Data/Time/Calendar/OrdinalDate.hs

    -- | ISO 8601 Ordinal Date format
    module Data.Time.Calendar.OrdinalDate where

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

    -- | convert to ISO 8601 Ordinal Date format. First element of result is year (proleptic Gregoran calendar),
    -- second is the day of the year, with 1 for Jan 1, and 365 (or 366 in leap years) for Dec 31.
    toOrdinalDate :: Day -> (Integer,Int)
    toOrdinalDate (ModifiedJulianDay mjd) = (year,yd) where
    	a = mjd + 678575
    	quadcent = div a 146097
    	b = mod a 146097
    	cent = min (div b 36524) 3
    	c = b - (cent * 36524)
    	quad = div c 1461
    	d = mod c 1461
    	y = min (div d 365) 3
    	yd = fromInteger (d - (y * 365) + 1)
    	year = quadcent * 400 + cent * 100 + quad * 4 + y + 1

    -- | convert from ISO 8601 Ordinal Date format.
    -- Invalid day numbers will be clipped to the correct range (1 to 365 or 366).
    fromOrdinalDate :: Integer -> Int -> Day
    fromOrdinalDate year day = ModifiedJulianDay mjd where
    	y = year - 1
    	mjd = (fromIntegral (clip 1 (if isLeapYear year then 366 else 365) day)) + (365 * y) + (div y 4) - (div y 100) + (div y 400) - 678576

    -- | convert from ISO 8601 Ordinal Date format.
    -- Invalid day numbers return Nothing
    fromOrdinalDateValid :: Integer -> Int -> Maybe Day
    fromOrdinalDateValid year day = do
    	day' <- clipValid 1 (if isLeapYear year then 366 else 365) day
    	let
    		y = year - 1
    		mjd = (fromIntegral day') + (365 * y) + (div y 4) - (div y 100) + (div y 400) - 678576
    	return (ModifiedJulianDay mjd)

    -- | show in ISO 8601 Ordinal Date format (yyyy-ddd)
    showOrdinalDate :: Day -> String
    showOrdinalDate date = (show4 (Just '0') y) ++ "-" ++ (show3 (Just '0') d) where
    	(y,d) = toOrdinalDate date

    -- | Is this year a leap year according to the proleptic Gregorian calendar?
    isLeapYear :: Integer -> Bool
    isLeapYear year = (mod year 4 == 0) && ((mod year 400 == 0) || not (mod year 100 == 0))

    -- | Get the number of the Monday-starting week in the year and the day of the week.
    -- The first Monday is the first day of week 1, any earlier days in the year are week 0 (as \"%W\" in 'Data.Time.Format.formatTime').
    -- Monday is 1, Sunday is 7 (as \"%u\" in 'Data.Time.Format.formatTime').
    mondayStartWeek :: Day -> (Int,Int)
    mondayStartWeek date = (fromInteger ((div d 7) - (div k 7)),fromInteger (mod d 7) + 1) where
    	yd = snd (toOrdinalDate date)
    	d = (toModifiedJulianDay date) + 2
    	k = d - (toInteger yd)

    -- | Get the number of the Sunday-starting week in the year and the day of the week.
    -- The first Sunday is the first day of week 1, any earlier days in the year are week 0 (as \"%U\" in 'Data.Time.Format.formatTime').
    -- Sunday is 0, Saturday is 6 (as \"%w\" in 'Data.Time.Format.formatTime').
    sundayStartWeek :: Day -> (Int,Int)
    sundayStartWeek date =(fromInteger ((div d 7) - (div k 7)),fromInteger (mod d 7)) where
    	yd = snd (toOrdinalDate date)
    	d = (toModifiedJulianDay date) + 3
    	k = d - (toInteger yd)

    -- | The inverse of 'mondayStartWeek'. Get a 'Day' given the year,
    -- the number of the Monday-starting week, and the day of the week.
    -- The first Monday is the first day of week 1, any earlier days in the year
    -- are week 0 (as \"%W\" in 'Data.Time.Format.formatTime').
    fromMondayStartWeek :: Integer -- ^ Year.
                        -> Int     -- ^ Monday-starting week number.
                        -> Int     -- ^ Day of week.
                                   -- Monday is 1, Sunday is 7 (as \"%u\" in 'Data.Time.Format.formatTime').
                        -> Day
    fromMondayStartWeek y w d = ModifiedJulianDay (firstDay + yd)
        where yd = firstMonday + 7 * toInteger (w-1) + toInteger d - 1
              -- first day of the year
              firstDay = toModifiedJulianDay (fromOrdinalDate y 1)
              -- 0-based year day of first monday of the year
              firstMonday = (5 - firstDay) `mod` 7

    fromMondayStartWeekValid :: Integer -- ^ Year.
                        -> Int     -- ^ Monday-starting week number.
                        -> Int     -- ^ Day of week.
                                   -- Monday is 1, Sunday is 7 (as \"%u\" in 'Data.Time.Format.formatTime').
                        -> Maybe Day
    fromMondayStartWeekValid year w d = do
    	d' <- clipValid 1 7 d
    	-- first day of the year
    	let firstDay = toModifiedJulianDay (fromOrdinalDate year 1)
    	-- 0-based year day of first monday of the year
    	let firstMonday = (5 - firstDay) `mod` 7
    	let yd = firstMonday + 7 * toInteger (w-1) + toInteger d'
    	yd' <- clipValid 1 (if isLeapYear year then 366 else 365) yd
    	return (ModifiedJulianDay (firstDay - 1 + yd'))

    -- | The inverse of 'sundayStartWeek'. Get a 'Day' given the year and
    -- the number of the day of a Sunday-starting week.
    -- The first Sunday is the first day of week 1, any earlier days in the
    -- year are week 0 (as \"%U\" in 'Data.Time.Format.formatTime').
    fromSundayStartWeek :: Integer -- ^ Year.
                        -> Int     -- ^ Sunday-starting week number.
                        -> Int     -- ^ Day of week
                                   -- Sunday is 0, Saturday is 6 (as \"%w\" in 'Data.Time.Format.formatTime').
                        -> Day
    fromSundayStartWeek y w d = ModifiedJulianDay (firstDay + yd)
        where yd = firstSunday + 7 * toInteger (w-1) + toInteger d
              -- first day of the year
              firstDay = toModifiedJulianDay (fromOrdinalDate y 1)
              -- 0-based year day of first sunday of the year
              firstSunday = (4 - firstDay) `mod` 7

    fromSundayStartWeekValid :: Integer -- ^ Year.
                        -> Int     -- ^ Monday-starting week number.
                        -> Int     -- ^ Day of week.
                                   -- Monday is 1, Sunday is 7 (as \"%u\" in 'Data.Time.Format.formatTime').
                        -> Maybe Day
    fromSundayStartWeekValid year w d = do
    	d' <- clipValid 1 7 d
    	-- first day of the year
    	let firstDay = toModifiedJulianDay (fromOrdinalDate year 1)
    	-- 0-based year day of first sunday of the year
    	let firstMonday = (4 - firstDay) `mod` 7
    	let yd = firstMonday + 7 * toInteger (w-1) + toInteger d'
    	yd' <- clipValid 1 (if isLeapYear year then 366 else 365) yd
    	return (ModifiedJulianDay (firstDay - 1 + yd'))