using System;
using System.Collections.Generic;
using System.Text.RegularExpressions;


namespace RobvanderWoude
{
	internal class Easter
	{
		static readonly string progver = "1.00";
		static readonly DateTime referencefullmoon = DateTime.Parse( "2022-01-17 23:48" );
		static readonly double mooncycleindays = 29.53;


		static int Main( string[] args )
		{
			int year;
			string dateformat = "yyyy-MM-dd";

			if ( args.Length == 0 )
			{
				year= DateTime.Now.Year;
			}
			else if ( args.Length > 2 || !int.TryParse( args[0], out year ) )
			{
				return ShowHelp( );
			}
			if ( args.Length == 2 )
			{
				// date delimiters allowed: hyphen, forward slash, dot or space
				Regex regex = new Regex( "^(short|long|[dMy/ .-]{6,10})", RegexOptions.IgnoreCase );
				if ( !regex.IsMatch( args[1] ) )
				{
					return ShowHelp( "Invalid date format \"{0}\"", args[1] );
				}
				dateformat = args[1].ToLower( ).Replace( 'm', 'M' ).Replace( "/", "'/'" ).Replace( "-", "'-'" ).Replace( ".", "'.'" ).Replace( " ", "' '" );
			}


			// Easter is at first Sunday after the first full moon at or after the Spring equinox (21 March)
			// Calculation explained: https://www.timeanddate.com/calendar/determining-easter-date.html

			// For Easter calculations, the Spring equinox is always assumed to be at 21 March
			DateTime springequinox = DateTime.Parse( string.Format( "{0}-03-21", year ) );

			// Calculate full moon cycles to first full moon after Spring equinox of specified year
			int fullcycles = (int)Math.Ceiling( springequinox.Subtract( referencefullmoon ).Days / mooncycleindays );

			// Date of first full moon after Spring equinox of specified year
			DateTime fullmoonafterequinox = referencefullmoon.AddDays( fullcycles * mooncycleindays );

			// First Sunday following first full moon at or after Spring equinox of specified year
			DateTime eastersunday = fullmoonafterequinox.AddDays( 7 - (int)fullmoonafterequinox.DayOfWeek ).Date;

			// Display reasult on screen
			string wasisorwillbe = "is today,";
			if ( DateTime.Compare( eastersunday, DateTime.Now.Date ) < 0 )
			{
				wasisorwillbe = "was at";
			}
			else if ( DateTime.Compare( eastersunday, DateTime.Now.Date ) > 0 )
			{
				wasisorwillbe = "will be at";
			}
			string easterdatestring;
			if ( dateformat == "short" )
			{
				easterdatestring = eastersunday.ToShortDateString( );
			}
			else if ( dateformat == "long" )
			{
				easterdatestring = eastersunday.ToLongDateString( );
			}
			else
			{
				easterdatestring = eastersunday.ToString( dateformat );
			}
			Console.WriteLine( "Easter Sunday {0} {1} {2}", year, wasisorwillbe, easterdatestring );

			// Return code equals Easter Sunday's DayOfYear
			return eastersunday.DayOfYear;
		}


		static int ShowHelp( params string[] errmsg )
		{
			#region Error Message

			if ( errmsg.Length > 0 )
			{
				List<string> errargs = new List<string>( errmsg );
				errargs.RemoveAt( 0 );
				Console.Error.WriteLine( );
				Console.ForegroundColor = ConsoleColor.Red;
				Console.Error.Write( "ERROR:\t" );
				Console.ForegroundColor = ConsoleColor.White;
				Console.Error.WriteLine( errmsg[0], errargs.ToArray( ) );
				Console.ResetColor( );
			}

			#endregion Error Message


			#region Help Text

			/*
			Easter.exe,  Version 1.00
			Calculate the date of Easter Sunday for the specified year
 
			Usage:    Easter.exe   [ year  [ format ] ]
 
			Where:    year         is the year to calculate Easter date for
			                       (default: current year; see notes for before 100 AD)
			          format       is the output format for the returned date: either
			                       "short", "long" or a combination of 6..10 "y", "M",
			                       "d" characters, hyphens, forward slashes, dots and/or
			                       spaces (default: "yyyy-MM-dd")
			
			Notes:    Easter Sunday is at the first Sunday after the first full moon at or
			          after the Spring equinox, which for Easter calculations is assumed
			          to be always at March 21.
			          Years before 100 AD will be interpreted as 2000 + specified year.
			          The program's return code equals the calculated date's DayOfYear
			          value, or -1 in case of (command line) errors.
 
			Credits:  Easter calculation explained:
			          https://www.timeanddate.com/calendar/determining-easter-date.html
			          Fix to use alternative date delimiter by Jon Skeet:
			          https://stackoverflow.com/a/5641084


			Written by Rob van der Woude
			https://www.robvanderwoude.com
			*/

			#endregion Help Text


			#region Display Help Text

			Console.Error.WriteLine( );

			Console.Error.WriteLine( "Easter.exe,  Version {0}", progver );

			Console.Error.WriteLine( "Calculate the date of Easter Sunday for the specified year" );

			Console.Error.WriteLine( );

			Console.Error.Write( "Usage:    " );
			Console.ForegroundColor = ConsoleColor.White;
			Console.Error.WriteLine( "Easter.exe   [ year  [ format ] ]" );
			Console.ResetColor( );

			Console.Error.WriteLine( );

			Console.Error.Write( "Where:    " );
			Console.ForegroundColor = ConsoleColor.White;
			Console.Error.Write( "year" );
			Console.ResetColor( );
			Console.Error.WriteLine( "         is the year to calculate Easter date for" );

			Console.Error.WriteLine( "                       (default: current year; see notes for before 100 AD)" );

			Console.ForegroundColor = ConsoleColor.White;
			Console.Error.Write( "          format" );
			Console.ResetColor( );
			Console.Error.WriteLine( "       is the output format for the returned date: either" );
			Console.ResetColor( );

			Console.Error.Write( "                       \"" );
			Console.ForegroundColor = ConsoleColor.White;
			Console.Error.Write( "short" );
			Console.ResetColor( );
			Console.Error.Write( "\", \"" );
			Console.ForegroundColor = ConsoleColor.White;
			Console.Error.Write( "long" );
			Console.ResetColor( );
			Console.Error.Write( "\" or a combination of 6..10 \"" );
			Console.ForegroundColor = ConsoleColor.White;
			Console.Error.Write( "y" );
			Console.ResetColor( );
			Console.Error.Write( "\", \"" );
			Console.ForegroundColor = ConsoleColor.White;
			Console.Error.Write( "M" );
			Console.ResetColor( );
			Console.Error.WriteLine( "\"," );

			Console.Error.Write( "                       \"" );
			Console.ForegroundColor = ConsoleColor.White;
			Console.Error.Write( "d" );
			Console.ResetColor( );
			Console.Error.WriteLine( "\" characters, hyphens, forward slashes, dots and/or" );

			Console.Error.Write( "                       spaces (default: \"" );
			Console.ForegroundColor = ConsoleColor.White;
			Console.Error.Write( "yyyy-MM-dd" );
			Console.ResetColor( );
			Console.Error.WriteLine( "\")" );

			Console.Error.WriteLine( );

			Console.Error.WriteLine( "Notes:    Easter Sunday is at the first Sunday after the first full moon at or" );

			Console.Error.WriteLine( "          after the Spring equinox, which for Easter calculations is assumed" );

			Console.Error.WriteLine( "          to be always at March 21." );

			Console.Error.WriteLine( "          Years before 100 AD will be interpreted as 2000 + specified year." );

			Console.Error.WriteLine( "          The program's return code equals the calculated date's DayOfYear" );

			Console.Error.WriteLine( "          value, or -1 in case of (command line) errors." );

			Console.Error.WriteLine( );

			Console.Error.WriteLine( "Credits:  Easter calculation explained:" );

			Console.ForegroundColor = ConsoleColor.DarkGray;
			Console.Error.WriteLine( "          https://www.timeanddate.com/calendar/determining-easter-date.html" );
			Console.ResetColor( );

			Console.Error.WriteLine( );

			Console.Error.WriteLine( "Written by Rob van der Woude" );

			Console.Error.WriteLine( "https://www.robvanderwoude.com" );

			#endregion Display Help Text


			return -1;
		}
	}
}