    {-# OPTIONS -fno-warn-orphans #-}

    -- #hide
    module Data.Time.Calendar.Gregorian
    (
        -- * Gregorian calendar
        toGregorian,fromGregorian,fromGregorianValid,showGregorian,gregorianMonthLength,

        -- calendrical arithmetic
        -- e.g. "one month after March 31st"
        addGregorianMonthsClip,addGregorianMonthsRollOver,
        addGregorianYearsClip,addGregorianYearsRollOver,

        -- re-exported from OrdinalDate
        isLeapYear
    ) where

    import Data.Time.Calendar.MonthDay
    import Data.Time.Calendar.OrdinalDate
    import Data.Time.Calendar.Days
    import Data.Time.Calendar.Private

    -- | Convert to proleptic Gregorian calendar. First element of result is year, second month number (1-12), third day (1-31).
    toGregorian :: Day -> (Integer,Int,Int)
    toGregorian date = (year,month,day) where
        (year,yd) = toOrdinalDate date
        (month,day) = dayOfYearToMonthAndDay (isLeapYear year) yd

    -- | Convert from proleptic Gregorian calendar. First argument is year, second month number (1-12), third day (1-31).
    -- Invalid values will be clipped to the correct range, month first, then day.
    fromGregorian :: Integer -> Int -> Int -> Day
    fromGregorian year month day = fromOrdinalDate year (monthAndDayToDayOfYear (isLeapYear year) month day)

    -- | Convert from proleptic Gregorian calendar. First argument is year, second month number (1-12), third day (1-31).
    -- Invalid values will return Nothing
    fromGregorianValid :: Integer -> Int -> Int -> Maybe Day
    fromGregorianValid year month day = do
        doy <- monthAndDayToDayOfYearValid (isLeapYear year) month day
        fromOrdinalDateValid year doy

    -- | Show in ISO 8601 format (yyyy-mm-dd)
    showGregorian :: Day -> String
    showGregorian date = (show4 y) ++ "-" ++ (show2 m) ++ "-" ++ (show2 d) where
        (y,m,d) = toGregorian date

    -- | The number of days in a given month according to the proleptic Gregorian calendar. First argument is year, second is month.
    gregorianMonthLength :: Integer -> Int -> Int
    gregorianMonthLength year = monthLength (isLeapYear year)

    rolloverMonths :: (Integer,Integer) -> (Integer,Int)
    rolloverMonths (y,m) = (y + (div (m - 1) 12),fromIntegral (mod (m - 1) 12) + 1)

    addGregorianMonths :: Integer -> Day -> (Integer,Int,Int)
    addGregorianMonths n day = (y',m',d) where
        (y,m,d) = toGregorian day
        (y',m') = rolloverMonths (y,fromIntegral m + n)

    -- | Add months, with days past the last day of the month clipped to the last day.
    -- For instance, 2005-01-30 + 1 month = 2005-02-28.
    addGregorianMonthsClip :: Integer -> Day -> Day
    addGregorianMonthsClip n day = fromGregorian y m d where
        (y,m,d) = addGregorianMonths n day

    -- | Add months, with days past the last day of the month rolling over to the next month.
    -- For instance, 2005-01-30 + 1 month = 2005-03-02.
    addGregorianMonthsRollOver :: Integer -> Day -> Day
    addGregorianMonthsRollOver n day = addDays (fromIntegral d - 1) (fromGregorian y m 1) where
        (y,m,d) = addGregorianMonths n day

    -- | Add years, matching month and day, with Feb 29th clipped to Feb 28th if necessary.
    -- For instance, 2004-02-29 + 2 years = 2006-02-28.
    addGregorianYearsClip :: Integer -> Day -> Day
    addGregorianYearsClip n = addGregorianMonthsClip (n * 12)

    -- | Add years, matching month and day, with Feb 29th rolled over to Mar 1st if necessary.
    -- For instance, 2004-02-29 + 2 years = 2006-03-01.
    addGregorianYearsRollOver :: Integer -> Day -> Day
    addGregorianYearsRollOver n = addGregorianMonthsRollOver (n * 12)

    -- orphan instance
    instance Show Day where
        show = showGregorian