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-rw-r--r--2023/16-The_Floor_Will_Be_Lava/second.hs124
1 files changed, 124 insertions, 0 deletions
diff --git a/2023/16-The_Floor_Will_Be_Lava/second.hs b/2023/16-The_Floor_Will_Be_Lava/second.hs
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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