using System;
using System.Globalization;

namespace Kreta.BusinessLogic.Classes
{
    public static class DateHelper
    {
        // This presumes that weeks start with Monday.
        // Week 1 is the 1st week of the year with a Thursday in it.
        public static int GetISO8601WeekOfYear(this DateTime date)
        {
            var calendar = CultureInfo.InvariantCulture.Calendar;

            // Seriously cheat.  If its Monday, Tuesday or Wednesday, then it’ll 
            // be the same week# as whatever Thursday, Friday or Saturday are,
            // and we always get those right
            DayOfWeek day = calendar.GetDayOfWeek(date);
            if (DayOfWeek.Monday <= day && day <= DayOfWeek.Wednesday)
            {
                date = date.AddDays(3);
            }

            // Return the week of our adjusted day
            return calendar.GetWeekOfYear(date, CalendarWeekRule.FirstFourDayWeek, DayOfWeek.Monday);
        }

        public static int GetISO8601WeekOfYear(this DateTime? date)
        {
            if (date.HasValue)
            {
                return date.Value.GetISO8601WeekOfYear();
            }

            return GetWeeksInYear();
        }

        private static int GetWeeksInYear()
        {
            return GetISO8601WeekOfYear(new DateTime(DateTime.Now.Year, 12, 31));
        }
    }
}