aboutsummaryrefslogtreecommitdiff
path: root/2020/11-Seating_System
diff options
context:
space:
mode:
Diffstat (limited to '')
-rw-r--r--2020/11-Seating_System/example10
-rw-r--r--2020/11-Seating_System/first-arrays.hs53
-rw-r--r--2020/11-Seating_System/first.hs43
-rw-r--r--2020/11-Seating_System/input90
-rw-r--r--2020/11-Seating_System/second.hs53
5 files changed, 249 insertions, 0 deletions
diff --git a/2020/11-Seating_System/example b/2020/11-Seating_System/example
new file mode 100644
index 0000000..1beaede
--- /dev/null
+++ b/2020/11-Seating_System/example
@@ -0,0 +1,10 @@
+L.LL.LL.LL
+LLLLLLL.LL
+L.L.L..L..
+LLLL.LL.LL
+L.LL.LL.LL
+L.LLLLL.LL
+..L.L.....
+LLLLLLLLLL
+L.LLLLLL.L
+L.LLLLL.LL
diff --git a/2020/11-Seating_System/first-arrays.hs b/2020/11-Seating_System/first-arrays.hs
new file mode 100644
index 0000000..fac8151
--- /dev/null
+++ b/2020/11-Seating_System/first-arrays.hs
@@ -0,0 +1,53 @@
+module Main (main) where
+import Control.Monad (when)
+import Data.Array qualified as A
+import Data.List (foldl')
+import Data.Maybe (catMaybes)
+import System.Exit (die)
+
+exampleExpectedOutput = 37
+
+type Seating = A.Array (Int, Int) (Maybe Bool)
+
+inputToArrayAssociations :: String -> [((Int, Int), Maybe Bool)]
+inputToArrayAssociations = snd . foldl' nextSeat ((0, 0), [])
+ where
+ nextSeat :: ((Int, Int), [((Int, Int), Maybe Bool)]) -> Char -> ((Int, Int), [((Int, Int), Maybe Bool)])
+ nextSeat ((x, y), acc) '\n' = ((0, y+1), acc)
+ nextSeat ((x, y), acc) '.' = ((x+1, y), acc ++ [((x, y), Nothing)])
+ nextSeat ((x, y), acc) 'L' = ((x+1, y), acc ++ [((x, y), Just False)])
+
+parseInput :: String -> IO Seating
+parseInput filename = do
+ input <- readFile filename
+ let ls = lines input
+ height = (length ls) - 1
+ width = (length (ls !! 0)) - 1
+ return $ A.array ((0,0), (width, height)) (inputToArrayAssociations input)
+
+compute :: Seating -> Int
+compute seating
+ | seating == seating' = length . filter id . catMaybes $ A.elems seating
+ | otherwise = compute seating'
+ where
+ (width, height) = snd $ A.bounds seating
+ seating' :: Seating
+ seating' = seating A.// (foldl' next [] (A.assocs seating))
+ next :: [((Int, Int), Maybe Bool)] -> ((Int, Int), Maybe Bool) -> [((Int, Int), Maybe Bool)]
+ next acc ((x, y), Just False) = ((x, y), Just $ around (x, y) == []) : acc
+ next acc ((x, y), Just True) = ((x, y), Just $ length (around (x,y)) < 4) : acc
+ next acc _ = acc
+ around :: (Int, Int) -> [Bool]
+ around (x, y) = filter id . catMaybes $ map lookup [(x-1, y-1), (x, y-1), (x+1, y-1), (x-1, y), (x+1, y), (x-1, y+1), (x, y+1), (x+1, y+1)]
+ lookup :: (Int, Int) -> Maybe Bool
+ lookup (x, y)
+ | x < 0 || y < 0 || x > width || y > height = Nothing
+ | otherwise = seating A.! (x, y)
+
+main :: IO ()
+main = do
+ example <- parseInput "example"
+ let exampleOutput = compute example
+ when (exampleOutput /= exampleExpectedOutput) (die $ "example failed: got " ++ show exampleOutput ++ " instead of " ++ show exampleExpectedOutput)
+ input <- parseInput "input"
+ print $ compute input
diff --git a/2020/11-Seating_System/first.hs b/2020/11-Seating_System/first.hs
new file mode 100644
index 0000000..d5871d5
--- /dev/null
+++ b/2020/11-Seating_System/first.hs
@@ -0,0 +1,43 @@
+module Main (main) where
+import Control.Monad (when)
+import Data.List (foldl')
+import Data.Maybe (mapMaybe)
+import qualified Data.Map as M
+import System.Exit (die)
+
+exampleExpectedOutput = 37
+
+type Seating = M.Map (Int, Int) Bool
+
+inputToList :: String -> [((Int, Int), Bool)]
+inputToList = snd . foldl' nextSeat ((0, 0), [])
+ where
+ nextSeat :: ((Int, Int), [((Int, Int), Bool)]) -> Char -> ((Int, Int), [((Int, Int), Bool)])
+ nextSeat ((x, y), acc) '\n' = ((0, y+1), acc)
+ nextSeat ((x, y), acc) '.' = ((x+1, y), acc)
+ nextSeat ((x, y), acc) 'L' = ((x+1, y), acc ++ [((x, y), False)])
+
+parseInput :: String -> IO Seating
+parseInput filename = do
+ input <- readFile filename
+ return $ M.fromList (inputToList input)
+
+compute :: Seating -> Int
+compute seating
+ | seating == seating' = length $ M.filter id seating
+ | otherwise = compute seating'
+ where
+ seating' = M.mapWithKey next seating
+ next :: (Int, Int) -> Bool -> Bool
+ next (x, y) False = around (x, y) == []
+ next (x, y) True = length (around (x,y)) < 4
+ around :: (Int, Int) -> [Bool]
+ around (x, y) = filter id $ mapMaybe (\v -> M.lookup v seating) [(x-1, y-1), (x, y-1), (x+1, y-1), (x-1, y), (x+1, y), (x-1, y+1), (x, y+1), (x+1, y+1)]
+
+main :: IO ()
+main = do
+ example <- parseInput "example"
+ let exampleOutput = compute example
+ when (exampleOutput /= exampleExpectedOutput) (die $ "example failed: got " ++ show exampleOutput ++ " instead of " ++ show exampleExpectedOutput)
+ input <- parseInput "input"
+ print $ compute input
diff --git a/2020/11-Seating_System/input b/2020/11-Seating_System/input
new file mode 100644
index 0000000..6e3d9b3
--- /dev/null
+++ b/2020/11-Seating_System/input
@@ -0,0 +1,90 @@
+LLLLLL.LLLL..LLLLLL.LLLLLLLLLLLLLLLLLLLLL.LLLLLL.LLLLLLLL.LLLLL.LLLLLL.L.LL.LLLLLL.LLLLLLLLLLLLLLLL
+LLLLLL.LLLLLLLLL.LLLLLLLLLLLLLL.LLLLLLLLLLLLLLLL.LLLLLLLLLL.LLL.LLLL.L.LLLLLL.LLLLLLLLLLLL.LLLLLLLL
+LLLLLLLL.LLL.LL..LLLLLLLLL.L.LLLLLLLLLLLLLLLLLLLLLLLLLLLL.LLLLLLLLLLLL.LLLL.LLLLLLLLLLLLLL.LLLLLLLL
+LLLLLL.LLLLL.LLL.LLLLLLLLL.LLLLLLLLLLL.LL.LLLLLL.LLLLLLLL.LLLLLLLLLLLL.LLLLLLLLLLL.LLLLLLLLL.LLLLLL
+LLLLLLLLLLLLL.LL.LLLLLLLLL..LLL.LLLLLLLLLLLLLLLLLLL.LLLLL.LLLLL.LLLLLL.LLLLLLLLLLL.LLLLLLLLL.LLLLLL
+LL..L.LLL.....L.L.L.....LL..L.LLLL..LL..LL..L.L.L.........LL.L..L..LL.L...L.LL..........LL....L.L..
+L.LLLLLLLLLLLLLL.LLLLLLLLL.LLLL.LLLLLLLLLLLLLLLL.LLLLLLLL.LLLLLLLLLLLLLLLLL.LLLLLL.LLLLLLLLL.LLLLLL
+.LLLLLLLLLLLLLLLL.LLLLLLLL.LLLL.LLLLLLLLL.LLLLLL.LLL.LLLL.LLLLL.LLLLLL.LLLL.LLLLLL.LLLLLLLLL.LLL.LL
+LLL.L..LLLLLLLLL..LLLLLLLLLLLLLLLLLLLLLLL.LLLLLLLLLLLLLLLLLLLLL.LL.LLL.LLLL.LLLLLL.LLLLLLLLL.LLLLLL
+.LLLLL.LLLLLLLLL.LLLLLLLLLLLLLL..LLLLLLLLLLLLLLL.LLLLLLLLLLLLLL.LLLLLL..LLL.LLLLLLLLLLLLLLLL.LLLLLL
+LLLLLLLLLLLLLLLL.LLLLLLLLL.LLLL.LLLLLLLLL.LLLLLLLLLLLLLLL.LLLLLLLLLLLL.LLLL.LLLLL..LLLLLLLLLLLLLLLL
+LLLLL.LLLLLLLLLLLL.LLLLLLLLLLLL.LLL.LLLLL.LLLLLL.LLL.LLLLLLLLLL.L.L.L..LLLL.LLLLLL.LLLLLLLLL.LLLLLL
+LLLLLL.LLLLLLLLL.LLLLLLLLL.LLLLLLLLLLLLLLLLLLLLL.LLLLLLLLLLLLLL.LLLLLL.LLLL.LLLLL..LLLLLLLLL.LLLLLL
+..L....L......L..L..L.......LL.L..L............LL.LL.L.L.L..........L..L.L.LL.L.LL......L.L....L..L
+LLLLLL.LLLLLLLLL.LLLLLLLLLLLLLL.LLLLLLLLL.LLLLLL.LLLLLLLL.L.LLLLLLLLLLLLLLL.LLLLLL.LLLL.LLLLLLLLLLL
+.LLLLL.LLLLLLLLL.LLLLLLLLL.LLLL.LLLLLLLLL.LLL.LLLLLLLLLL.LLLLLL.LL.LLL.LLLL.LLLLLL.LLLLLLLLL.LLL.L.
+LLLLL..LLLLLLLLL.LLLLLLLLLLLLLL.LLLLLLLLL.LL.LLLLLLLLLLLLLLLLLL.LLLLLL.LLLL.LLLLLL.LLLLLLLLL.LLLLLL
+LLLLLL.LLLLLLL.L.L.LLLLLLLLLLLL.LLLLLLLLL.LLLLLL.LLLLLLLL.LLLLL.LLLL.LLLLLL.LLLLLL.LLLLLLLLLLLLLLLL
+LLLLLL.LLLLLLLLL.LLLLLLLLLLLLLLLLLLLLLLLL.LLLLLL.LLLLLLLL.LLLLLLLLL.LL.LLLL.LLLLLLLLLLLL.LLL.LLLLLL
+L.LLLL.L.LLLLLLLLLLLLLLLLL.LLLL.LLLLLLLLLLLLLLLL.LLLLLLLL.LLLLL.LLLLLLLLLLL.LLLLLLLLLLLLLLLL.LLLLLL
+L.L...L...LL....LL....L..L.L.LLLL..LL..L.L....L.LL.L..L.L..LL.L..L..L..LLL..L..L..........L........
+LLLLLL.LLL.LLL.L.LLLLLLLLL.LLLLLLL.LLLLLL.LLL.LL.LLLLLLLL.LLLLL.LLLLLLLLLLLLLLLLLLLLLLLLLLLL.LLLLLL
+LLLLLL.LLLLLLLLLLLLLLLLLLL.L.LL.LLLLLLLLL.L.LLLL.LLLLLLLLLLLLLLLLLLLLLLLLLL.LLLLLLLLLLLLLLLLLLLLLLL
+LLLLLL.LLLLLLLLLLLLLLLLLLL.LLLL.LLLLL.LLL.LLLLLLLLLLLLLLLLLLLLL.LLLLLL.LLLLLLLLLLLLLLLLLLLLL.LLLLLL
+LLLLLL..LLLLLLLL.LL.LLLLLL.LLLL.LLLLLLLLL.LLLLL...L.LL.LL.LLLLL.LLLLLLLL.LL.LLLL.L.LL.LLLLLL.LLLLLL
+LLLLLL.LLLLLLLLLLLLLLLLLLL.LLLLLLL.LLLLLL..LLLLL.LLLLLLLL.LLLLLLLLLLLL.LLLLLLLLLLLLLLLL.LLLLLLL.LLL
+L..LL...............L...L.......LLL.....LL...L.........L.....L...L..L.......L.LLLL..L.L..LL.LL....L
+LLLLLL.LLLLLLLLL.LLLLLL.LL.LLLL.LLLLLLLLL.LLLLLL.LLLLLL.LLLLLLL.LLL.LL.LLLLLLLLLLL.LLLLLLLLL.LLLLLL
+LLLLLL.LLLLLLLLLLLLLLLL.LL.LLLL.LLLLLLLLL.LLLLLL.LLLLLLLL.LLLLLLLLLLLL.LLLL.LLLLLL.LLLLL.LLL.LLLLLL
+LLLLLL.LLLLLL.LLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLL.LLLLLLLL.LLLLL.LLLLLL.LLLLLLLLLLL..LLLLLLLLLLLLLLL
+.LLLLLLL.LLLLLLL.LLLLLLLLL..LLL.LLLLLLLLL..LLLLL.LLLLLLLL.LLLLL.LLLLLL.LLLL.L.L.LL..LLLLLLLL.L.LLLL
+LLLLLLLLLLLLLLLL.LLLLLLLLL.LLLL.LLLLLLLLL.LLLLLL.LLLLLLLL.LLLLLLLLLLL..LLLL.LLL.LL.LLLLLLLLL.LLLLLL
+LLLLLLLLLLLLLLLLLLLLLLLLLL.LLLL.LLLLLLLLL.LLLLLL.LLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLL.LLL.LLL.L.LLLLL.
+LLLLLLLLLLLLLLLLLLLLLLLLLL.LLLL.LL.LLLLLLLLLLLLLLLLLLLLLL.LLLLLLLLLLL..LLLLLLLLLLL.LLLLLLLLL.LLLLLL
+....L....L.L...L....L..L........L.L..LLL..L.L.L.L..L..L.....L.L....LL...LL..L..LL..LLL....LL...LL.L
+.LLLLLLLLLLLLLLL.LLLLLLLLL..LLLLLLLLL.LL..LLLLLL.L.LLLLLL.LLLLLLL.LLLL.LLLLLLLLLL..LLLLLLLLL.LLLLLL
+LLLLLL.LLLLLLLLL.LLLLLLLLL.LLLL.LLLLLLLLLLLLLLLLLLLLLLLLLLLLLLL...LLLL.LLLL.LLLLLLLLLL.LLLLLLLLLLLL
+LLLLLL.LLL.LLLLL.LLLLLLLLL.LLLL.LLLLLLLLL.LLLLLLLLLLLLLLL.LLLLLLLLLLLL.LLLL.LLLLLL.LL.LLLLLLLLLLLLL
+LLLLLLLL.LLLLLLLLLLLL.LLLL.LLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLL.L.LLLLLLLLL.LLLLLLLLL.LLLLLL.LLLLLL
+LLLLLLLLLLLLL.LLL.LLLLLLLLLLLLL.LLLLLLLLL.LLLLLLLLLLLLLLL.LLLLL.LLL.L..LLLLLLLLLLL.LLLLLLLLL.LLLLLL
+LLLLLLL.LLLLLL.L.LLLLLLLL.LLLLL.LLLLLLLLL.LLLLLL.LLLLL.LL.LLLLLLLL.LLL.LLLL.LLLLLL.LLLLLLLLL.LLLLLL
+LLLLLL.LLLLLLLLL.LLLLLLL.L.LLLL.LLLLLLLLL.LLLLLLLLLLL.LLL.LLLLL.LLLLLL.LLLL.LLLLLL.LLLLLLLLLLL.LLLL
+LLLLLL.LLLLLLLLL.LLLLLLLLL.L.LLL.LLLLLLLL.LLLLLL.LLLLLLLL.LLLLL.LLLLLL..LLLLLLLLLL.LL.LLLLLL.LLLLLL
+LLLLLL.LLLLLLLLL.L.LLLLLLL.LLLLLLLLLLLL.LLLLLLLL.LLLLLLLLLL.LLL.LLLLLL.LLLL.LLLLLL.L.LLL.LLL.LLLLLL
+...........L...L.......L.LL...LL.L.L.L..L..L.L.LL.LLLL.LLL..LLL.L.L..L.L.LL.L..L....LLLLLL...L.....
+LLLL.L.LLLLLLL.LLLLLLLLLLL.LLLLLLLLLLLLLL.LLLLLL..L.LLLLL.LLLLL.LLLLLL.LLLLLLLLLLLLLLLLLLL.L.L.LLLL
+LLLLLLLLLL..LLLL.LLLLLLLLL.LLLLLLLLLL.LLL.LLLLLLLLLLLLLLLLLLLLL.LLLLLL.LLLL.LLLLLL.LLLLLLLLL.L.LLLL
+LLLLLL.LLLLLLLLL.LLLLLLLLL.LLLLLLLLLLLLLL.LLLLL..LLLLLLLL.LLLLL.LLLLLL.LLLLLLLLLLLLLLLLLLLLL.LLLLL.
+LL.LLLLLLLLLLLLL.LLLLLLLLL.LLL..LLLLLLLLLLLLLLLL.LLL.L.LL.LLLLL.LLLLLL.LLLL.LLLLLL.LLLL.LLLL.LLL.LL
+LLLLLL.LLLLLLLLL.LLLLLLLLL.LLLL.LLLLLLLLL..LLLLLLLLLLLLLL.LLLLL.LLLLLL.LLL.LLLLLLL.LLLLLLL.L.LLLLLL
+LLLLLL.LLLLLLLLL.LLLLLLLLL.LLLLLLLLLLLLLLLLLLLLLLLLLLLLLL.LLL.LLLLLLLL.LLLLLLLLLLL.LLLLLLLLL.LLLLLL
+L...LLLLL.L.L.L.L........L...L...L.LLL...L..LL.LLL.L..LL..........L.LL.LL......L.L.........LLL.LLL.
+LLLLLLLLLLLLLLLL.LLLLLLLLLLLLLL.LLLLLLLLL.LLLLLLLLLLLLLL..LLLLL.LLLLLLLLLLL.LLLLLL.LLLLLLLLL.LLLLLL
+LLLLLLLLLLLLLLLL.LLLLLLL.LLLLLL.LLLLLLLLL.LLLL.L.LLLLLL.L..LLLL.LLLLLLLLLLL.LLLLLLLLLLLLLLLL.LLLLLL
+LLLL.L.LLLLLL.LLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLL.LLL.LLLLLLLLLL.LLLLLL.L.LLLLLLLLLLLLLLLLLL..LLLLLL
+LLLLLL.LLLLLL..L.LLLLLLLLL.L.LL.LLLLLLLLL.LLLLLLLLLL.LLLL..LLLLLLLLLLL.LLLL.LLLL.L.LLLLLLLLLLLLLLLL
+LLLLLLLLLLLLLLLL.LLLLLLLLLLLLLL.LL..LLLLL.LLLLL.LLLLLLLLLLLLLL..LLLL.L.LLLLLLLLLLL.LLLLLLLLL.LLLLLL
+LLLLLL.LLLLLLLLLLLLLLLL.LLLLLLL.LLLLLLLLLLLLLLLLLLLLLLLLL.LLLLL.LLLLLL.LLLL.LLLLLL.LLLLLLLLL.LLLLLL
+LLLLLLLLLLLLLLLL.LLLLLLLLL.LLLL.LLLLLLLLLLLLLLLL.LLLLLLLLL.LL.L.LL.LLL.LLLL.LLLLLLLLLLLLLLLL.LLLLLL
+L...LL.L.LL.LL..L.....L...L.L.L.L..LL....LL.L.L......L..L...L..LLL....LL.L.LLLL.L.LL.L..LLL.....L.L
+LLLLLL.LLLLLLLLLLLLLLLLLLLLLLLLLLLL.LLLLLLLLL.LL.LLLLLLLLLLLLLL.LLLLLLLLLLLLLLLLLL.LLL.L.LLLLLLLLLL
+.LLLLL.LLLLLLLLL.LLLLLLLLL.LL.LLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLL.LLLLLLLLLLL.LLLLLL.LLLLLLLLL.LLLLLL
+LLLLLL.LL.LLLLLL.LLLLLLLLL.LLLLLLLLLLLLLLLLLLLLL.LLLLLLLL.LLLLLLLLLLLL.LLLLLLLLLLL.LLLLLLLLL.LLLLLL
+LLLLLL..LLLLLLLL.LLLLLLLLLLLLLL.LLLLLLLLLLLLLLLLLLL.LLLLLLLLLLLLLLLLLLLLLLLLLLLLLL.LLLLLLLLL.LLLLLL
+LLLLLL.LLLLLLLLL.LLLLL.LLL.LLLL.LLLLLLLLLLLLL.LLLLLLLL.LL.L.LLLLLLLLLL.LLLL.LLLLLLLLLLLLLLLL.LLLLL.
+.L.......L....L.LL....L..L.......LL..L..L..L..LL..L...L......LLLL.L.......L.L.L...LL.LL.L.L...LLL..
+LLLLLL.L.LLLLLLL.LLLLLLLLLLL.LLLLL..LLLLL.LLLLLL.LLLLLLLL.LLLLL.LLLLLLLLLLL.LLLLLL.LLLLLLLLL.LLLLLL
+LLLLLL.LLLLLLLLL.LL.LLLLLLLLLLLLL..LLLLLL.LLLLLL.LLLLLLLL.LLLLL.LLLLLL.LLLLLLLLLLL.LLLLLLLLL.LLLLLL
+LLLLLL.LLLLLLLLL.LLLLLLLLL.L.LL.LLLLLLL.L.LLLLLL.LL..LLLL.LLLLLL.L.LLLLLLLLLLLLLLLLLLLLLLLLL.LLLLLL
+LLLLLL.L.L.LLLLL.LLLLLLLLLLLLLL.L.LLLLLLL.LLLLLLL.LLLLLLL.LLLLLLLLL.LLLLLLLLLLLLLL.LLLLLLLLL.LLLLL.
+LLLLLL.LLLLLLLLLLL.LL.LLLL.LLLL.LLLLLLLLL.LLLLLL.LLLLLLLLLL.LLL.LLLLLL.LLLL.LLLLL..LLLLLLLLL.LLLLLL
+LLL..LLLLLLLLLLLLLLLLLLLLL.LLLL.LLLLLLLLL.LLLLLL.LLLLLLLL.LLLLLLLLLLLLLLLLL.LLLLLL..LLLLLLLLLLLLLLL
+LLLLLL.L.LLLLLLL.LLLLLLLLL..LLL.LLLLLLLLLLLL.LLLLLLLLLLLL.LLL.LLLLLLLL.LLLLLLLLLLLLLLLLLLLLLLLLLLLL
+.....LLL..........L.....L..L...LL.....LL...L...L.L....L..L.LL.L.L.LL.L.L.LLL.L.L.L..L.LL.L....LL...
+LLLLL..LLLLLLLLL.LLLLLLLLL.L.LLLLLLL.LLLL.LLLLLLLLLLLLLLLLLL.LL.LLLLLL.LLLL.LLLL.LLLLLLLL.LL.LLLLLL
+L.LLLL.LLLLLLLLL.LL.LLLLLL.LL.L..LLLLLLLL.LLLLLL.LLLLL.LLLLLLLLLLLLLLLLLLLL..LLLLL..LLLLLLLL.L.L.LL
+LL.LLLLLLLLLLLLL.LLLLLLLL.LLLLL.LLLLLLLLL.LLLLL.L.LLLLLLL.LLLLL.LLLLLL.LLLL.LLLLL..LLLLLLLLL.LLLLLL
+LLLLL..LLLLLLLLL.LLLLLLLLLLLLLLLLLL.LLLLL.LLLLLL.LLLLLLLLLLLLLL.LLL.LL.LLLL.LLLLLL.LLLLLLLLL.LLLLLL
+LL.LLL.LLLLLLLLLLLLLLLLLLL.LLLLLLLLLLLLLLLLLLLLL.L.LLLLLL.LLLLL.LLLLLL.LLLLLLLLLLL.LLLLLLLLL.LLLLLL
+L.L.....LL........L..L....L.....LLL.L.L.L........L.........L........L..L..L..L.....L....LLL..L..LL.
+L.LLLL.LLL.LLLLLLL.LLLLLLL.LLLL.LLLLLLLLL.LLLLLL.LLLLLLL..LLLLL.LLLLLLLLLLLLLLLLL..LLLLLLL.L.LLLLLL
+LLLLLL.LLLL.LLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLL.LLLLLLLLLLLL.L.LLLLLL.LLLL.LLLLLL.LLLLLLLLLLLLLLLL
+LLLLLL.LLLLLLLLL.LLLLLLLLLL.LLLLLLLLLLLL.LLLLLLLLLLLLLLL...LLLL.LLLLLL.LLLLLLLLLLL.LLLLLLLLL.LLLLLL
+LLLL.L.LL.L.LLLL.LLLLLLLLL.LLLL.LLLLLLLLL.LLLLLL.LLLLLLLL.LLLLL.LLLLLLLLLLLLLLLLLLLLLLLLLL.L.L.LLLL
+LL.L.L.L..L..L..L....L....L.L.LLL.L.LLL.......L..LL.....LLL......L.L.L.L..L.LLL...L.......L........
+LLLLLL.LLLLLLLLL.LLLL.LLLL.LLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLL.LLL.LLLLLL.LLLL.LLLLLL.LLLLLLLLL.LLLLLL
+LLLLLL.LLLLLLLLL.LL.LLLLLLLLLLLLLLL..LLLL.LLLLLL.LLLLLLLLLLLLLL.LLLLLLLLLLLLLLLLLLLLLLLLLLLL.LLLLLL
+LLLLLL.LLLLLLL.L.L.LLLLLLL.LLLL.LLLLLLLLL.LLLLLLLLLLLL.LL.LLLLL.LLLLLL.LLLL.LLLLLLLLLLLLLLLLLLLLLLL
+LLLLLLLLLLLLLLLL.LLLLLLLLLLLLLL.LLLLLLLLL.LLLLLL.LLLLL..LLLLLLLLLL.LLL.LLLL..LLLLL.LLLLLLL.LL.LLLLL
+LLLLLLLLLLLLLLLL.LLLL.LLLL.LLLLLLL.LLLLLL.LLLLLLLLLLLLLLL.LLLLL.LLLLLLLLLLL.LLLLLLLLLLLLLLLLLLLLLLL
diff --git a/2020/11-Seating_System/second.hs b/2020/11-Seating_System/second.hs
new file mode 100644
index 0000000..4b60a8a
--- /dev/null
+++ b/2020/11-Seating_System/second.hs
@@ -0,0 +1,53 @@
+module Main (main) where
+import Control.Monad (when)
+import Data.List (foldl')
+import Data.Maybe (catMaybes, mapMaybe)
+import qualified Data.Map as M
+import System.Exit (die)
+
+exampleExpectedOutput = 26
+
+type Seating = M.Map (Int, Int) (Maybe Bool)
+
+inputToList :: String -> [((Int, Int), Maybe Bool)]
+inputToList = snd . foldl' nextSeat ((0, 0), [])
+ where
+ nextSeat :: ((Int, Int), [((Int, Int), Maybe Bool)]) -> Char -> ((Int, Int), [((Int, Int), Maybe Bool)])
+ nextSeat ((x, y), acc) '\n' = ((0, y+1), acc)
+ nextSeat ((x, y), acc) '.' = ((x+1, y), ((x, y), Nothing) : acc)
+ nextSeat ((x, y), acc) 'L' = ((x+1, y), ((x, y), Just False) : acc)
+
+parseInput :: String -> IO Seating
+parseInput filename = do
+ input <- readFile filename
+ return $ M.fromList (inputToList input)
+
+compute :: Seating -> Int
+compute seating
+ | seating == seating' = length . filter id . catMaybes $ map snd (M.toList seating)
+ | otherwise = compute seating'
+ where
+ seating' :: Seating
+ seating' = M.mapWithKey next seating
+ next :: (Int, Int) -> Maybe Bool -> Maybe Bool
+ next _ Nothing = Nothing
+ next (x, y) (Just False) = Just $ around (x, y) == []
+ next (x, y) (Just True) = Just $ length (around (x,y)) < 5
+ around :: (Int, Int) -> [Bool]
+ around (x, y) = filter id . catMaybes $ mapMaybe lookup [(-1, -1), (0, -1), (1, -1), (-1, 0), (1, 0), (-1, 1), (0, 1), (1, 1)]
+ where
+ lookup :: (Int, Int) -> Maybe (Maybe Bool)
+ lookup (dx, dy) = lookup' (1, dx, dy)
+ lookup' :: (Int, Int, Int) -> Maybe (Maybe Bool)
+ lookup' (i, dx, dy) = case M.lookup (x + i * dx, y + i * dy) seating of
+ Just (Just a) -> Just $ Just a
+ Just Nothing -> lookup' (i+1, dx, dy)
+ Nothing -> Nothing
+
+main :: IO ()
+main = do
+ example <- parseInput "example"
+ let exampleOutput = compute example
+ when (exampleOutput /= exampleExpectedOutput) (die $ "example failed: got " ++ show exampleOutput ++ " instead of " ++ show exampleExpectedOutput)
+ input <- parseInput "input"
+ print $ compute input