51 lines
1.6 KiB
Haskell
51 lines
1.6 KiB
Haskell
-- 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.List as L
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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 = 18
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type Line = [Char]
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type Input = [Line]
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type Parser = Parsec Void String
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parseInput' :: Parser Input
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parseInput' = some (some letterChar <* eol) <* eof
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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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rotate90 = reverse . L.transpose
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rotate180 = rotate90 . rotate90
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diagonals = (++) <$> L.transpose . zipWith drop [0..]
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<*> L.transpose . zipWith drop [1..] . rotate180
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countInString :: String -> Int
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countInString [] = 0
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countInString s@('X':'M':'A':'S':_) = 1 + (countInString $ tail s)
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countInString s@('S':'A':'M':'X':_) = 1 + (countInString $ tail s)
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countInString s = countInString $ tail s
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compute :: Input -> Int
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compute lines = sum $ map countInString combinations
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where
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columns = rotate90 lines
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combinations = lines ++ columns ++ diagonals lines ++ diagonals columns
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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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