Day 11, Part 2 + improved Part 1

Day11/puzzle2.hs

@@ -0,0 +1,17 @@
+import qualified Data.Map.Strict as M
+
+blinkStone :: (Int, Int) -> [(Int, Int)]
+blinkStone (0, n) = [(1, n)]
+blinkStone (s, n) = let ss = show s
+                        nDigit = length ss
+                    in  if even nDigit
+                            then zip (map read [take (nDigit `div` 2) ss, drop (nDigit `div` 2) ss]) [n, n]
+                        else [(s * 2024, n)]
+
+blink :: Int -> M.Map Int Int -> M.Map Int Int
+blink 0 m = m
+blink n m = blink (n - 1) $ M.fromListWith (+) $ concatMap blinkStone $ M.toList m
+
+main = do
+    contents <- M.fromListWith (+) . flip zip (repeat 1) . map read . words <$> readFile "day11.txt"
+    print . M.foldl (+) 0 $ blink 75 contents