ISOWeek.cs

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namespace System.Globalization;
 
public static class ISOWeek
{
    public static int GetWeekOfYear(DateTime date)
    {
        int weekNumber = GetWeekNumber(date);
        if (weekNumber < 1)
        {
            return GetWeeksInYear(date.Year - 1);
        }
        if (weekNumber > GetWeeksInYear(date.Year))
        {
            return 1;
        }
        return weekNumber;
    }
 
    public static int GetYear(DateTime date)
    {
        int weekNumber = GetWeekNumber(date);
        if (weekNumber < 1)
        {
            return date.Year - 1;
        }
        if (weekNumber > GetWeeksInYear(date.Year))
        {
            return date.Year + 1;
        }
        return date.Year;
    }
 
    public static DateTime GetYearStart(int year)
    {
        return ToDateTime(year, 1, DayOfWeek.Monday);
    }
 
    public static DateTime GetYearEnd(int year)
    {
        return ToDateTime(year, GetWeeksInYear(year), DayOfWeek.Sunday);
    }
 
    public static int GetWeeksInYear(int year)
    {
        if (year < 1 || year > 9999)
        {
            throw new ArgumentOutOfRangeException("year", SR.ArgumentOutOfRange_Year);
        }
        if (P(year) == 4 || P(year - 1) == 3)
        {
            return 53;
        }
        return 52;
        static int P(int y)
        {
            return (y + y / 4 - y / 100 + y / 400) % 7;
        }
    }
 
    public static DateTime ToDateTime(int year, int week, DayOfWeek dayOfWeek)
    {
        if (year < 1 || year > 9999)
        {
            throw new ArgumentOutOfRangeException("year", SR.ArgumentOutOfRange_Year);
        }
        if (week < 1 || week > 53)
        {
            throw new ArgumentOutOfRangeException("week", SR.ArgumentOutOfRange_Week_ISO);
        }
        if (dayOfWeek < DayOfWeek.Sunday || dayOfWeek > (DayOfWeek)7)
        {
            throw new ArgumentOutOfRangeException("dayOfWeek", SR.ArgumentOutOfRange_DayOfWeek);
        }
        int num = GetWeekday(new DateTime(year, 1, 4).DayOfWeek) + 3;
        int num2 = week * 7 + GetWeekday(dayOfWeek) - num;
        return new DateTime(year, 1, 1).AddDays(num2 - 1);
    }
 
    private static int GetWeekNumber(DateTime date)
    {
        return (date.DayOfYear - GetWeekday(date.DayOfWeek) + 10) / 7;
    }
 
    private static int GetWeekday(DayOfWeek dayOfWeek)
    {
        if (dayOfWeek != 0)
        {
            return (int)dayOfWeek;
        }
        return 7;
    }
}