using System;
using System.Diagnostics;
using System.Globalization;

namespace StartOfCurrentWeek {
    class Program {
        static void Main(string[] args) {


            // monday result
            DateTimeFormatInfo fi = new CultureInfo("nl-NL").DateTimeFormat;
            DateTime mondayResult = new DateTime(2008, 4, 28);

            DateTime test1 = StartOfWeek(new DateTime(2008, 5, 4), fi);
            Debug.Assert(test1.Date == mondayResult.Date);


            DateTime test2 = StartOfWeek(new DateTime(2008, 4, 28), fi);
            Debug.Assert(test2.Date == mondayResult.Date);

            DateTime test3 = StartOfWeek(new DateTime(2008, 4, 29), fi);
            Debug.Assert(test3.Date == mondayResult.Date);


            DateTime test4 = StartOfWeek(new DateTime(2008, 4, 30), fi);
            Debug.Assert(test4.Date == mondayResult.Date);

            DateTime test5 = StartOfWeek(new DateTime(2008, 5, 1), fi);
            Debug.Assert(test5.Date == mondayResult.Date);

            DateTime test6 = StartOfWeek(new DateTime(2008, 5, 2), fi);
            Debug.Assert(test6.Date == mondayResult.Date);

            DateTime test7 = StartOfWeek(new DateTime(2008, 5, 3), fi);
            Debug.Assert(test7.Date == mondayResult.Date);


            // sunday result

            DateTimeFormatInfo fi2 = new CultureInfo("en-US").DateTimeFormat;
            DateTime sundayResult = new DateTime(2008, 4, 27);

            DateTime test8 = StartOfWeek(new DateTime(2008, 4, 27), fi2);
            Debug.Assert(test8.Date == sundayResult.Date);


            DateTime test9 = StartOfWeek(new DateTime(2008, 4, 28), fi2);
            Debug.Assert(test9.Date == sundayResult.Date);

            DateTime test10 = StartOfWeek(new DateTime(2008, 4, 29), fi2);
            Debug.Assert(test10.Date == sundayResult.Date);


            DateTime test11 = StartOfWeek(new DateTime(2008, 4, 30), fi2);
            Debug.Assert(test11.Date == sundayResult.Date);

            DateTime test12 = StartOfWeek(new DateTime(2008, 5, 1), fi2);
            Debug.Assert(test12.Date == sundayResult.Date);

            DateTime test13 = StartOfWeek(new DateTime(2008, 5, 2), fi2);
            Debug.Assert(test13.Date == sundayResult.Date);

            DateTime test14 = StartOfWeek(new DateTime(2008, 5, 3), fi2);
            Debug.Assert(test14.Date == sundayResult.Date);

        }



        public static DateTime StartOfWeek(DateTime date, DateTimeFormatInfo dateTimeFormat) {

            Debug.Assert(dateTimeFormat != null, "dateTimeFormat != null");
            if (dateTimeFormat == null) {
                throw new ArgumentNullException("dateTimeFormat");
            }

            DayOfWeek currentDay = date.DayOfWeek;
            DayOfWeek firstDay = dateTimeFormat.FirstDayOfWeek;

            int difference = (int)firstDay - (int)currentDay;

            // we always have to move back, 
            // since we're interested in the first day of the week
            if (difference > 0) { difference -= 7; }

            return date.AddDays(difference);
        }
    }
}