75 lines
2.6 KiB
Haskell
75 lines
2.6 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.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 = 22
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type Coord = (Int, Int)
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type Input = [Coord]
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type Parser = Parsec Void String
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parseNumber :: Parser Int
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parseNumber = read <$> some digitChar
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parseCoord :: Parser Coord
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parseCoord = (,) <$> parseNumber <* char ','
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<*> parseNumber <* eol
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parseInput' :: Parser Input
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parseInput' = some parseCoord <* eof
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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 Position = Position Coord Cost deriving 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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instance Eq Position where
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(Position p1 _ ) == (Position p2 _ ) = p1 == p2
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type Visited = M.Map Coord Cost
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type Maze = M.Map Coord ()
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type Candidates = H.MinHeap Position
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compute :: Int -> Int -> Input -> Int
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compute size cutoff input = walk (M.singleton (0, 0) 0) $ H.singleton (Position (0, 0) 0)
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where
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walk :: Visited -> Candidates -> Int
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walk v h | x == size && y == size = c
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| otherwise = walk v' $ H.union h' $ H.fromList n
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where
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([pos@(Position p@(x, y) c)], h') = H.splitAt 1 h
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n = nexts v pos
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v' = L.foldl' (\acc (Position a b) -> M.insert a b acc) v n
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nexts :: Visited -> Position -> [Position]
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nexts v (Position p c) = L.filter (valid v) . map (\p' -> Position p' (c+1)) $ candidates p
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valid :: Visited -> Position -> Bool
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valid v (Position p@(x, y) c) = x >= 0 && x <= size && y >= 0 && y <= size && not (M.member p maze) && 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 :: Coord -> [Coord]
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candidates (x, y) = [ (x-1, y), (x+1, y), (x, y-1), (x, y+1) ]
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maze = M.fromList $ zip (take cutoff input) (L.repeat ())
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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 6 12 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 70 1024 input
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