using System;
using System.Globalization;

public struct WeekAndYear
{
	public int Week
	{
		get;
		private set;
	}
	
	public int Year
	{
		get;
		private set;
	}

	public WeekAndYear(int week, int year) : this()
	{
		Week = week;
		Year = year;
	}

	public DateTime StartDate
	{
		get
		{
			DateTime jan1 = new DateTime(Year, 1, 1);
			int day = (int)jan1.DayOfWeek - 1;
			int delta = (day < 4 ? -day : 7 - day) + 7 * (Week - 1);
			return jan1.AddDays(delta);
		}
	}

	public static WeekAndYear operator ++(WeekAndYear value)
	{
		if (value.Week >= 52)
		{
			// December 28th is always in the last week of a year when using ISO week-numbering
			Calendar cal = CultureInfo.CurrentCulture.Calendar;
			int lastWeekOfYear = cal.GetWeekOfYear(new DateTime(value.Year, 12, 28),
				CalendarWeekRule.FirstFourDayWeek, DayOfWeek.Monday);

			if (value.Week >= lastWeekOfYear)
				return new WeekAndYear() { Week = 1, Year = value.Year + 1 };
		}

		return new WeekAndYear() { Week = value.Week + 1, Year = value.Year };
	}

	public static WeekAndYear operator --(WeekAndYear value)
	{
		if (value.Week == 1)
		{
			// December 28th is always in the last week of a year when using ISO week-numbering
			Calendar cal = CultureInfo.CurrentCulture.Calendar;
			int lastWeekOfYear = cal.GetWeekOfYear(new DateTime(value.Year - 1, 12, 28),
				CalendarWeekRule.FirstFourDayWeek, DayOfWeek.Monday);

			return new WeekAndYear(lastWeekOfYear, value.Year - 1);
		}

		return new WeekAndYear(value.Week - 1, value.Year);
	}

	public static bool operator >(WeekAndYear v1, WeekAndYear v2)
	{
		if (v1.Year == v2.Year)
			return v1.Week > v2.Week;
		else
			return v1.Year > v2.Year;
	}

	public static bool operator <(WeekAndYear v1, WeekAndYear v2)
	{
		return v2 > v1;
	}

	public override string ToString()
	{
		return String.Format("W{0:D2} {1}", Week, Year);
	}

	/// <summary>
	/// Construct a WeekAndYear from any date (returning the week this date falls into).
	/// Corrects year number in case of W53 dates in January.
	/// </summary>
	public static WeekAndYear FromDate(DateTime date)
	{
		WeekAndYear result = new WeekAndYear();

		result.Year = date.Year;
		result.Week = CultureInfo.CurrentCulture.Calendar.GetWeekOfYear(date, CalendarWeekRule.FirstFourDayWeek, DayOfWeek.Monday);

		// If the date in January falls into the last week of the previous
		// year, decrement the year extracted from the DateTime
		if (result.Week >= 52 && date.DayOfYear <= 3)
			result.Year--;

		return result;
	}
}