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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.Map as M
import Data.Maybe
import qualified Data.Vector as V
import Data.Void (Void)
import Text.Megaparsec
import Text.Megaparsec.Char
exampleExpectedOutput = 41
data Tile = Floor | Wall | Guard deriving (Eq, Show)
type Line = V.Vector Tile
type Input = V.Vector Line
type Parser = Parsec Void String
parseTile :: Parser Tile
parseTile = char '.' $> Floor
<|> char '#' $> Wall
<|> char '^' $> Guard
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 Visited = M.Map (Int, Int) ()
data Direction = N | S | E | W
next (x, y) N = (x, y-1)
next (x, y) S = (x, y+1)
next (x, y) W = (x-1, y)
next (x, y) E = (x+1, y)
nextRot N = E
nextRot E = S
nextRot S = W
nextRot W = N
compute :: Input -> Int
compute input = M.size $ step (startx, starty) N M.empty
where
Just (starty, Just startx) = let startx (y, v) = (y, V.elemIndex Guard v)
in V.find (isJust . snd) $ V.map startx $ V.indexed input
step :: (Int, Int) -> Direction -> Visited -> Visited
step (x, y) h v | ahead == Nothing = v'
| ahead == Just Wall = step (x, y) (nextRot h) v'
| otherwise = step (x', y') h v'
where
v' = M.insert (x, y) () v
(x', y') = next (x, y) h
ahead = case input V.!? y' of
Just l -> l V.!? x'
Nothing -> Nothing
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
|
