(//) = posit
infixl 8  //  

main :: IO ()
main = do
         putStrLn $ dateString 2023 2 24 ++ " JDN = " ++ show (jdnGr 2023 2 24) 
         putStrLn $ dateString 2023 2 28 ++ " JDN = " ++ show (jdnGr 2023 2 28) 
         putStrLn $ dateString 2023 3 1  ++ " JDN = " ++ show (jdnGr 2023 3 1)
         putStrLn $ dateString 2023 3 3  ++ " weekday is " ++ show (weekday 2023 3 3)
         putStrLn $ show (daysUntilXmas)  ++ " days until Xmas"


posit h k = 
     if h >= 0 then (div h k)
     else  negate (div (-h) k)


jdnGr y m d = (1461 * (y + 4800 + (m - 14)//12))//4
        + (367 * (m - 2 - 12 * ((m - 14)//12)))//12
        - ((3 * ((y + 4900 + (m - 14)//12)//100))//4)
        + d - 32075


dateXmas = jdnGr 2023 12 24
daysUntilXmas = dateXmas - (jdnGr 2023 3 3)

weekday y m d = 
    let dayNumber = mod (1 + jdnGr y m d) 7
    in  
    case dayNumber of
        0 ->
            "Sunday"
        1 ->
            "Monday"
        2 ->
            "Tuesday"
        3 ->
            "Wednesday"
        4 ->
            "Thursday"
        5 ->
            "Friday"
        6 ->
            "Saturday"
        _ ->
            "Unknown day"


-- Converts the date (year, month, day) to string

dateString
  :: (Show a1, Show a2, Show a3) => a1 -> a2 -> a3 -> String
dateString y m d =
   (show y) ++ "-"
      ++ (show m) ++ "-"
      ++ (show d) :: String