content: -- | ISO 8601 Week Date format
  module Data.Time.Calendar.WeekDate where
  
  import Data.Time.Calendar.OrdinalDate
  import Data.Time.Calendar.Days
  import Data.Time.Calendar.Private
  
  -- | convert to ISO 8601 Week Date format. First element of result is year, second week number (1-53), third day of week (1 for Monday to 7 for Sunday).
  -- Note that \"Week\" years are not quite the same as Gregorian years, as the first day of the year is always a Monday.
  -- The first week of a year is the first week to contain at least four days in the corresponding Gregorian year.
  toWeekDate :: Day -> (Integer,Int,Int)
  toWeekDate date@(ModifiedJulianDay mjd) = (y1,fromInteger (w1 + 1),fromInteger d_mod_7 + 1) where
          (d_div_7, d_mod_7) = d `divMod` 7
  	(y0,yd) = toOrdinalDate date
  	d = mjd + 2
  	foo :: Integer -> Integer
  	foo y = bar (toModifiedJulianDay (fromOrdinalDate y 6))
  	bar k = d_div_7 - k `div` 7
  	(y1,w1) = case bar (d - toInteger yd + 4) of
  	            -1 -> (y0 - 1, foo (y0 - 1))
  	            52 -> if foo (y0 + 1) == 0
  	                  then (y0 + 1, 0)
  	                  else (y0, 52)
  	            w0  -> (y0, w0)
  	
  -- | convert from ISO 8601 Week Date format. First argument is year, second week number (1-52 or 53), third day of week (1 for Monday to 7 for Sunday).
  -- Invalid week and day values will be clipped to the correct range.
  fromWeekDate :: Integer -> Int -> Int -> Day
  fromWeekDate y w d = ModifiedJulianDay (k - (mod k 7) + (toInteger (((clip 1 (if longYear then 53 else 52) w) * 7) + (clip 1 7 d))) - 10) where
  		k = toModifiedJulianDay (fromOrdinalDate y 6)
  		longYear = case toWeekDate (fromOrdinalDate y 365) of
  			(_,53,_) -> True
  			_ -> False
  
  -- | convert from ISO 8601 Week Date format. First argument is year, second week number (1-52 or 53), third day of week (1 for Monday to 7 for Sunday).
  -- Invalid week and day values will return Nothing.
  fromWeekDateValid :: Integer -> Int -> Int -> Maybe Day
  fromWeekDateValid y w d = do
  	d' <- clipValid 1 7 d
  	let
  		longYear = case toWeekDate (fromOrdinalDate y 365) of
  			(_,53,_) -> True
  			_ -> False
  	w' <- clipValid 1 (if longYear then 53 else 52) w
  	let
  		k = toModifiedJulianDay (fromOrdinalDate y 6)
  	return (ModifiedJulianDay (k - (mod k 7) + (toInteger ((w' * 7) + d')) - 10))
  
  -- | show in ISO 8601 Week Date format as yyyy-Www-d (e.g. \"2006-W46-3\").
  showWeekDate :: Day -> String
  showWeekDate date = (show4 (Just '0') y) ++ "-W" ++ (show2 (Just '0') w) ++ "-" ++ (show d) where
  	(y,w,d) = toWeekDate date
  