model

class CustomCalendar {
// number of days in month
  //[JAN, FEB, MAR, APR, MAY, JUN, JUL, AUG, SEP, OCT, NOV, DEC]
  final List<int> _monthDays = [31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31];

// check for leap year
  bool _isLeapYear(int year) {
    if (year % 4 == 0) {
      if (year % 100 == 0) {
        if (year % 400 == 0) return true;
        return false;
      }
      return true;
    }
    return false;
  }

  List<CalendarModel> getMonthCalendar(int month, int year,
      {StartWeekDay startWeekDay = StartWeekDay.sunday}) {
    // validate
    if (year == null || month == null || month < 1 || month > 12) {
      throw ArgumentError('Invalid year or month');
    }

    List<CalendarModel> calendar = [];
    // used for previous and next month's calendar days
    int otherYear;
    int otherMonth;
    int leftDays;

    // get no. of days in the month
    // month-1 because _monthDays starts from index 0 and month starts from 1
    int totalDays = _monthDays[month - 1];
    // if this is a leap year and the month is february, increment the total days by 1
    if (_isLeapYear(year) && month == DateTime.february) totalDays++;

    // get this month's calendar days
    for (int i = 0; i < totalDays; i++) {
      calendar.add(CalendarModel(
        // i+1 because day starts from 1 in DateTime class
          date: DateTime(year, month, i + 1),
          thisMonth: true));
    }

    // fill the unfilled starting weekdays of this month with the previous month days
    if ((startWeekDay == StartWeekDay.sunday &&
        calendar.first.date!.weekday != DateTime.sunday) ||
        (startWeekDay == StartWeekDay.monday &&
            calendar.first.date!.weekday != DateTime.monday)) {
      if(month == DateTime.january){
        otherMonth = DateTime.december; // _monthDays index starts from 0 (11 for december)
        otherYear = year-1;
      }
      else{
        otherMonth = month - 1;
        otherYear = year;
      }
      // month-1 because _monthDays starts from index 0 and month starts from 1
      totalDays = _monthDays[otherMonth - 1];
      if(_isLeapYear(otherYear) && otherMonth == DateTime.february) totalDays++;

      leftDays = totalDays - calendar.first.date!.weekday + ((startWeekDay == StartWeekDay.sunday) ? 0 : 1);
      for(int i=totalDays;i>leftDays;i--){
        calendar.insert(0, CalendarModel(
            date: DateTime(otherYear,otherMonth,i),
            prevMonth: true
        ));
      }
    }

    // fill the unfilled ending weekdays of this month with the next month days

    if(
    (startWeekDay == StartWeekDay.sunday && calendar.last.date!.weekday != DateTime.saturday) ||
        (startWeekDay == StartWeekDay.monday && calendar.last.date!.weekday != DateTime.sunday)
    ){
      // if this month is december, then next month would be january of next year
      if(month == DateTime.december){
        otherMonth = DateTime.january;
        otherYear = year+1;
      }
      else{
        otherMonth = month+1;
        otherYear = year;
      }
      // month-1 because _monthDays starts from index 0 and month starts from 1
      totalDays = _monthDays[otherMonth-1];
      if(_isLeapYear(otherYear) && otherMonth == DateTime.february) totalDays++;

      leftDays = 7 - calendar.last.date!.weekday - ((startWeekDay == StartWeekDay.sunday) ? 1 : 0);
      if(leftDays == -1) leftDays = 6;

      for(int i=0; i<leftDays; i++){
        calendar.add(
          CalendarModel(
            date: DateTime(otherYear, otherMonth, i+1),
            nextMonth: true,
          ),
        );
      }
    }
    return calendar;
  }
}

enum StartWeekDay { sunday, monday }






class CalendarModel{
  final DateTime ?date;
  final bool thisMonth;
  final bool prevMonth;
  final bool nextMonth;
  CalendarModel({
    this.date,
    this.thisMonth = false,
    this.prevMonth = false,
    this.nextMonth = false
  });
}