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diff --git a/2022/23-Unstable-Diffusion/second.hs b/2022/23-Unstable-Diffusion/second.hs
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+++ b/2022/23-Unstable-Diffusion/second.hs
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+-- requires cabal install --lib megaparsec parser-combinators vector
+module Main (main) where
+import Control.Monad (void, when)
+import Data.Functor
+import Data.List qualified as L
+import Data.Map qualified as M
+import Data.Void (Void)
+import Text.Megaparsec
+import Text.Megaparsec.Char
+import System.Exit (die)
+
+exampleExpectedOutput = 20
+
+type Input = M.Map (Int, Int) Bool -- True if there is an elf
+
+type Parser = Parsec Void String
+
+inputToList :: String -> [((Int, Int), Bool)]
+inputToList = snd . L.foldl'  nextElf  ((0, 0), [])
+  where
+    nextElf :: ((Int, Int), [((Int, Int), Bool)]) -> Char -> ((Int, Int), [((Int, Int), Bool)])
+    nextElf ((x, y), acc) '\n' = ((0, y+1), acc)
+    nextElf ((x, y), acc) '.' = ((x+1, y), acc)
+    nextElf ((x, y), acc) '#' = ((x+1, y), acc ++ [((x, y), True)])
+
+parseInput :: String -> IO Input
+parseInput filename = do
+  input <- readFile filename
+  return $ M.fromList (inputToList input)
+
+type Proposals = M.Map (Int, Int) (Int, Int, Bool)
+
+firstHalfStep :: Input -> Int -> Proposals
+firstHalfStep input iter = M.filter (\(_, _, e) -> e) $ M.foldrWithKey eval M.empty input
+  where
+    eval :: (Int, Int) -> Bool -> Proposals -> Proposals
+    eval (x, y) _ = eval' (x, y, 0)
+    eval' :: (Int, Int, Int) -> Proposals -> Proposals
+    eval' (x, y, 4) acc = acc
+    eval' (x, y, r) acc | alone = acc
+                        | otherwise = case (iter + r) `mod` 4 of
+      0 -> if M.member (x-1, y-1) input || M.member (x, y-1) input || M.member (x+1, y-1) input
+           then next
+           else if M.member (x, y-1) acc
+                then M.insert (x, y-1) (x, y, False) acc -- two or more elves attempted to move to the same spot
+                else M.insert (x, y-1) (x, y, True) acc
+      1 -> if M.member (x-1, y+1) input || M.member (x, y+1) input || M.member (x+1, y+1) input
+           then next
+           else if M.member (x, y+1) acc
+                then M.insert (x, y+1) (x, y, False) acc -- two or more elves attempted to move to the same spot
+                else M.insert (x, y+1) (x, y, True) acc
+      2 -> if M.member (x-1, y-1) input || M.member (x-1, y) input || M.member (x-1, y+1) input
+           then next
+           else if M.member (x-1, y) acc
+                then M.insert (x-1, y) (x, y, False) acc -- two or more elves attempted to move to the same spot
+                else M.insert (x-1, y) (x, y, True) acc
+      3 -> if M.member (x+1, y-1) input || M.member (x+1, y) input || M.member (x+1, y+1) input
+           then next
+           else if M.member (x+1, y) acc
+                then M.insert (x+1, y) (x, y, False) acc -- two or more elves attempted to move to the same spot
+                else M.insert (x+1, y) (x, y, True) acc
+      where
+        alone = M.notMember (x-1, y-1) input
+             && M.notMember (x, y-1) input
+             && M.notMember (x+1, y-1) input
+             && M.notMember (x-1, y) input
+             && M.notMember (x+1, y) input
+             && M.notMember (x-1, y+1) input
+             && M.notMember (x, y+1) input
+             && M.notMember (x+1, y+1) input
+        next = eval' (x, y, r+1) acc
+
+secondHalfStep :: Input -> Proposals -> Input
+secondHalfStep = M.foldrWithKey eval
+  where
+    eval :: (Int, Int) -> (Int, Int, Bool) -> Input -> Input
+    eval (x', y') (x, y, _) acc = M.insert (x', y') True $ M.delete (x, y) acc
+
+compute :: Input -> Int
+compute = compute' 0
+  where
+    compute' :: Int -> Input -> Int
+    compute' i input | next == input = i + 1
+                     | otherwise = compute' (i+1) next
+     where
+       next = secondHalfStep input (firstHalfStep input i)
+       keys = M.keys input
+       xs = L.map fst keys
+       ys = L.map snd keys
+       xmax = maximum xs
+       xmin = minimum xs
+       ymax = maximum ys
+       ymin = minimum ys
+
+main :: IO ()
+main = do
+  example <- parseInput "example"
+  putStr $ showMap example
+  let exampleOutput = compute example
+  when  (exampleOutput /= exampleExpectedOutput)  (die $ "example failed: got " ++ show exampleOutput ++ " instead of " ++ show exampleExpectedOutput)
+  input <- parseInput "input"
+  print $ compute input
+
+showMap :: Input -> String
+showMap input = printMap' xmin ymin ""
+  where
+    printMap' :: Int -> Int -> String -> String
+    printMap' x y acc | M.member (x, y) input = printMap' (x+1) y (acc ++ "#")
+                      | y > ymax = acc
+                      | x <= xmax = printMap' (x+1) y (acc ++ ".")
+                      | otherwise = printMap' xmin (y+1) (acc ++ "\n")
+    keys = M.keys input
+    xs = L.map fst keys
+    ys = L.map snd keys
+    xmax = maximum xs
+    xmin = minimum xs
+    ymax = maximum ys
+    ymin = minimum ys