Wochentagsberechnung

data Month = January | February | March | April | May | June | July | August | September | October | November | December deriving (Show, Eq, Enum)
data DoW = Sunday | Monday | Tuesday | Wednesday | Thursday | Friday | Saturday deriving (Show, Enum)

(.:) :: Month -> Int -> Month
a .: b = toEnum $ flip mod 12 $ 12 + fromEnum a - b

calc_leapyear :: Int -> Bool
calc_leapyear y = (y `mod` 4 == 0 && y `mod` 100 /= 0) || (y `mod` 4 == 0 && y `mod` 400 == 0)

get_max_days :: Month -> Int
get_max_days m  | m == January || m == March || m == May || m == July || m == August || m == October || m == December = 31
                | m == April || m == June || m == September || m == November = 30
                | otherwise = 28

day_digit :: Int -> Int
day_digit d = d `mod` 7

month_digit :: Month -> Int
month_digit January = 0
month_digit a = flip mod 7 $ (get_max_days $ a .: 1) `mod` 7 + (month_digit $ a .: 1)

year_digit :: Int -> Int
year_digit y = ((y `mod` 100) + ((y `mod` 100) `quot` 4)) `mod` 7

century_digit :: Int -> Int
century_digit y = (3 - ((y `quot` 100) `mod` 4)) * 2

ly_correction :: Month -> Int -> Int
ly_correction m y   | calc_leapyear(y) && (m == January || m == February) = -1
                    | otherwise = 0

calc_day_of_week :: Int -> Month -> Int -> DoW
calc_day_of_week d m y = toEnum $ flip mod 7 $ day_digit d + month_digit m + year_digit y + century_digit y + ly_correction m y