lib/Data/Time/Calendar/JulianYearDay.hs

    -- #hide
    module Data.Time.Calendar.JulianYearDay
        (
        -- * Year and day format
        module Data.Time.Calendar.JulianYearDay
        ) where

    import Data.Time.Calendar.Days
    import Data.Time.Calendar.Private

    -- | Convert to proleptic Julian year and day format. First element of result is year (proleptic Julian calendar),
    -- second is the day of the year, with 1 for Jan 1, and 365 (or 366 in leap years) for Dec 31.
    toJulianYearAndDay :: Day -> (Integer,Int)
    toJulianYearAndDay (ModifiedJulianDay mjd) = (year,yd) where
        a = mjd + 678577
        quad = div a 1461
        d = mod a 1461
        y = min (div d 365) 3
        yd = fromInteger (d - (y * 365) + 1)
        year = quad * 4 + y + 1

    -- | Convert from proleptic Julian year and day format.
    -- Invalid day numbers will be clipped to the correct range (1 to 365 or 366).
    fromJulianYearAndDay :: Integer -> Int -> Day
    fromJulianYearAndDay year day = ModifiedJulianDay mjd where
        y = year - 1
        mjd = (fromIntegral (clip 1 (if isJulianLeapYear year then 366 else 365) day)) + (365 * y) + (div y 4) - 678578

    -- | Convert from proleptic Julian year and day format.
    -- Invalid day numbers will return Nothing
    fromJulianYearAndDayValid :: Integer -> Int -> Maybe Day
    fromJulianYearAndDayValid year day = do
        day' <- clipValid 1 (if isJulianLeapYear year then 366 else 365) day
        let
            y = year - 1
            mjd = (fromIntegral day') + (365 * y) + (div y 4) - 678578
        return (ModifiedJulianDay mjd)

    -- | Show in proleptic Julian year and day format (yyyy-ddd)
    showJulianYearAndDay :: Day -> String
    showJulianYearAndDay date = (show4 y) ++ "-" ++ (show3 d) where
        (y,d) = toJulianYearAndDay date

    -- | Is this year a leap year according to the proleptic Julian calendar?
    isJulianLeapYear :: Integer -> Bool
    isJulianLeapYear year = (mod year 4 == 0)