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module Day11B where
import System.Environment (getArgs)
type AreaMap = [[Space]];
data Space = Occupied | Empty | Floor
deriving (Show, Eq, Ord);
-- Keep applying f to a until the output stops changing
untilUnchanged :: Eq a => (a -> a) -> a -> a
untilUnchanged f a | n /= a = untilUnchanged f n
| otherwise = n
where n = f a
-- Find if an occupied seat is visible from (x,y) looking along (dx, dy)
occupiedAlongDir :: AreaMap -> (Int, Int) -> (Int, Int) -> Bool
occupiedAlongDir ss (x, y) (dx, dy) | x >= length (head ss) || y >= length ss || x < 0 || y < 0 = False
| ss!!y!!x == Empty = False
| ss!!y!!x == Occupied = True
| otherwise = occupiedAlongDir ss (x + dx, y + dy) (dx, dy)
-- Count the number of visible occupied seats from (x, y), looking along all directions incl diagonals.
visibleOccupied :: AreaMap -> (Int, Int) -> Int
visibleOccupied a (x, y) = length [(x', y') | x' <- [x-1..x+1], y' <- [y-1..y+1], x' /= x || y' /= y, occupiedAlongDir a (x', y') (x'-x, y'-y)]
-- Mutate a space given its current state and its adjacent spaces
mutateSpace :: Space -> Int -> Space
mutateSpace Empty adj | adj == 0 = Occupied
| otherwise = Empty
mutateSpace Occupied adj | adj >= 5 = Empty
| otherwise = Occupied
mutateSpace x _ = x
-- Mutate all spaces on the map
mutateAll :: AreaMap -> AreaMap
mutateAll rs = [[mutateSpace s (visibleOccupied rs (c, r)) | (c, s) <- zip [0..] ss] | (r, ss) <- zip [0..] rs]
-- Read a map in from its string representation
readMap :: String -> AreaMap
readMap = map readLine . lines
where readLine [] = []
readLine ('.':xs) = Floor : readLine xs
readLine ('L':xs) = Empty : readLine xs
readLine ('#':xs) = Occupied : readLine xs
-- Read a map from the filename
mapFromFile :: String -> IO AreaMap
mapFromFile s = do
contents <- readFile s;
return $ readMap contents;
main :: IO ()
main = do
args <- getArgs;
m <- mapFromFile (head args);
let final = untilUnchanged mutateAll m;
let occ = sum $ map (length . filter (== Occupied)) final;
putStrLn $ "Occupied Seats: " ++ show occ;
return ();
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