84 lines
2.9 KiB
Haskell
84 lines
2.9 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.List as L
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import qualified Data.Map as M
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import Data.Maybe
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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 = 5
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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 Coord = (Int, Int)
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type Visited = M.Map Coord Int
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type Cost = Int
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compute :: Input -> Int -> Int
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compute input minGain = findAllShortcut
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where
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findAllShortcut :: Int
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findAllShortcut = M.foldrWithKey findShortcutFrom 0 costs
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findShortcutFrom :: Coord -> Int -> Int -> Int
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findShortcutFrom (x, y) c acc = M.foldrWithKey validShortcut acc candidates
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where
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candidates = M.filterWithKey (\(x', y') _ -> (abs $ x - x') + (abs $ y - y') <= 2) costs
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validShortcut :: Coord -> Int -> Int -> Int
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validShortcut (x', y') c' acc' = let gain = c' - c - (abs $ x - x') - (abs $ y - y')
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in if gain >= minGain then acc' + 1 else acc'
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(costs, len) = walk M.empty 0 (sx, sy)
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where
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walk :: Visited -> Int -> Coord -> (Visited, Int)
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walk m c (x, y) | t == End = (m', c)
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| otherwise = walk m' (c+1) next
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where
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[next] = filter (\p@(i, j) -> and [input V.! j V.! i /= Wall, not $ M.member p m']) candidates
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candidates = [(x-1, y), (x+1, y), (x, y-1), (x, y+1)]
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m' = M.insert (x, y) c m
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t = input V.! y V.! x
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height = V.length input
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width = V.length (input V.! 0)
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sx = fst $ (V.filter (\(_, t) -> t == Start) $ V.indexed $ input V.! sy) V.! 0
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sy = fst $ (V.filter (\(_, v) -> V.length (V.filter (== Start) v) == 1) $ V.indexed input) V.! 0
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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 20
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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 100
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