Source code for easter.cs

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using System;
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using System.Collections.Generic;
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using System.Text.RegularExpressions;
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namespace RobvanderWoude
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{
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	internal class Easter
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	{
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		static readonly string progver = "1.00";

		static readonly DateTime referencefullmoon = DateTime.Parse( "2022-01-17 23:48" );

		static readonly double mooncycleindays = 29.53;

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		static int Main( string[] args )
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		{
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			int year;
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			string dateformat = "yyyy-MM-dd";
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			if ( args.Length == 0 )
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			{
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				year= DateTime.Now.Year;
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			}
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			else if ( args.Length > 2 || !int.TryParse( args[0], out year ) )

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			{
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				return ShowHelp( );
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			}
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			if ( args.Length == 2 )
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			{
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				// date delimiters allowed: hyphen, forward slash, dot or space

				Regex regex = new Regex( "^(short|long|[dMy/ .-]{6,10})", RegexOptions.IgnoreCase );

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				if ( !regex.IsMatch( args[1] ) )
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				{
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					return ShowHelp( "Invalid date format \"{0}\"", args[1] );

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				}
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				dateformat = args[1].ToLower( ).Replace( 'm', 'M' ).Replace( "/", "'/'" ).Replace( "-", "'-'" ).Replace( ".", "'.'" ).Replace( " ", "' '" );

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			}
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			// 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

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			// For Easter calculations, the Spring equinox is always assumed to be at 21 March

			DateTime springequinox = DateTime.Parse( string.Format( "{0}-03-21", year ) );

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			// Calculate full moon cycles to first full moon after Spring equinox of specified year

			int fullcycles = (int)Math.Ceiling( springequinox.Subtract( referencefullmoon ).Days / mooncycleindays );

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			// Date of first full moon after Spring equinox of specified year

			DateTime fullmoonafterequinox = referencefullmoon.AddDays( fullcycles * mooncycleindays );

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			// First Sunday following first full moon at or after Spring equinox of specified year

			DateTime eastersunday = fullmoonafterequinox.AddDays( 7 - (int)fullmoonafterequinox.DayOfWeek ).Date;

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			// Display reasult on screen
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			string wasisorwillbe = "is today,";
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			if ( DateTime.Compare( eastersunday, DateTime.Now.Date ) < 0 )

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			{
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				wasisorwillbe = "was at";
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			}
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			else if ( DateTime.Compare( eastersunday, DateTime.Now.Date ) > 0 )

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			{
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				wasisorwillbe = "will be at";
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			}
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			string easterdatestring;
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			if ( dateformat == "short" )
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			{
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				easterdatestring = eastersunday.ToShortDateString( );

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			}
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			else if ( dateformat == "long" )
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			{
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				easterdatestring = eastersunday.ToLongDateString( );

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			}
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			else
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			{
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				easterdatestring = eastersunday.ToString( dateformat );

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			}
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			Console.WriteLine( "Easter Sunday {0} {1} {2}", year, wasisorwillbe, easterdatestring );

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			// Return code equals Easter Sunday's DayOfYear

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			return eastersunday.DayOfYear;
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		}
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		static int ShowHelp( params string[] errmsg )

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		{
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			#region Error Message
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			if ( errmsg.Length > 0 )
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			{
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				List<string> errargs = new List<string>( errmsg );

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				errargs.RemoveAt( 0 );
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				Console.Error.WriteLine( );
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				Console.ForegroundColor = ConsoleColor.Red;

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				Console.Error.Write( "ERROR:\t" );
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				Console.ForegroundColor = ConsoleColor.White;

				Console.Error.WriteLine( errmsg[0], errargs.ToArray( ) );

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				Console.ResetColor( );
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			}
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			#endregion Error Message
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			#region Help Text
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			/*
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			Easter.exe,  Version 1.00
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			Calculate the date of Easter Sunday for the specified year

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			Usage:    Easter.exe   [ year  [ format ] ]

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```

			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

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			          to be always at March 21.
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			          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

```
 
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			Written by Rob van der Woude
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			https://www.robvanderwoude.com
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			*/
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			#endregion Help Text
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			#region Display Help Text
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			Console.Error.WriteLine( );
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			Console.Error.WriteLine( "Easter.exe,  Version {0}", progver );

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

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			Console.Error.WriteLine( );
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			Console.Error.Write( "Usage:    " );
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			Console.ForegroundColor = ConsoleColor.White;

			Console.Error.WriteLine( "Easter.exe   [ year  [ format ] ]" );

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			Console.ResetColor( );
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			Console.Error.WriteLine( );
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			Console.Error.Write( "Where:    " );
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			Console.ForegroundColor = ConsoleColor.White;

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			Console.Error.Write( "year" );
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			Console.ResetColor( );
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			Console.Error.WriteLine( "         is the year to calculate Easter date for" );

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

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			Console.ForegroundColor = ConsoleColor.White;

			Console.Error.Write( "          format" );

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			Console.ResetColor( );
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			Console.Error.WriteLine( "       is the output format for the returned date: either" );

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			Console.ResetColor( );
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			Console.Error.Write( "                       \"" );

			Console.ForegroundColor = ConsoleColor.White;

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			Console.Error.Write( "short" );
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			Console.ResetColor( );
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			Console.Error.Write( "\", \"" );
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			Console.ForegroundColor = ConsoleColor.White;

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			Console.Error.Write( "long" );
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			Console.ResetColor( );
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			Console.Error.Write( "\" or a combination of 6..10 \"" );

			Console.ForegroundColor = ConsoleColor.White;

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			Console.Error.Write( "y" );
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			Console.ResetColor( );
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			Console.Error.Write( "\", \"" );
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			Console.ForegroundColor = ConsoleColor.White;

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			Console.Error.Write( "M" );
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			Console.ResetColor( );
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			Console.Error.WriteLine( "\"," );
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			Console.Error.Write( "                       \"" );

			Console.ForegroundColor = ConsoleColor.White;

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

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			Console.Error.Write( "                       spaces (default: \"" );

			Console.ForegroundColor = ConsoleColor.White;

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			Console.Error.Write( "yyyy-MM-dd" );
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			Console.ResetColor( );
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			Console.Error.WriteLine( "\")" );
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			Console.Error.WriteLine( );
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			Console.Error.WriteLine( "Notes:    Easter Sunday is at the first Sunday after the first full moon at or" );

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

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

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

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

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

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

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			Console.ForegroundColor = ConsoleColor.DarkGray;

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

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

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

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			#endregion Display Help Text
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			return -1;
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		}
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	}
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}
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