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