1 files changed, 53 insertions, 0 deletions
diff --git a/2020/11-Seating_System/first-arrays.hs b/2020/11-Seating_System/first-arrays.hs
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+++ b/2020/11-Seating_System/first-arrays.hs
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+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