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-- requires cabal install --lib megaparsec parser-combinators
module Main (main) where
import Control.Applicative.Permutations
import Control.Monad (void, when)
import Data.Char qualified as C
import Data.Either
import Data.Functor
import Data.List qualified as L
import Data.Map qualified as M
import Data.Maybe
import Data.Set qualified as S
import Data.Vector qualified as V
import qualified Data.Vector.Unboxed as VU
import Data.Void (Void)
import Text.Megaparsec
import Text.Megaparsec.Char
import Debug.Trace
exampleExpectedOutput = 51
data Tile = Backslash | Empty | Horizontal | Slash | Vertical deriving Eq
instance Show Tile where
show Backslash = "\\"
show Empty = "."
show Horizontal = "-"
show Slash = "/"
show Vertical = "|"
type Line = V.Vector Tile
type Input = V.Vector Line
type Parser = Parsec Void String
parseTile :: Parser Tile
parseTile = char '\\' $> Backslash
<|> char '.' $> Empty
<|> char '-' $> Horizontal
<|> char '/' $> Slash
<|> char '|' $> Vertical
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 LineOutput = VU.Vector Bool
type Output = V.Vector LineOutput
data Heading = N | S | E | W deriving (Eq, Ord, Show)
data Cursor = Cursor Int Int Heading deriving (Eq, Ord, Show)
type Visited = M.Map Cursor ()
next :: Cursor -> Cursor
next (Cursor x y N) = Cursor x (y-1) N
next (Cursor x y S) = Cursor x (y+1) S
next (Cursor x y W) = Cursor (x-1) y W
next (Cursor x y E) = Cursor (x+1) y E
compute' :: Input -> Cursor -> Int
compute' input startingCursor = V.foldl' count 0 . fst $ walk startingOutput M.empty startingCursor
where
count :: Int -> LineOutput -> Int
count i l = i + VU.length (VU.filter id l)
startingOutput = V.replicate size $ VU.replicate size False
size = V.length input
walk :: Output -> Visited -> Cursor -> (Output, Visited)
walk output v c | isNothing nextTile = (output, v)
| justNextTile == Empty = walk output' v c'
| visited = (output, v)
| justNextTile == Horizontal = if h == W || h == E then walk output' v c'
else let (output'', v'') = walk output' v' (Cursor x y E) in walk output'' v'' $ Cursor x y W
| justNextTile == Vertical = if h == N || h == S then walk output' v c'
else let (output'', v'') = walk output' v' (Cursor x y N) in walk output'' v'' $ Cursor x y S
| justNextTile == Backslash = case h of
N -> walk output' v' $ Cursor x y W
S -> walk output' v' $ Cursor x y E
W -> walk output' v' $ Cursor x y N
E -> walk output' v' $ Cursor x y S
| justNextTile == Slash = case h of
N -> walk output' v' $ Cursor x y E
S -> walk output' v' $ Cursor x y W
W -> walk output' v' $ Cursor x y S
E -> walk output' v' $ Cursor x y N
where
c'@(Cursor x y h) = next c
nextTile = case input V.!? y of
Just l -> l V.!? x
Nothing -> Nothing
justNextTile = fromJust nextTile
line = output V.! y
visited = M.member c' v
v' = M.insert c' () v
output' = if visited then output
else output V.// [(y, line VU.// [(x, True)])]
compute :: Input -> Int
compute input = maximum $ map (compute' input) cursors
where
cursors = [Cursor (-1) y E | y <- [0..l]]
++ [Cursor l y W | y <- [0..l]]
++ [Cursor x (-1) S | x <- [0..l]]
++ [Cursor x l N | x <- [0..l]]
l = V.length input
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
|
