//-------------------------------------------------------------
// Example to illustrate user-defined functions
//-------------------------------------------------------------

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

//-------------------------------------------------------------
// LeapYear calculates for leap years.
//-------------------------------------------------------------
// Returns:  true if it is a leap year and false if it is not.
//-------------------------------------------------------------

bool LeapYear (int year)
{
 // calculating if there is a leap year or not
  if (year % 4 != 0)
      return false;     
   else if (year % 100 == 0 && year % 400 != 0)
           return false;
        else
           return true;
 
}                     

//-------------------------------------------------------------
// DaysInMonth determines the number of days of the month
// Note:  ignores leap year
//-------------------------------------------------------------
// Accepts:  number (1 - 12) representing month
// Returns:  number of days in that month
//-------------------------------------------------------------
    
int DaysInMonth (int monthnum, int year)
{
    int numdays;  // temp variable for days in month

    switch (monthnum)
    {
        case 1:
        case 3:
        case 5:
        case 7:
        case 8:
        case 10:
        case 12:  numdays = 31;  break;
        case 2:   if (LeapYear (year) == true)    // modify this case so that february has 
                      numdays = 29;               // the right amount of days with or 
                  else                            // without a leapyear
                      numdays = 28; 
                  break;
        case 4:
        case 6:
        case 9:
        case 11:   numdays = 30;  break;
    }

    return numdays;
}

//-------------------------------------------------------------
// DisplayMonthName displays the month name to standard output
//-------------------------------------------------------------
// Accepts:  number (1 - 12) representing month
// Returns:  nothing
//-------------------------------------------------------------

void DisplayMonthName (int monthnum)
{
    switch (monthnum)
    {
        case 1:  cout << "January";  break;
        case 2:  cout << "February";  break;
        case 3:  cout << "March";  break;
        case 4:  cout << "April";  break;
        case 5:  cout << "May";  break;
        case 6:  cout << "June";  break;
        case 7:  cout << "July";  break;
        case 8:  cout << "August";  break;
        case 9:  cout << "September";  break;
        case 10:  cout << "October";  break;
        case 11:  cout << "November";  break;
        case 12:  cout << "December";  break;
    }
}

//---------------------------------------------------
// Display first line displays the first line of the 
// week in the calendar by finding the startdayofweek
//---------------------------------------------------

void DisplayFirstLine (int startdayofweek)
{
  int i;
   cout << endl;
  for (i=0; i < startdayofweek; i++)
   cout << "      ";
  for (i=1; i<=(7-startdayofweek); i++)
   cout << setw(6) << i;
}

//--------------------------------------------------
// Display full week calculates what the next couple
// of weeks should start on by looking at the last 
// number displayed
//--------------------------------------------------

void DisplayFullWeek (int startdateofweek)
{
  int i;
  cout << "\n";
  for (i=startdateofweek; i<= (startdateofweek+6) ; i++)
   cout << setw(6) << i;
}

//-------------------------------------------------
// Display last week has to calculate what the week
// should start on and also calculate where it 
// should stop based on how many days are in the 
// month.
//-------------------------------------------------

void DisplayLastWeek (int startdateofweek,
                      int numdaysinmonth)
{
  int i;
  cout << "\n";
  for (i=startdateofweek; i<= numdaysinmonth; i++)
  cout << setw(6) << i;
}

//------------------------------------------------
// Calculate day of week calculates the day of the 
// week of the date passed in via the parameters 
// month, day, and year. 
//------------------------------------------------
// Accepts: calls the leap year function
// Returns: an integer that indicates the day of 
//          the week.
//-----------------------------------------------

int CalculateDayofWeek (int month, int day, int year)
{
  int code,
      lasttwo,          // last two digits
      total,            // add day, monthcode, and lasttwodigits of the year
      quarterofyear,    // last two digits of the year + 1/4 of the last two digits
      centuries;        // assignment statement for centuries

 
// switch statment to declare the month code for each month.
 switch (month)
  {
    case 1: code = 6; break;
    case 2: code = 2; break;
    case 3: code = 2; break;
    case 4: code = 5; break;
    case 5: code = 0; break;
    case 6: code = 3; break;
    case 7: code = 5; break;
    case 8: code = 1; break;
    case 9: code = 4; break;
    case 10: code = 6; break; 
    case 11: code = 2; break;
    case 12: code = 4; break;
  }
  
  
  lasttwo = year % 100;                     // calculate the last 2 digits of the year
  quarterofyear = (lasttwo/4) + lasttwo;    // adding a quarter of the year onto itself
  total = quarterofyear + code + day;       // adding the total of all three numbers together

  // calculating for centuries
  centuries = year / 100;
  if (centuries == 17 || centuries == 21)
     total = total + 5;
    else if (centuries == 18 || centuries == 22)
     total = total + 3;
      else if (centuries == 19)
         total = total + 1;  

   // calculating the leap year for january and february to add 1 to the total
  if (LeapYear (year) == true)
     if (month == 1 || month == 2)
        total = total - 1;
   
   // add a mod so that the number correspnds to the day of the week
  total = total % 7;

 return total; 

}

//--------------------------------------------------
// Display title and headings displays the month 
// name and the days of the week.
//--------------------------------------------------

void DisplayTitleAndHeadings (int month)
{
 cout << "                   ";
 DisplayMonthName (month);
 cout << "\n" << setw(6) <<"    Sun" << setw(6) << "Mon" << setw(6) << "Tue" << setw(6) << "Wed" << setw(6) << "Thu" << setw(6) << "Fri"; 
 cout <<  setw(6) << "Sat";
}


//-------------------------------------------------------------
//  Main Function
//-------------------------------------------------------------

int main ( )
{

// --- Variables

    int monthnum,         // month number (1 - 12) entered by user
        begdayofmonth,    // begining day of the month.
        begdateofnextweek,// begining date of the next week
        year,             // the year the user entered
        numdays;          // number of days in month
     
    
// --- Get month number from user

    cout << "\nEnter a month number (1-12): ";
    cin >> monthnum;
    cout << "\n Enter the Year  ";
    cin >> year;
    begdayofmonth = CalculateDayofWeek (monthnum, 1, year);
     
// --- Validate month number and year

    while (monthnum < 1 || monthnum > 12)
    {
       cout << "\n\n!!! Month number must be in range 1 - 12 !!!";
       cout << "\n\nPlease re-enter month number: ";
       cin >> monthnum;
    }
    
    // add a while to validate the year
    while (year < 1700 || year > 2299)
    {
      cout << "\n\n!!! Year must be in range 1700 - 2299 !!!";
      cout << "\n\n Please re-enter year:   ";
      cin >> year;
    }
 
// --- Get number of days in month
    LeapYear(year);
    numdays = DaysInMonth(monthnum, year);
    
// --- Display output sentence    

    DisplayTitleAndHeadings(monthnum);
    cout << "\n";
    DisplayFirstLine(begdayofmonth);
    begdateofnextweek =(8 - begdayofmonth);
    DisplayFullWeek(begdateofnextweek);
    begdateofnextweek += 7;
    DisplayFullWeek(begdateofnextweek);
    begdateofnextweek += 7;
    DisplayFullWeek(begdateofnextweek);
    begdateofnextweek += 7;
    if (numdays-begdateofnextweek >= 7)
    {     
      DisplayFullWeek(begdateofnextweek);
      begdateofnextweek +=7;
    }
    DisplayLastWeek(begdateofnextweek,numdays);    
    cout << "\n";
 return 0;
}