Data commit

Task/Palindrome-dates/Haskell/palindrome-dates.hs

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+import Data.List (unfoldr)
+import Data.List.Split (chunksOf)
+import Data.Maybe (mapMaybe)
+import Data.Time.Calendar (Day, fromGregorianValid)
+import Data.Tuple (swap)
+
+-------------------- PALINDROMIC DATES -------------------
+
+palinDates :: [Day]
+palinDates = mapMaybe palinDay [2021 .. 9999]
+
+palinDay :: Integer -> Maybe Day
+palinDay y = fromGregorianValid y m d
+  where
+    [m, d] = unDigits <$> chunksOf 2
+      (reversedDecimalDigits (fromInteger y))
+
+
+--------------------------- TEST -------------------------
+main :: IO ()
+main =
+  let n = length palinDates
+   in putStrLn ("Count of palindromic dates [2021..9999]: "
+        <> show n) >>
+      putStrLn "\nFirst 15:" >>
+      mapM_ print (take 15 palinDates) >>
+      putStrLn "\nLast 15:" >>
+      mapM_ print (take 15 (drop (n - 15) palinDates))
+
+
+------------------------- GENERIC ------------------------
+
+reversedDecimalDigits :: Int -> [Int]
+reversedDecimalDigits = unfoldr go
+  where
+    go n
+      | 0 < n = Just $ swap (quotRem n 10)
+      | otherwise = Nothing
+
+unDigits :: [Int] -> Int
+unDigits = foldl ((+) . (10 *)) 0