Add Doomsday algorithm

Doomsday/doomsday.hs

@@ -0,0 +1,54 @@
+import Data.List
+import System.IO
+
+main :: IO ()
+main = return()
+
+{-
+The Doomsday algorithm is a simple algorithm used to determine the day of the week for a given date
+
+Besides being really simple to use mentally, making it a cool party trick, it's really interesting
+
+For more info https://en.wikipedia.org/wiki/Doomsday_rule
+-}
+
+
+doomsdayLeap = [4,1,7,4,2,6,4,1,5,3,7,5]
+doomsdayNotLeap = [3,7,7,4,2,6,4,1,5,3,7,5]
+
+printWeekday :: Int -> String
+printWeekday 0 = "Sunday"
+printWeekday 1 = "Monday"
+printWeekday 2 = "Tuesday"
+printWeekday 3 = "Wednesday"
+printWeekday 4 = "Thursday"
+printWeekday 5 = "Friday"
+printWeekday 6 = "Sunday"
+
+getWeekday :: (Int,Int,Int) -> Int
+getWeekday (d,m,y)
+-- Return -1 if there's any obvious error in the input
+ | (d<1 || d>31) = -1
+ | (m<1 || m>12) = -1
+ | (y<1990 || y>2019) = -1
+ | (m == 2 && d > 29) = -1
+-- If there's no error go ahead and calculate the weekday using Doomsday algorithm
+ | otherwise = dday
+ where dday = (d1 + (d - h)) `mod` 7
+ d1 = c4 `mod` 7
+ c4 = c1 + c2 + c3 + k
+ c3 = c2 `div` 4
+ c2 = c0 - (c1 * 12)
+ c1 = c0 `div` 12
+ c0 = y `mod` 100
+ h = getAnchorDay m y
+ k = getYearValue y
+
+getYearValue x
+ | (x<2000) = 3
+ | otherwise = 2
+
+getAnchorDay m y
+ | (y `mod` 100 == 0) && not (y `mod` 400 == 0) = doomsdayNotLeap !! (m-1)
+ | (y `mod` 4 == 0) = doomsdayLeap !! (m-1)
+ | otherwise = doomsdayNotLeap !! (m-1)