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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
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