content: module Data.Time.Calendar.Easter
  	(
  	sundayAfter,
  	orthodoxPaschalMoon,orthodoxEaster,
  	gregorianPaschalMoon,gregorianEaster
  	) where
  
  -- formulae from Reingold & Dershowitz, _Calendrical Calculations_, ch. 8.
  	
  import Data.Time.Calendar
  import Data.Time.Calendar.Julian
  
  -- | The next Sunday strictly after a given day.
  sundayAfter :: Day -> Day
  sundayAfter day = addDays (7 - (mod (toModifiedJulianDay day + 3) 7)) day
  
  -- | Given a year, find the Paschal full moon according to Orthodox Christian tradition
  orthodoxPaschalMoon :: Integer -> Day
  orthodoxPaschalMoon year  = addDays (- shiftedEpact) (fromJulian jyear 4 19) where
  	shiftedEpact = mod (14 + 11 * (mod year 19)) 30
  	jyear = if year > 0 then year else year - 1
  
  -- | Given a year, find Easter according to Orthodox Christian tradition
  orthodoxEaster :: Integer -> Day
  orthodoxEaster = sundayAfter . orthodoxPaschalMoon
  
  -- | Given a year, find the Paschal full moon according to the Gregorian method
  gregorianPaschalMoon :: Integer -> Day
  gregorianPaschalMoon year  = addDays (- adjustedEpact) (fromGregorian year 4 19) where
  	century = (div year 100) + 1
  	shiftedEpact = mod (14 + 11 * (mod year 19) - (div (3 * century) 4) + (div (5 + 8 * century) 25)) 30
  	adjustedEpact = if shiftedEpact == 0 || ((shiftedEpact == 1) && (mod year 19 < 10)) then shiftedEpact + 1 else shiftedEpact
  
  -- | Given a year, find Easter according to the Gregorian method
  gregorianEaster :: Integer -> Day
  gregorianEaster = sundayAfter . gregorianPaschalMoon
  