94 lines
3.6 KiB
Haskell
94 lines
3.6 KiB
Haskell
-- requires cabal install --lib megaparsec parser-combinators heap vector
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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.Heap as H
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import qualified Data.List as L
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import qualified Data.Map as M
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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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exampleExpectedOutput = 64
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data Tile = Wall | Floor | Start | End deriving (Eq, 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 '#' $> Wall
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<|> char '.' $> Floor
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<|> char 'E' $> End
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<|> char 'S' $> Start
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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 Cost = Int
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data Heading = N | S | E | W deriving (Eq, Ord, Show)
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type Coord = (Int, Int, Heading)
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type Path = S.Set Coord
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data Position = Position Coord Cost Path deriving (Eq, Show)
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instance Ord Position where
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compare (Position _ c1 _) (Position _ c2 _) = c1 `compare` c2
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type Visited = M.Map Coord Int
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type Candidates = H.MinHeap Position
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compute :: Input -> Int
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compute input = S.size $ S.map (\(x, y, _) -> (x, y)) $ walk infinity M.empty $ H.singleton (Position start 0 S.empty)
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where
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walk :: Int -> Visited -> Candidates -> S.Set Coord
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walk l v h | H.size h == 0 = S.empty
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| c > l = walk l v' h'
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| t == End = S.union s' $ walk c v' h'
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| otherwise = walk l v' $ H.union h' $ H.fromList $ nexts v' $ Position p c s'
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where
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([(Position p@(x, y, d) c s)], h') = H.splitAt 1 h
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t = input V.! y V.! x
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v' = case M.lookup p v of
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Just c' -> if c < c' then M.insert p c v else v
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Nothing -> M.insert p c v
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s' = S.insert p s
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nexts :: Visited -> Position -> [Position]
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nexts v p = L.filter (valid v) $ candidates p
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valid :: Visited -> Position -> Bool
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valid v (Position p@(x, y, _) c _) = input V.! y V.! x /= Wall && case M.lookup p v of
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Just c' -> c <= c'
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Nothing -> True
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candidates :: Position -> [Position]
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candidates (Position (x, y, N) c s) = [ Position (x-1, y, W) (c+1001) s, Position (x+1, y, E) (c+1001) s, Position (x, y-1, N) (c+1) s ]
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candidates (Position (x, y, S) c s) = [ Position (x-1, y, W) (c+1001) s, Position (x+1, y, E) (c+1001) s, Position (x, y+1, S) (c+1) s ]
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candidates (Position (x, y, E) c s) = [ Position (x, y-1, N) (c+1001) s, Position (x, y+1, S) (c+1001) s, Position (x+1, y, E) (c+1) s ]
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candidates (Position (x, y, W) c s) = [ Position (x, y-1, N) (c+1001) s, Position (x, y+1, S) (c+1001) s, Position (x-1, y, W) (c+1) s ]
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height = V.length input
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width = V.length (input V.! 0)
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start = (1, height - 2, E)
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infinity = maxBound :: Int
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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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