2023-16 part 2 in haskell
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2023/16-The_Floor_Will_Be_Lava/second.hs
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2023/16-The_Floor_Will_Be_Lava/second.hs
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-- requires cabal install --lib megaparsec parser-combinators
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module Main (main) where
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import Control.Applicative.Permutations
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import Control.Monad (void, when)
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import Data.Char qualified as C
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import Data.Either
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import Data.Functor
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import Data.List qualified as L
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import Data.Map qualified as M
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import Data.Maybe
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import Data.Set qualified as S
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import Data.Vector qualified as V
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import qualified Data.Vector.Unboxed as VU
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import Data.Void (Void)
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import Text.Megaparsec
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import Text.Megaparsec.Char
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import Debug.Trace
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exampleExpectedOutput = 51
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data Tile = Backslash | Empty | Horizontal | Slash | Vertical deriving Eq
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instance Show Tile where
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show Backslash = "\\"
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show Empty = "."
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show Horizontal = "-"
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show Slash = "/"
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show Vertical = "|"
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type Line = V.Vector Tile
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type Input = V.Vector Line
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type Parser = Parsec Void String
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parseTile :: Parser Tile
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parseTile = char '\\' $> Backslash
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<|> char '.' $> Empty
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<|> char '-' $> Horizontal
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<|> char '/' $> Slash
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<|> char '|' $> Vertical
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parseLine :: Parser Line
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parseLine = do
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line <- some parseTile <* eol
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return $ V.generate (length line) (line !!)
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parseInput' :: Parser Input
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parseInput' = do
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line <- some parseLine <* eof
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return $ V.generate (length line) (line !!)
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parseInput :: String -> IO Input
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parseInput filename = do
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input <- readFile filename
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case runParser parseInput' filename input of
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Left bundle -> error $ errorBundlePretty bundle
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Right input' -> return input'
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type LineOutput = VU.Vector Bool
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type Output = V.Vector LineOutput
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data Heading = N | S | E | W deriving (Eq, Ord, Show)
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data Cursor = Cursor Int Int Heading deriving (Eq, Ord, Show)
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type Visited = M.Map Cursor ()
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next :: Cursor -> Cursor
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next (Cursor x y N) = Cursor x (y-1) N
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next (Cursor x y S) = Cursor x (y+1) S
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next (Cursor x y W) = Cursor (x-1) y W
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next (Cursor x y E) = Cursor (x+1) y E
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compute' :: Input -> Cursor -> Int
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compute' input startingCursor = V.foldl' count 0 . fst $ walk startingOutput M.empty startingCursor
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where
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count :: Int -> LineOutput -> Int
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count i l = i + VU.length (VU.filter id l)
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startingOutput = V.replicate size $ VU.replicate size False
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size = V.length input
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walk :: Output -> Visited -> Cursor -> (Output, Visited)
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walk output v c | isNothing nextTile = (output, v)
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| justNextTile == Empty = walk output' v c'
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| visited = (output, v)
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| justNextTile == Horizontal = if h == W || h == E then walk output' v c'
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else let (output'', v'') = walk output' v' (Cursor x y E) in walk output'' v'' $ Cursor x y W
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| justNextTile == Vertical = if h == N || h == S then walk output' v c'
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else let (output'', v'') = walk output' v' (Cursor x y N) in walk output'' v'' $ Cursor x y S
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| justNextTile == Backslash = case h of
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N -> walk output' v' $ Cursor x y W
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S -> walk output' v' $ Cursor x y E
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W -> walk output' v' $ Cursor x y N
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E -> walk output' v' $ Cursor x y S
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| justNextTile == Slash = case h of
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N -> walk output' v' $ Cursor x y E
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S -> walk output' v' $ Cursor x y W
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W -> walk output' v' $ Cursor x y S
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E -> walk output' v' $ Cursor x y N
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where
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c'@(Cursor x y h) = next c
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nextTile = case input V.!? y of
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Just l -> l V.!? x
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Nothing -> Nothing
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justNextTile = fromJust nextTile
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line = output V.! y
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visited = M.member c' v
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v' = M.insert c' () v
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output' = if visited then output
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else output V.// [(y, line VU.// [(x, True)])]
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compute :: Input -> Int
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compute input = maximum $ map (compute' input) cursors
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where
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cursors = [Cursor (-1) y E | y <- [0..l]]
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++ [Cursor l y W | y <- [0..l]]
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++ [Cursor x (-1) S | x <- [0..l]]
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++ [Cursor x l N | x <- [0..l]]
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l = V.length input
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main :: IO ()
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main = do
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example <- parseInput "example"
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let exampleOutput = compute example
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when (exampleOutput /= exampleExpectedOutput) (error $ "example failed: got " ++ show exampleOutput ++ " instead of " ++ show exampleExpectedOutput)
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input <- parseInput "input"
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print $ compute input
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