2024-15 in haskell

2024/15-Warehouse_Woes/first.hs

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+-- requires cabal install --lib megaparsec parser-combinators heap vector
+module Main (main) where
+
+import           Control.Monad        (void, when)
+import           Data.Functor
+import qualified Data.List            as L
+import qualified Data.Vector          as V
+import           Data.Void            (Void)
+import           Text.Megaparsec
+import           Text.Megaparsec.Char
+
+exampleExpectedOutput = 2028
+example2ExpectedOutput = 10092
+
+data Tile = Wall | Box | Floor | Robot deriving (Eq, Show)
+type Line = V.Vector Tile
+type Warehouse = V.Vector Line
+data Op = N | S | E | W deriving Show
+data Input = Input Warehouse [Op] deriving Show
+
+type Parser = Parsec Void String
+
+parseTile :: Parser Tile
+parseTile = char '#' $> Wall
+        <|> char 'O' $> Box
+        <|> char '.' $> Floor
+        <|> char '@' $> Robot
+
+parseLine :: Parser Line
+parseLine = do
+  line <- some parseTile <* eol
+  return $ V.generate (length line) (line !!)
+
+parseOp :: Parser Op
+parseOp = char '^' $> N
+      <|> char 'v' $> S
+      <|> char '>' $> E
+      <|> char '<' $> W
+
+parseInput' :: Parser Input
+parseInput' = do
+  line <- some parseLine <* eol
+  ops <- some (parseOp <* optional eol) <* eof
+  return $ Input (V.generate (length line) (line !!)) ops
+
+parseInput :: String -> IO Input
+parseInput filename = do
+  input <- readFile filename
+  case runParser parseInput' filename input of
+    Left bundle  -> error $ errorBundlePretty bundle
+    Right input' -> return input'
+
+type Coord = (Int, Int)
+
+next :: Coord -> Op -> Coord
+next (x, y) N = (x, y-1)
+next (x, y) S = (x, y+1)
+next (x, y) E = (x+1, y)
+next (x, y) W = (x-1, y)
+
+compute :: Input -> Int
+compute (Input warehouse ops) = V.ifoldl' scoreBoxes 0 warehouse'
+  where
+    scoreBoxes :: Int -> Int -> Line -> Int
+    scoreBoxes acc y line = V.ifoldl' (scoreBox y) acc line
+    scoreBox :: Int -> Int -> Int -> Tile -> Int
+    scoreBox y acc x Box = acc + 100 * y + x
+    scoreBox _ acc _ _   = acc
+    warehouse' = fst $ L.foldl' step (warehouse, start) ops
+    step :: (Warehouse, Coord) -> Op -> (Warehouse, Coord)
+    step a@(w, r@(x, y)) op | t == Wall = a
+                            | t == Box = case push w r' op of
+                                Just w' -> let line = w' V.! y'
+                                               line' = line V.// [(x', Floor)]
+                                               w'' = w' V.// [(y', line')]
+                                           in (w'', r')
+                                Nothing -> a
+                            | otherwise = (w, (x', y'))
+      where
+        r'@(x', y') = next r op
+        t = w V.! y' V.! x'
+    push :: Warehouse -> Coord -> Op -> Maybe Warehouse
+    push w r@(x, y) op | t == Wall = Nothing
+                       | t == Box = push w (x', y') op
+                       | otherwise = Just w'
+      where
+        (x', y') = next r op
+        t = w V.! y' V.! x'
+        line = w V.! y'
+        line' = line V.// [(x', Box)]
+        w' = w V.// [(y', line')]
+    start = V.ifoldl' findRobot (0, 0) warehouse
+    findRobot :: (Int, Int) -> Int -> Line -> (Int, Int)
+    findRobot (0, _) y line = (V.ifoldl' findRobotInLine 0 line, y)
+    findRobot a _ _         = a
+    findRobotInLine :: Int -> Int -> Tile -> Int
+    findRobotInLine 0 x Robot = x
+    findRobotInLine a _ _     = a
+
+main :: IO ()
+main = do
+  example <- parseInput "example"
+  let exampleOutput = compute example
+  when  (exampleOutput /= exampleExpectedOutput)  (error $ "example failed: got " ++ show exampleOutput ++ " instead of " ++ show exampleExpectedOutput)
+  example2 <- parseInput "example2"
+  let example2Output = compute example2
+  when  (example2Output /= example2ExpectedOutput)  (error $ "example2 failed: got " ++ show example2Output ++ " instead of " ++ show example2ExpectedOutput)
+  input <- parseInput "input"
+  print $ compute input