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-- requires cabal install --lib megaparsec parser-combinators heap vector
-- very slow with runghc, use ghc -O3 -o second second.hs and get the result in seconds
module Main (main) where
import Control.Monad (void, when)
import Data.Functor
import qualified Data.Map as M
import Data.Maybe
import qualified Data.Set as S
import qualified Data.Vector as V
import Data.Void (Void)
import Text.Megaparsec
import Text.Megaparsec.Char
import Debug.Trace
exampleExpectedOutput = 6
data Tile = Floor | Wall | Guard deriving (Eq, Ord, 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, Direction) ()
data Direction = N | S | E | W deriving (Eq, Ord, Show)
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
validCandidate :: (Int, Int) -> Input -> Bool
validCandidate (startx, starty) input = step (startx, starty) N M.empty
where
step :: (Int, Int) -> Direction -> Visited -> Bool
step (x, y) h v | M.member (x', y', h) v = True
| ahead == Nothing = False
| ahead == Just Wall = step (x, y) (nextRot h) v'
| otherwise = step (x', y') h v'
where
v' = M.insert (x, y, h) () v
(x', y') = next (x, y, h)
ahead = case input V.!? y' of
Just l -> l V.!? x'
Nothing -> Nothing
compute :: Input -> Int
compute input = S.size loops
where
loops = S.filter (validCandidate (startx, starty)) candidates
candidates = S.map placeWall . S.fromList . map next $ M.keys steps
placeWall (x, y) = let line = (input V.! y) V.// [(x, Wall)] in input V.// [(y, line)]
steps = step (startx, starty) N M.empty
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 -- do not add the last step so we do not go outside in placeWall
| ahead == Just Wall = step (x, y) (nextRot h) v'
| otherwise = step (x', y') h v'
where
v' = M.insert (x, y, h) () 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
|
