 7 == 0)
		startDay = 0;
	if (totalDays % 7 == 1)
		startDay = 1;
	if (totalDays % 7 == 2)
		startDay = 2;
	if (totalDays % 7 == 3)
		startDay = 3;
	if (totalDays % 7 == 4)
		startDay = 4;
	if (totalDays % 7 == 5)
		startDay = 5;
	if (totalDays % 7 == 6)
		startDay = 6;
	else
		startDay = 7;
	return startDay;

}

StartDay = the day of the week the month starts on (0=Sun, 1=Mon, etc.
EndDay = number of days of the month
CurrDay = 0, the current day-of-week (always starts with Sunday)

Loop from 0 to StartDay - 1
    output an empty day 
    increment CurrDay

Loop from 0 to EndDay-1
    output the day based on loop counter
    increment CurrDay
    if CurrDay = 7
        go to the next line.
        set CurrDay to 0

Loop from CurrDay to 6
    output an empty day