using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

public class IsoDate1
{
    public DateTime date { get; private set; }
    public int Year { get; private set; }
    public int Week { get; private set; }

    public IsoDate1(int _year, int _week)
    {
        DateTime newYear;
        int NewYearWeek;
        int WeeksBetween;
        int dayOfWeek;
        Year = _year;
        Week = _week;
        if (Week == 1)
        {
            DateTime newYearsDay = new DateTime(Year, 1, 1);
            if (newYearsDay.DayOfWeek == DayOfWeek.Monday)
                date = newYearsDay;
            else if (newYearsDay.DayOfWeek == DayOfWeek.Friday
                || newYearsDay.DayOfWeek == DayOfWeek.Saturday
                || newYearsDay.DayOfWeek == DayOfWeek.Sunday)
            {
                dayOfWeek = (int)newYearsDay.DayOfWeek - 1;
                if (dayOfWeek == -1)
                    dayOfWeek = 6;
                date = newYearsDay.AddDays(7 - dayOfWeek);
            }
            else
            {
                newYear = new DateTime(Year - 1, 12, 31);
                dayOfWeek = (int)newYear.DayOfWeek - 1;
                if (dayOfWeek == -1)
                    dayOfWeek = 6;
                date = newYear.AddDays(-dayOfWeek);
            }
        }
        else
        {
            newYear = new DateTime(Year, 12, 31);
            NewYearWeek = GetNewYearWeek(newYear);
            if (NewYearWeek == 1)
            {
                IsoDate1 lateDecember = new IsoDate1(newYear.AddDays(-7));
                WeeksBetween = lateDecember.Week - Week;
                date = lateDecember.date.AddDays(-7 * WeeksBetween);
                dayOfWeek = (int)lateDecember.date.DayOfWeek - 1;
                if (dayOfWeek == -1)
                    dayOfWeek = 6;
                date = date.AddDays(-dayOfWeek);
            }
            else
            {
                WeeksBetween = NewYearWeek - Week;
                date = newYear.AddDays(-7 * WeeksBetween);
                dayOfWeek = (int)newYear.DayOfWeek - 1;
                if (dayOfWeek == -1)
                    dayOfWeek = 6;
                date = date.AddDays(-dayOfWeek);
            }
        }
    }
    private int GetNewYearWeek(DateTime newYear)
    {
        int NewYearWeek;
        if (DateTime.IsLeapYear(newYear.Year))
        {
            if (newYear.DayOfWeek == DayOfWeek.Saturday
                || newYear.DayOfWeek == DayOfWeek.Sunday)
                NewYearWeek = 52;
            else if (newYear.DayOfWeek == DayOfWeek.Friday
                || newYear.DayOfWeek == DayOfWeek.Thursday)
                NewYearWeek = 53;
            else
                NewYearWeek = 1;
        }
        else
        {
            if (newYear.DayOfWeek == DayOfWeek.Friday
                || newYear.DayOfWeek == DayOfWeek.Saturday
                || newYear.DayOfWeek == DayOfWeek.Sunday)
                NewYearWeek = 52;
            else if (newYear.DayOfWeek == DayOfWeek.Thursday)
                NewYearWeek = 53;
            else
                NewYearWeek = 1;
        }
        return NewYearWeek;
    }
    public IsoDate1(DateTime _date)
    {
        date = _date;
        DateTime newYear = new DateTime(date.Year, 12, 31);
        int NewYearWeek;
        DateTime lastMonday;
        int lastMondayWeek;
        int daysBetween;
        int weeksBetween;
        int dayOfWeek;

        NewYearWeek = GetNewYearWeek(newYear);
        if (date == newYear)
        {
            Week = NewYearWeek;
            if (Week == 1)
                Year = newYear.Year + 1;
            else
                Year = newYear.Year;
            return;
        }
        if (NewYearWeek == 1)
        {
            dayOfWeek = (int)newYear.DayOfWeek - 1;
            if (dayOfWeek == -1)
                dayOfWeek = 6;
            lastMonday = newYear.AddDays(-7 - dayOfWeek);
            lastMondayWeek = 52;
        }
        else
        {
            dayOfWeek = (int)newYear.DayOfWeek - 1;
            if (dayOfWeek == -1)
                dayOfWeek = 6;
            lastMonday = newYear.AddDays(-dayOfWeek);
            lastMondayWeek = NewYearWeek;
        }
        if (date <= lastMonday)
        {
            daysBetween = (lastMonday - date).Days;
            if (daysBetween == 0)
            {
                Week = lastMondayWeek;
                Year = lastMonday.Year;
            }
            else
            {
                weeksBetween = (daysBetween - 1) / 7;
                Week = lastMondayWeek - weeksBetween - 1;
                if (Week > 0)
                    Year = date.Year;
                else
                {
                    IsoDate1 lastNewYear = new IsoDate1(new DateTime(date.Year - 1, 12, 31));
                    Week = lastNewYear.Week;
                    Year = lastNewYear.Year;
                }
            }
        }
        else
        {
            daysBetween = (date - lastMonday).Days;
            if (daysBetween < 7)
            {
                Week = lastMondayWeek;
                Year = lastMonday.Year;
            }
            else
            {
                Week = NewYearWeek;
                if (Week == 1)
                    Year = newYear.Year + 1;
                else
                    Year = newYear.Year;
            }
        }
    }
}