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diff --git a/2020/17-Conway_Cubes/second.hs b/2020/17-Conway_Cubes/second.hs
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+++ b/2020/17-Conway_Cubes/second.hs
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+module Main (main) where
+import Control.Monad (when)
+import Data.List (foldl')
+import Data.Map qualified as M
+import System.Exit (die)
+
+exampleExpectedOutput = 848
+
+type Coord = (Int, Int, Int, Int)
+type Input = M.Map Coord Bool
+
+parseInput :: String -> IO Input
+parseInput filename = do
+  input <- readFile filename
+  return . M.fromList . fst $ foldl' process ([], (0, 0, 0, 0)) input
+  where
+    process :: ([(Coord, Bool)], Coord) -> Char -> ([(Coord, Bool)], Coord)
+    process (acc, (x, y, z, w)) c = case c of
+      '.' -> (acc, (x+1, y, z, w))
+      '#' -> (((x, y, z, w), True):acc, (x+1, y, z, w))
+      '\n' -> (acc, (0, y+1, z, w))
+
+r :: [Int]
+r = [-1..1]
+
+complete :: Input -> Input
+complete input = M.foldrWithKey process M.empty input
+  where
+    process :: Coord -> Bool -> Input -> Input
+    process (x, y, z, w) False acc = acc
+    process (x, y, z, w) True acc = M.unions [M.singleton (x, y, z, w) True, acc, M.fromList [((x+a, y+b, z+c, w+d), False) | a<-r, b<-r, c<-r, d<-r]]
+
+step :: Int -> Input -> Input
+step 0 input = input
+step n input = step (n-1) (M.foldrWithKey evolve M.empty input')
+  where
+    input' = complete input
+    evolve :: Coord -> Bool -> Input -> Input
+    evolve (x, y, z, w) s acc = case s of
+      False -> M.insert (x, y, z, w) (actives == 3) acc
+      True -> M.insert (x, y, z, w) (actives == 3 || actives == 4) acc
+      where
+        actives = length . filter id $ [lookup (x+a, y+b, z+c, w+d) | a<-r, b<-r, c<-r, d<-r]
+    lookup :: Coord -> Bool
+    lookup c = case M.lookup c input' of
+      Just True -> True
+      otherwise -> False
+
+compute :: Input -> Int
+compute input = M.size . M.filter id $ step 6 input
+
+main :: IO ()
+main = do
+  example <- parseInput "example"
+  let exampleOutput = compute example
+  when  (exampleOutput /= exampleExpectedOutput)  (die $ "example failed: got " ++ show exampleOutput ++ " instead of " ++ show exampleExpectedOutput)
+  input <- parseInput "input"
+  print $ compute input