-- Copyright 2007 Ashish Hanwadikar
 
module DateStream where
import Data.Time.Calendar (gregorianMonthLength)

-- Basic idea taken from Ketil Malde (http://www.haskell.org/pipermail/libraries/2005-January/002929.html)
-- converted into Monad

data Year = Y Int

data MonthEnum = January | February | March | April | May | June | July | August
					| September | October | November | December   deriving (Show, Eq, Enum, Bounded, Ord)

data DayEnum = Sunday | Monday | Tuesday | Wednesday | Thursday | Friday | Saturday   deriving (Show, Eq, Enum, Bounded)

data Month = M MonthEnum Year
data Day   = Dm Int Month

instance Show Month where
	show (M m t) = show m ++ " " ++ show t
	
instance Show Year where
	show (Y y) = " " ++ show y

instance Show Day where
	show (Dm d t) = " " ++ show d ++ " " ++ show t
				
dayofweek :: Day -> DayEnum
dayofweek (Dm d (M m (Y y))) = 
			let t = [Sunday, Wednesday, Tuesday, Friday, Sunday, Wednesday, Friday, Monday, Thursday, Saturday, Tuesday, Thursday]
			    y1 = (if m < March then (y - 1) else y)
			    dw = mod (y1 + (div y1  4) - (div y1  100) + (div y1  400) + (fromEnum(t !! (fromEnum ( m))) + d))  7 in
			    toEnum dw

isDayOfWeek dw dt =(dayofweek dt) == dw

months (Y y) = [M m (Y y) | m <- [January .. December]]

monthLength y m = gregorianMonthLength (toInteger y) (fromEnum m+1)

days (M m yy@(Y y)) = [Dm d (M m yy) | d <- [1..monthLength y m]]

years = [Y y | y <- [2007..]]