{-# LANGUAGE Safe #-}

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

module Data.Time.Calendar.Gregorian (
    -- * Year, month and day
    Year,
    MonthOfYear,
    pattern January,
    pattern February,
    pattern March,
    pattern April,
    pattern May,
    pattern June,
    pattern July,
    pattern August,
    pattern September,
    pattern October,
    pattern November,
    pattern December,
    DayOfMonth,

    -- * Gregorian calendar
    toGregorian,
    fromGregorian,
    pattern YearMonthDay,
    fromGregorianValid,
    showGregorian,
    gregorianMonthLength,
    -- calendrical arithmetic
    -- e.g. "one month after March 31st"
    addGregorianMonthsClip,
    addGregorianMonthsRollOver,
    addGregorianYearsClip,
    addGregorianYearsRollOver,
    addGregorianDurationClip,
    addGregorianDurationRollOver,
    diffGregorianDurationClip,
    diffGregorianDurationRollOver,
    -- re-exported from OrdinalDate
    isLeapYear,
) where

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

-- | Convert to proleptic Gregorian calendar.
toGregorian :: Day -> (Year, MonthOfYear, DayOfMonth)
toGregorian date = (year, month, day)
  where
    (year, yd) = toOrdinalDate date
    (month, day) = dayOfYearToMonthAndDay (isLeapYear year) yd

-- | Convert from proleptic Gregorian calendar.
-- Invalid values will be clipped to the correct range, month first, then day.
fromGregorian :: Year -> MonthOfYear -> DayOfMonth -> Day
fromGregorian year month day = fromOrdinalDate year (monthAndDayToDayOfYear (isLeapYear year) month day)

-- | Bidirectional abstract constructor for the proleptic Gregorian calendar.
-- Invalid values will be clipped to the correct range, month first, then day.
pattern YearMonthDay :: Year -> MonthOfYear -> DayOfMonth -> Day
pattern YearMonthDay y m d <-
    (toGregorian -> (y, m, d))
    where
        YearMonthDay y m d = fromGregorian y m d

{-# COMPLETE YearMonthDay #-}

-- | Convert from proleptic Gregorian calendar.
-- Invalid values will return Nothing
fromGregorianValid :: Year -> MonthOfYear -> DayOfMonth -> 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.
gregorianMonthLength :: Year -> MonthOfYear -> DayOfMonth
gregorianMonthLength year = monthLength (isLeapYear year)

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

addGregorianMonths :: Integer -> Day -> (Year, MonthOfYear, DayOfMonth)
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)

-- | Add months (clipped to last day), then add days
addGregorianDurationClip :: CalendarDiffDays -> Day -> Day
addGregorianDurationClip (CalendarDiffDays m d) day = addDays d $ addGregorianMonthsClip m day

-- | Add months (rolling over to next month), then add days
addGregorianDurationRollOver :: CalendarDiffDays -> Day -> Day
addGregorianDurationRollOver (CalendarDiffDays m d) day = addDays d $ addGregorianMonthsRollOver m day

-- | Calendrical difference, with as many whole months as possible
diffGregorianDurationClip :: Day -> Day -> CalendarDiffDays
diffGregorianDurationClip day2 day1 =
    let (y1, m1, d1) = toGregorian day1
        (y2, m2, d2) = toGregorian day2
        ym1 = y1 * 12 + toInteger m1
        ym2 = y2 * 12 + toInteger m2
        ymdiff = ym2 - ym1
        ymAllowed =
            if day2 >= day1
                then
                    if d2 >= d1
                        then ymdiff
                        else ymdiff - 1
                else
                    if d2 <= d1
                        then ymdiff
                        else ymdiff + 1
        dayAllowed = addGregorianDurationClip (CalendarDiffDays ymAllowed 0) day1
     in CalendarDiffDays ymAllowed $ diffDays day2 dayAllowed

-- | Calendrical difference, with as many whole months as possible.
-- Same as 'diffGregorianDurationClip' for positive durations.
diffGregorianDurationRollOver :: Day -> Day -> CalendarDiffDays
diffGregorianDurationRollOver day2 day1 =
    let (y1, m1, d1) = toGregorian day1
        (y2, m2, d2) = toGregorian day2
        ym1 = y1 * 12 + toInteger m1
        ym2 = y2 * 12 + toInteger m2
        ymdiff = ym2 - ym1
        ymAllowed =
            if day2 >= day1
                then
                    if d2 >= d1
                        then ymdiff
                        else ymdiff - 1
                else
                    if d2 <= d1
                        then ymdiff
                        else ymdiff + 1
        dayAllowed = addGregorianDurationRollOver (CalendarDiffDays ymAllowed 0) day1
     in CalendarDiffDays ymAllowed $ diffDays day2 dayAllowed

-- orphan instance
instance Show Day where
    show = showGregorian