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width :: Int
width = 100
width2 :: Int
width2 = width + 2
golStep :: [[Int]] -> [[Int]]
golStep bd = replicate width2 0 : (map golLine $ zip [1..] $ init (tail bd)) ++ [replicate width2 0]
where golLine (y,l) = map (golCell y) $ zip [0..] l
golCell y (x,c) = if x == 0 || x == width2 - 1
then 0
else if golSum x y == 3 || (c == 1 && golSum x y == 2)
then 1
else 0
golSum x y = bd!!(y-1)!!(x-1)+
bd!!(y-1)!!(x )+
bd!!(y-1)!!(x+1)+
bd!!(y )!!(x+1)+
bd!!(y+1)!!(x+1)+
bd!!(y+1)!!(x )+
bd!!(y+1)!!(x-1)+
bd!!(y )!!(x-1)
setCorners :: [[Int]] -> [[Int]]
setCorners bd = bd!!0 : set2 (bd!!1) : (init $ init $ drop 2 bd) ++ [set2 (bd!!(width2-2)),last bd]
where set2 line = head line : 1 : (init $ init $ drop 2 line) ++ [1,last line]
day18 :: IO ()
day18 = do
input <- readFile "day18.txt"
let bd = map (\l -> 0 : l ++ [0]) $ replicate width 0 : [map (\c -> if c == '#' then 1 else 0) l | l <- lines input] ++ [replicate width 0]
print $ sum $ map sum $ iterate (setCorners . golStep) bd !! 100 -- part 1: setCorners -> id
main = day18
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