Improved project structure.

haskell/e019.hs

@@ -0,0 +1,41 @@
+data Weekday = Monday | Tuesday | Wednesday | Thursday | Friday | Saturday | Sunday
+	deriving (Show, Eq, Ord, Enum)
+
+data Month = January | February | March | April | May | June | July | August | September | October | November | December
+	deriving (Show, Eq, Ord, Enum)
+
+type Day = Int
+type Year = Int
+
+data Date = Date Year Month Day Weekday
+	deriving (Show, Eq, Ord)
+
+nextWeekday :: Weekday -> Weekday
+nextWeekday Sunday = Monday
+nextWeekday d = succ d
+
+nextMonth :: Month -> Month
+nextMonth December = January
+nextMonth m = succ m
+
+getDays :: Month -> Year -> Int
+getDays m y
+	| elem m [September, April, June, November] = 30
+  | m == February = if rem y 4 == 0 && if rem y 100 == 0 && rem y 400 /= 0 then False else True
+										then 29 else 28
+	| otherwise = 31
+
+nextDate :: Date -> Date
+nextDate (Date y m d wd)
+	| d < getDays m y = Date y m (d+1) (nextWeekday wd)
+	| m == December   = Date (y+1) January 1 (nextWeekday wd)
+	| otherwise       = Date y (nextMonth m) 1 (nextWeekday wd)
+
+
+daysTwentieth :: [Date]
+daysTwentieth = takeWhile (\(Date y _ _ _) -> y /=2001) (dates start)
+	where 
+		dates d = d:dates (nextDate d)
+		start = Date 1901 January 1 Tuesday
+
+e019 = length . filter (\(Date y m d wd) -> d == 1 && wd == Sunday) $ daysTwentieth