diff --git a/2021/day20.hs b/2021/day20.hs
new file mode 100644
index 0000000..56deff6
--- /dev/null
+++ b/2021/day20.hs
@@ -0,0 +1,80 @@
+module Day20 where
+import qualified Data.Array as A
+import qualified Data.Set as S
+import qualified Data.Map as M
+import Debug.Trace (traceShow, trace)
+import Data.Maybe (catMaybes)
+
+type Algorithm = A.Array Int Bool
+data Image = Img Bool (S.Set (Int,Int)) deriving (Show)
+
+getNd :: Image -> S.Set (Int,Int)
+getNd (Img _ s) = s
+
+getLit :: Image -> Int
+getLit (Img True s) = error "all elems lit"
+getLit (Img False s) = length $ S.elems s
+
+newDefault :: Algorithm -> Bool -> Bool
+newDefault algo False = algo A.! 0
+newDefault algo True  = algo A.! 511
+
+insertManyWith :: Ord k => (a -> a -> a) -> [(k,a)] -> M.Map k a -> M.Map k a
+insertManyWith f [] = id
+insertManyWith f ((k,v):xs) = insertManyWith f xs . M.insertWith f k v
+
+binToDec :: [Bool] -> Int
+binToDec = foldr (\x acc -> acc * 2 + fromEnum x) 0
+
+next :: Algorithm -> Image -> Image
+next algo (Img def diff) = Img (newDefault algo def) newDiff where
+    nonDefault = foldr (\(x,y) -> insertManyWith (++) (neighs x y)) M.empty diff
+    neighs x y = map (\i -> ((x-1 + i `mod` 3, y-1 + i `div` 3), [i])) [0..8]
+    calcNonDef = M.filter (/= newDefault algo def) $ M.map (\ns -> algo A.! binToDec (nonDefToNum ns)) nonDefault
+    nonDefToNum ns = map (\i -> if i `elem` ns then not def else def) [0..8]
+    newDiff = S.fromList $ M.keys calcNonDef
+
+printImage :: Image -> IO ()
+printImage img = do
+    mapM_ (\y -> do
+        mapM_ (\x -> if (x,y) `S.member` getNd img then putChar '#' else putChar '.') [minx..maxx];
+        putStrLn "") [miny..maxy] where
+    minx = minimum $ map fst $ S.elems $ getNd img
+    maxx = maximum $ map fst $ S.elems $ getNd img
+    miny = minimum $ map snd $ S.elems $ getNd img
+    maxy = maximum $ map snd $ S.elems $ getNd img
+
+
+main :: IO()
+main = do
+    algorithm <- A.listArray (0,511) . map (=='#') <$> getLine
+    getLine
+    content <- lines <$> getContents
+    let nonDef = catMaybes $ concat $ zipWith (\j -> zipWith (\i e -> if e =='#' then Just (i,j) else Nothing) [0..]) [0..] content
+    let init = Img False $ S.fromList nonDef
+
+    mapM_ print content
+    putStr "initial: "
+    print init
+    printImage init
+
+    putStr "after 1: "
+    let first = next algorithm init
+    print first
+    printImage first
+
+    putStr "after 2: "
+    let second = next algorithm first
+    print second
+    printImage second
+
+    putStr "part 1: "
+    print $ getLit second
+
+    let enhanced = iterate (next algorithm) init !! 50
+    putStr "part 2: "
+    print $ getLit enhanced
+
+
+
+    print "bye"
\ No newline at end of file
diff --git a/2021/inputs/day20.input b/2021/inputs/day20.input
new file mode 100644
index 0000000..eb70350
--- /dev/null
+++ b/2021/inputs/day20.input
@@ -0,0 +1,102 @@
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\ No newline at end of file
diff --git a/2021/inputs/day20.sample b/2021/inputs/day20.sample
new file mode 100644
index 0000000..8fa4bd4
--- /dev/null
+++ b/2021/inputs/day20.sample
@@ -0,0 +1,7 @@
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