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