using System;

using System.Globalization;

​

public class Program

{

    public static void Main()

    {

        Console.WriteLine("Hello World");

        System.Globalization.ISOWeek.GetWeeksInYear(2004);

        for (var y = 2000; y < 2031; y++)

        {

            var weeksInYear = System.Globalization.ISOWeek.GetWeeksInYear(y);

            DateTime dt = new DateTime(y, 12, 31);

            int week = CultureInfo.CurrentCulture.Calendar.GetWeekOfYear(dt, CalendarWeekRule.FirstFourDayWeek, DayOfWeek.Monday);

            var dateOfLastMonday = ISOWeek.ToDateTime(y, weeksInYear, System.DayOfWeek.Monday);

            var dateOfFirstMonday = ISOWeek.ToDateTime(y, 1, System.DayOfWeek.Monday);

            Console.WriteLine($"{y} : {weeksInYear} .NET5 weeks - {Program.weeks(y)} manual weeks  - date of first and last Monday = {dateOfFirstMonday:dd-MM-yyyy} - {dateOfLastMonday:dd-MM-yyyy}");

        }

    }

​

    /*

            func weeks(in year: Int) -> Int {

                func p(_ year: Int) -> Int {

                    return (year + year/4 - year/100 + year/400) % 7

                }

                return (p(year) == 4 || p(year-1) == 3) ? 53 : 52

            }

​

            */

    // see : https://stackoverflow.com/a/40491108/735753

    public static int weeks(int year)

    {

        Func<int, int> p = y => (y + y / 4 - y / 100 + y / 400) % 7;

        return p(year) == 4 || p(year - 1) == 3 ? 53 : 52;

    }

}