Source code for easter.bat

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@ECHO OFF
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ECHO.
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:: Localize environment and enable delayed variable expansion (XP and later only)

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IF "%OS%"=="Windows_NT" (
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	SETLOCAL ENABLEDELAYEDEXPANSION
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) ELSE (
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	GOTO Syntax
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)
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:: Command line check
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IF NOT "%~2"=="" GOTO Syntax
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:: REG.EXE version 3 or later required
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IF NOT EXIST "%SystemRoot%\System32\reg.exe" GOTO Syntax

FOR /F "tokens=5" %%A IN ('FILEVER.EXE "%SystemRoot%\System32\reg.exe"') DO (

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	FOR /F "delims=." %%B IN ("%%A") DO (
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		IF %%B0 LSS 30 GOTO Syntax
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	)
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)
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:: Determine current year
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CALL :ThisYear
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:: If no year is specified, use current year

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IF "%~1"=="" (
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	SET Y=%ThisYear%
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) ELSE (
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	SET /A Y=%~1
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	IF ERRORLEVEL 1 GOTO Syntax
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)
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:: Is the specified year valid?
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:: Check if number
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FOR /F "tokens=1 delims=0123456789" %%A IN ("%Y%") DO IF NOT "%%~A"=="" GOTO Syntax

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:: check if in range
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IF %Y%0 LSS 17520 GOTO Syntax
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IF %Y%0 GTR 30000 GOTO Syntax
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:: Array of month names used
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SET Month.03=March
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SET Month.3=March
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SET Month.04=April
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SET Month.4=April
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SET Month.05=May
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SET Month.5=May
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SET Month.06=June
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SET Month.6=June
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```

:: Calculate Easter Day using the instructions found at

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:: Simon Kershaw's "KEEPING THE FEAST"
```

:: http://www.oremus.org/liturgy/etc/ktf/app/easter.html

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SET /A G  = ( %Y% %% 19 ) + 1
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SET /A S  = (( %Y% - 1600 ) / 100 ) - (( %Y% - 1600 ) / 400 )

SET /A L  = ((( %Y% - 1400 ) / 100 ) * 8 ) / 25

SET /A P1 = ( 30003 - 11 * %G% + %S% - %L% ) %% 30

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SET P=%P1%
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IF %P%==28 IF %G% GTR 11 SET P=27
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IF %P%==29 SET P=28
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SET /A D  = ( %Y% + ( %Y% / 4 ) - ( %Y% / 100 ) + ( %Y% / 400 )) %% 7

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SET /A D1 = ( 8 - %D% ) %% 7
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SET /A P2 = ( 70003 + %P% ) %% 7
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SET /A X  = (( 70004 - %D% - %P% ) %% 7 ) + 1

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SET /A E  = %P% + %X%
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IF %E% LSS 11 (
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	SET /A ED = %E% + 21
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	SET EM=3
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) ELSE (
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	SET /A ED = %E% - 10
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	SET EM=4
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)
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IF %Y% LSS %ThisYear% SET IS=was
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IF %Y% EQU %ThisYear% SET IS=is
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IF %Y% GTR %ThisYear% SET IS=will be
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:: Calculate Ascension and Pentecost dates

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CALL :JDate %Y% %EM% %ED%
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SET /A ADJ = %JDate% + 39
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SET /A PDJ = %JDate% + 49
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CALL :GDate %ADJ%
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FOR /F "tokens=2,3" %%A IN ("%GDate%") DO (

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	SET AM=%%A
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	SET AD=%%B
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)
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CALL :GDate %PDJ%
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FOR /F "tokens=2,3" %%A IN ("%GDate%") DO (

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	SET PM=%%A
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	SET PD=%%B
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)
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:: Display the result
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ECHO In %Y% Easter Day %IS% !Month.%EM%! %ED%

ECHO         Ascension Day %IS% !Month.%AM%! %AD%

ECHO         Pentecost %IS% !Month.%PM%! %PD%

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:: Done
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GOTO End
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:GDate
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:: Convert Julian date back to "normal" Gregorian date

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:: Argument : Julian date
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:: Returns  : YYYY MM DD
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::
```

:: Algorithm based on Fliegel-Van Flandern

:: algorithm from the Astronomical Almanac,

:: provided by Doctor Fenton on the Math Forum

:: (http://mathforum.org/library/drmath/view/51907.html),

:: and converted to batch code by Ron Bakowski.

```
::
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SET /A P      = %1 + 68569
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SET /A Q      = 4 * %P% / 146097
```

SET /A R      = %P% - ( 146097 * %Q% +3 ) / 4

SET /A S      = 4000 * ( %R% + 1 ) / 1461001

SET /A T      = %R% - 1461 * %S% / 4 + 31

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SET /A U      = 80 * %T% / 2447
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SET /A V      = %U% / 11
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SET /A GYear  = 100 * ( %Q% - 49 ) + %S% + %V%

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SET /A GMonth = %U% + 2 - 12 * %V%
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SET /A GDay   = %T% - 2447 * %U% / 80
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:: Clean up the mess
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FOR %%A IN (P Q R S T U V) DO SET %%A=
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:: Add leading zeroes
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IF 1%GMonth% LSS 20 SET GMonth=0%GMonth%

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IF 1%GDay%   LSS 20 SET GDay=0%GDay%
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:: Return value
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SET GDate=%GYear% %GMonth% %GDay%
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GOTO:EOF
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:JDate
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SETLOCAL
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:: Convert date to Julian
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:: Arguments : YYYY MM DD
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:: Returns   : Julian date
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::
```

:: Algorithm based on Fliegel-Van Flandern

:: algorithm from the Astronomical Almanac,

:: provided by Doctor Fenton on the Math Forum

:: (http://mathforum.org/library/drmath/view/51907.html),

:: and converted to batch code by Ron Bakowski.

```
::
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SET Month0=%2
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SET Day0=%3
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:: Strip leading zeroes from month and day

IF 1%Month0% GTR 100 SET /A Month0 = 1%Month0% - 100

IF 1%Day0%   GTR 100 SET /A Day0   = 1%Day0%   - 100

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SET /A Month1 = ( %Month0% - 14 ) / 12
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SET /A Year1  = %1 + 4800
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SET /A JDate  = 1461 * ( %Year1% + %Month1% ) / 4 + 367 * ( %Month0% - 2 -12 * %Month1% ) / 12 - ( 3 * ( ( %Year1% + %Month1% + 100 ) / 100 ) ) / 4 + %3 - 32075

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ENDLOCAL & SET JDate=%JDate%
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GOTO:EOF
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:ThisYear
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:: Read iDate and sDate from the registry; more info on iDate and sDate:

:: https://www.itprotoday.com/windows-78/jsi-tip-0311-regional-settings-registry

FOR /F "tokens=3" %%A IN ('REG.EXE Query "HKEY_CURRENT_USER\Control Panel\International" /v iDate') DO SET iDate=%%A

FOR /F "tokens=3" %%A IN ('REG.EXE Query "HKEY_CURRENT_USER\Control Panel\International" /v sDate') DO SET sDate=%%A

:: Detemine current year depending on registry settings

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FOR %%A IN (%Date%) DO SET Today=%%A
```

IF %iDate%==2 (SET Token=1) ELSE (SET Token=3)

FOR /F "tokens=%Token% delims=%sDate%" %%A IN ("%Today%") DO SET ThisYear=%%A

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GOTO:EOF
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:Syntax
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ECHO Easter.bat,  Version 3.01 for Windows XP and later

ECHO Calculate Easter day, Ascension day and Pentecost dates for the specified year.

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ECHO.
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ECHO Usage:  EASTER  [ year ]
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ECHO.
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ECHO Where:  year should be within the range of 1752..3000 (default: current year)

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ECHO.
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ECHO Note:   Easter day calculation based on Simon Kershaw's "KEEPING THE FEAST"

ECHO         (http://www.oremus.org/liturgy/etc/ktf/app/easter.html).

ECHO         Julian date math algorithms based on Fliegel-Van Flandern algorithm

ECHO         from the Astronomical Almanac, provided by Doctor Fenton on the Math

ECHO         Forum (http://mathforum.org/library/drmath/view/51907.html), and

ECHO         converted to batch code by Ron Bakowski.

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ECHO.
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ECHO Written by Rob van der Woude
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ECHO http://www.robvanderwoude.com
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:End
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IF "%OS%"=="Windows_NT" ENDLOCAL
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```