57 lines
1.7 KiB
Haskell
57 lines
1.7 KiB
Haskell
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-- requires cabal install --lib megaparsec parser-combinators heap vector
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module Main (main) where
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import Control.Monad (void, when)
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import qualified Data.Vector 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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exampleExpectedOutput = 9
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type Line = VU.Vector Char
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type Input = V.Vector Line
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type Parser = Parsec Void String
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parseLine :: Parser Line
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parseLine = do
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line <- some letterChar <* eol
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return $ VU.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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compute :: Input -> Int
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compute g = length $ filter xmas as
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where
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l = V.length g
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as = [(x, y) | x<-[1..l-2], y<-[1..l-2], g V.! x VU.! y == 'A']
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xmas :: (Int, Int) -> Bool
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xmas (x, y) = d1 && d2
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where
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tl = g V.! (x-1) VU.! (y-1)
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tr = g V.! (x+1) VU.! (y-1)
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bl = g V.! (x-1) VU.! (y+1)
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br = g V.! (x+1) VU.! (y+1)
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d1 = tl == 'M' && br == 'S' || tl == 'S' && br == 'M'
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d2 = bl == 'M' && tr == 'S' || bl == 'S' && tr == 'M'
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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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