100 lines
3.4 KiB
Haskell
100 lines
3.4 KiB
Haskell
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-- requires cabal install --lib megaparsec parser-combinators heap vector
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-- very slow with runghc, use ghc -O3 -o second second.hs and get the result in seconds
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module Main (main) where
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import Control.Monad (void, when)
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import Data.Functor
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import qualified Data.Map as M
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import Data.Maybe
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import qualified Data.Set as S
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import qualified Data.Vector as V
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import Data.Void (Void)
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import Text.Megaparsec
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import Text.Megaparsec.Char
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import Debug.Trace
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exampleExpectedOutput = 6
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data Tile = Floor | Wall | Guard deriving (Eq, Ord, Show)
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type Line = V.Vector Tile
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type Input = V.Vector Line
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type Parser = Parsec Void String
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parseTile :: Parser Tile
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parseTile = char '.' $> Floor
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<|> char '#' $> Wall
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<|> char '^' $> Guard
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parseLine :: Parser Line
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parseLine = do
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line <- some parseTile <* eol
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return $ V.generate (length line) (line !!)
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parseInput' :: Parser Input
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parseInput' = do
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line <- some parseLine <* eof
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return $ V.generate (length line) (line !!)
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parseInput :: String -> IO Input
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parseInput filename = do
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input <- readFile filename
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case runParser parseInput' filename input of
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Left bundle -> error $ errorBundlePretty bundle
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Right input' -> return input'
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type Visited = M.Map (Int, Int, Direction) ()
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data Direction = N | S | E | W deriving (Eq, Ord, Show)
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next (x, y, N) = (x, y-1)
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next (x, y, S) = (x, y+1)
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next (x, y, W) = (x-1, y)
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next (x, y, E) = (x+1, y)
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nextRot N = E
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nextRot E = S
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nextRot S = W
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nextRot W = N
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validCandidate :: (Int, Int) -> Input -> Bool
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validCandidate (startx, starty) input = step (startx, starty) N M.empty
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where
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step :: (Int, Int) -> Direction -> Visited -> Bool
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step (x, y) h v | M.member (x', y', h) v = True
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| ahead == Nothing = False
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| ahead == Just Wall = step (x, y) (nextRot h) v'
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| otherwise = step (x', y') h v'
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where
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v' = M.insert (x, y, h) () v
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(x', y') = next (x, y, h)
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ahead = case input V.!? y' of
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Just l -> l V.!? x'
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Nothing -> Nothing
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compute :: Input -> Int
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compute input = S.size loops
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where
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loops = S.filter (validCandidate (startx, starty)) candidates
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candidates = S.map placeWall . S.fromList . map next $ M.keys steps
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placeWall (x, y) = let line = (input V.! y) V.// [(x, Wall)] in input V.// [(y, line)]
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steps = step (startx, starty) N M.empty
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Just (starty, Just startx) = let startx (y, v) = (y, V.elemIndex Guard v)
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in V.find (isJust . snd) $ V.map startx $ V.indexed input
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step :: (Int, Int) -> Direction -> Visited -> Visited
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step (x, y) h v | ahead == Nothing = v -- do not add the last step so we do not go outside in placeWall
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| ahead == Just Wall = step (x, y) (nextRot h) v'
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| otherwise = step (x', y') h v'
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where
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v' = M.insert (x, y, h) () v
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(x', y') = next (x, y, h)
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ahead = case input V.!? y' of
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Just l -> l V.!? x'
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Nothing -> Nothing
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main :: IO ()
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main = do
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example <- parseInput "example"
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let exampleOutput = compute example
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when (exampleOutput /= exampleExpectedOutput) (error $ "example failed: got " ++ show exampleOutput ++ " instead of " ++ show exampleExpectedOutput)
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input <- parseInput "input"
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print $ compute input
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