using System;
using System.Collections.Generic;
using System.Globalization;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace EgwCoreLib.Razor.Data
{
    public class WeekData
    {
        public int anno { get; set; } = DateTime.Today.Year;

        public DateTime inizio { get; set; } = DateTime.Today;
        public DateTime fine { get; set; } = DateTime.Today.AddDays(1);

        public int weekNumber { get; set; } = 1;

        /// <summary>
        /// Calcola estremi settimana dato un giorno come lunedì-domenica
        /// </summary>
        /// <param name="dtRif"></param>
        public WeekData(DateTime dtRif)
        {
            DayOfWeek day = CultureInfo.InvariantCulture.Calendar.GetDayOfWeek(dtRif);
            // attenzione: è domenica = 0 --> in quel caso va portato a 7!!!
            int numDay = day == DayOfWeek.Sunday ? 7 : (int)day;
            this.weekNumber = GetIso8601WeekOfYear(dtRif);
            this.inizio = dtRif.Date.AddDays(1 - numDay);
            this.anno = inizio.Year;
            this.fine = dtRif.Date.AddDays(7 - numDay);
        }

        /// <summary>
        /// Calcola estremi settimana dato numero + anno
        /// </summary>
        /// <param name="numWeek"></param>
        public WeekData(int year, int numWeek)
        {
            this.anno = year;
            this.weekNumber = numWeek;
            DateTime dtRif = FirstDateOfWeekISO8601(year, numWeek);
            DayOfWeek day = CultureInfo.InvariantCulture.Calendar.GetDayOfWeek(dtRif);
            this.inizio = dtRif.Date.AddDays(1 - (int)day);
            this.fine = dtRif.Date.AddDays(7 - (int)day);
        }

        /// <summary>
        /// Calcolo settimana dell'anno ISO 8601
        /// 
        /// This presumes that weeks start with Monday.
        /// Week 1 is the 1st week of the year with a Thursday in it.
        /// 
        /// rif: https://stackoverflow.com/questions/11154673/get-the-correct-week-number-of-a-given-date
        /// </summary>
        /// <param name="time"></param>
        /// <returns></returns>
        public static int GetIso8601WeekOfYear(DateTime time)
        {
            // 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 = CultureInfo.InvariantCulture.Calendar.GetDayOfWeek(time);
            if (day >= DayOfWeek.Monday && day <= DayOfWeek.Wednesday)
            {
                time = time.AddDays(3);
            }

            // Return the week of our adjusted day
            return CultureInfo.InvariantCulture.Calendar.GetWeekOfYear(time, CalendarWeekRule.FirstFourDayWeek, DayOfWeek.Monday);
        }

        public static DateTime FirstDateOfWeekISO8601(int year, int weekOfYear)
        {
            DateTime jan1 = new DateTime(year, 1, 1);
            int daysOffset = DayOfWeek.Thursday - jan1.DayOfWeek;

            // Use first Thursday in January to get first week of the year as
            // it will never be in Week 52/53
            DateTime firstThursday = jan1.AddDays(daysOffset);
            var cal = CultureInfo.CurrentCulture.Calendar;
            int firstWeek = cal.GetWeekOfYear(firstThursday, CalendarWeekRule.FirstFourDayWeek, DayOfWeek.Monday);

            var weekNum = weekOfYear;
            // As we're adding days to a date in Week 1,
            // we need to subtract 1 in order to get the right date for week #1
            if (firstWeek == 1)
            {
                weekNum -= 1;
            }

            // Using the first Thursday as starting week ensures that we are starting in the right year
            // then we add number of weeks multiplied with days
            var result = firstThursday.AddDays(weekNum * 7);

            // Subtract 3 days from Thursday to get Monday, which is the first weekday in ISO8601
            return result.AddDays(-3);
        }
    }
}