2020-11 in haskell

1 files changed, 53 insertions, 0 deletions

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