150 lines
5.3 KiB
Haskell
150 lines
5.3 KiB
Haskell
-- 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 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 = 4
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data Direction = N | S | E | W deriving (Eq, Show)
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data Tile = NS | NE | NW | EW | ES | WS | Floor | Start | Loop deriving (Eq, Show)
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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 '|' $> NS
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<|> char 'L' $> NE
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<|> char 'J' $> NW
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<|> char '-' $> EW
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<|> char 'F' $> ES
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<|> char '7' $> WS
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<|> char '.' $> Floor
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<|> char 'S' $> Start
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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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compatible :: Tile -> Direction -> Bool
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compatible NS N = True
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compatible NS S = True
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compatible NE S = True
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compatible NE W = True
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compatible NW S = True
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compatible NW E = True
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compatible EW E = True
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compatible EW W = True
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compatible ES W = True
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compatible ES N = True
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compatible WS E = True
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compatible WS N = True
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compatible _ _ = False
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type Position = (Int, Int)
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type Heading = (Position, Direction)
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data Where = Outside | Inside
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compute :: Input -> Int
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compute input = V.sum $ V.zipWith (count 0 False) inputWithStartReplaced inputLooped
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where
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count :: Int -> Bool -> Line -> Line -> Int
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count i w line line' | V.length line == 0 = i
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| u /= Loop = count (if w then i+1 else i) w r s
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| t == NS = count i (not w) r s
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| (t == NE && t'== NW) || (t == ES && t' == WS) = count i w r'' s'
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| otherwise = count i (not w) r'' s'
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where
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(t, u) = (line V.! 0, line' V.!0)
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(r, s) = (V.tail line, V.tail line')
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r' = V.dropWhile (== EW) r
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s' = V.drop (1 + V.length s - V.length r') s
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t' = r' V.! 0
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r'' = V.tail r'
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inputWithStartReplaced = input V.// [(sy, (input V.! sy) V.// [(sx, startingPipe startn starte starts startw)])]
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startingPipe Nothing Nothing _ _ = WS
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startingPipe Nothing _ Nothing _ = EW
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startingPipe Nothing _ _ Nothing = ES
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startingPipe _ Nothing Nothing _ = NW
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startingPipe _ Nothing _ Nothing = NS
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startingPipe _ _ Nothing Nothing = NE
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inputLooped = walk start input
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walk :: Heading -> Input -> Input -- walk the loop, marking each point in the loop to a Loop tile
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walk h@((x, y), _) i | tile (fst h) == Just Start = i'
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| otherwise = walk h' i'
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where
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h' = step h
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i' = i V.// [(y, (i V.! y) V.// [(x, Loop)])]
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step :: Heading -> Heading
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step (p@(x, y), N) | tile p == Just NS = ((x, y-1), N)
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| tile p == Just ES = ((x+1, y), E)
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| tile p == Just WS = ((x-1, y), W)
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step (p@(x, y), S) | tile p == Just NS = ((x, y+1), S)
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| tile p == Just NE = ((x+1, y), E)
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| tile p == Just NW = ((x-1, y), W)
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step (p@(x, y), E) | tile p == Just NW = ((x, y-1), N)
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| tile p == Just EW = ((x+1, y), E)
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| tile p == Just WS = ((x, y+1), S)
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step (p@(x, y), W) | tile p == Just NE = ((x, y-1), N)
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| tile p == Just EW = ((x-1, y), W)
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| tile p == Just ES = ((x, y+1), S)
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start = head $ catMaybes [startn, starte, starts, startw]
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startn = nearStart (sx, sy-1) N
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starts = nearStart (sx, sy+1) S
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starte = nearStart (sx+1, sy) E
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startw = nearStart (sx-1, sy) W
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nearStart :: Position -> Direction -> Maybe Heading
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nearStart p d = case tile p of
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Just t -> if compatible t d then Just (p, d) else Nothing
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Nothing -> Nothing
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(sx, sy) = (x, y) -- start
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where
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hasNoStart :: Line -> Bool
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hasNoStart = V.all (/= Start)
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y = length $ V.takeWhile hasNoStart input
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x = length $ V.takeWhile (/= Start) (input V.! y)
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tile :: Position -> Maybe Tile
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tile (x, y) = case input V.!? y of
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Just line -> line V.!? x
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Nothing -> Nothing
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main :: IO ()
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main = do
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example <- parseInput "example3"
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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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example4 <- parseInput "example4"
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let exampleOutput4 = compute example4
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when (exampleOutput4 /= 8) (error $ "example failed: got " ++ show exampleOutput4 ++ " instead of " ++ show 8)
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example5 <- parseInput "example5"
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let exampleOutput5 = compute example5
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when (exampleOutput5 /= 10) (error $ "example failed: got " ++ show exampleOutput5 ++ " instead of " ++ show 10)
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input <- parseInput "input"
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print $ compute input
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