Solve day 4 part 1

haskell/Puzzles/Day4.hs

@@ -1,14 +1,70 @@
 module Puzzles.Day4 (puzzle) where
 
+import Control.Applicative
+import Control.Monad
+import Data.Bifunctor
+import Data.Functor
+import Data.List.Extra
+import Data.Sequence qualified as Seq
+import Data.Text qualified as T
+import Linear
 import Puzzle
+import Text.Megaparsec hiding (some)
+import Text.Megaparsec.Char
 
 puzzle :: Puzzle
 puzzle =
     Puzzle
         { number = 4
-        , parser = pure ()
+        , parser = flip sepEndBy newline $ some $ asum $ enumerate <&> \t -> char (inToChar t) $> t
         , parts =
-            [ \() ->
-                "TODO"
+            [ \input ->
+                let inputWithCoords = zipWith (map . first . V2) [0 ..] $ map (zip [0 ..]) input
+                    inputSeq = Seq.fromList $ map Seq.fromList input
+                 in T.show
+                        . length
+                        . concatMap (filter (== OutAccessible))
+                        $ inputWithCoords <<&>> \(v, t) -> case t of
+                            InEmpty -> OutEmpty
+                            InRoll ->
+                                if length (filter (== Just InRoll) neighbours) < 4
+                                    then OutAccessible
+                                    else OutRoll
+                              where
+                                neighbours = do
+                                    x <- [-1 .. 1]
+                                    y <- [-1 .. 1]
+                                    guard $ not (x == 0 && y == 0)
+                                    let V2 x' y' = v + V2 x y
+                                    pure $ Seq.lookup x' inputSeq >>= Seq.lookup y'
             ]
         }
+
+data InTile
+    = InEmpty
+    | InRoll
+    deriving (Eq, Ord, Show, Enum, Bounded)
+inToChar :: InTile -> Char
+inToChar = \case
+    InEmpty -> '.'
+    InRoll -> '@'
+drawGridIn :: [[InTile]] -> String
+drawGridIn = unlines . map (map inToChar)
+
+data OutTile
+    = OutEmpty
+    | OutRoll
+    | OutAccessible
+    deriving (Eq, Ord, Show, Enum, Bounded)
+outToChar :: OutTile -> Char
+outToChar = \case
+    OutEmpty -> '.'
+    OutRoll -> '@'
+    OutAccessible -> 'x'
+drawGridOut :: [[OutTile]] -> String
+drawGridOut = unlines . map (map outToChar)
+
+(<<$>>) :: (Functor f1, Functor f2) => (a -> b) -> f1 (f2 a) -> f1 (f2 b)
+(<<$>>) = fmap . fmap
+(<<&>>) :: (Functor f1, Functor f2) => f1 (f2 a) -> (a -> b) -> f1 (f2 b)
+(<<&>>) = flip (<<$>>)

outputs/real/4/1

@@ -0,0 +1 @@
+1437