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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.Heap as H
import qualified Data.List as L
import qualified Data.Map as M
import qualified Data.Vector as V
import Data.Void (Void)
import Text.Megaparsec
import Text.Megaparsec.Char
exampleExpectedOutput = 11048
data Tile = Wall | Floor | Start | End deriving (Eq, Show)
type Line = V.Vector Tile
type Input = V.Vector Line
type Parser = Parsec Void String
parseTile :: Parser Tile
parseTile = char '#' $> Wall
<|> char '.' $> Floor
<|> char 'E' $> End
<|> char 'S' $> Start
parseLine :: Parser Line
parseLine = do
line <- some parseTile <* eol
return $ V.generate (length line) (line !!)
parseInput' :: Parser Input
parseInput' = do
line <- some parseLine <* eof
return $ V.generate (length line) (line !!)
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 Cost = Int
data Heading = N | S | E | W deriving (Eq, Show)
type Coord = (Int, Int)
data Position = Position Coord Heading Cost deriving Show
instance Ord Position where
compare (Position _ _ c1) (Position _ _ c2) = c1 `compare` c2
instance Eq Position where
(Position _ _ c1) == (Position _ _ c2) = c1 == c2
type Visited = M.Map Coord Int
type Candidates = H.MinHeap Position
compute :: Input -> Int
compute input = walk M.empty $ H.singleton (Position start E 0)
where
walk :: Visited -> Candidates -> Int
walk v h | t == End = c
| otherwise = walk v' $ H.union h' $ H.fromList $ nexts v' $ Position p d c
where
([(Position p@(x, y) d c)], h') = H.splitAt 1 h
t = input V.! y V.! x
v' = case M.lookup p v of
Just c' -> if c < c' then M.insert p c v else v
Nothing -> M.insert p c v
nexts :: Visited -> Position -> [Position]
nexts v p = L.filter (valid v) $ candidates p
valid :: Visited -> Position -> Bool
valid v (Position p@(x, y) _ c) = input V.! y V.! x /= Wall && case M.lookup p v of
Just c' -> c < c'
Nothing -> True
candidates :: Position -> [Position]
candidates (Position (x, y) N c) = [ Position (x-1, y) W (c+1001), Position (x+1, y) E (c+1001), Position (x, y-1) N (c+1) ]
candidates (Position (x, y) S c) = [ Position (x-1, y) W (c+1001), Position (x+1, y) E (c+1001), Position (x, y+1) S (c+1) ]
candidates (Position (x, y) E c) = [ Position (x, y-1) N (c+1001), Position (x, y+1) S (c+1001), Position (x+1, y) E (c+1) ]
candidates (Position (x, y) W c) = [ Position (x, y-1) N (c+1001), Position (x, y+1) S (c+1001), Position (x-1, y) W (c+1) ]
height = V.length input
width = V.length (input V.! 0)
start = (1, height - 2)
main :: IO ()
main = do
example <- parseInput "example"
let exampleOutput = compute example
when (exampleOutput /= exampleExpectedOutput) (error $ "example failed: got " ++ show exampleOutput ++ " instead of " ++ show exampleExpectedOutput)
input <- parseInput "input"
print $ compute input
|
