51 lines
1.3 KiB
Haskell
51 lines
1.3 KiB
Haskell
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-- requires cabal install --lib megaparsec
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module Main (main) where
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import Control.Monad (void, when)
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import Data.List (foldl')
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import Data.Void
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import Text.Megaparsec
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import Text.Megaparsec.Char
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import System.Exit (die)
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exampleExpectedOutput = 7
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type Line = [Bool]
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type Field = [Line]
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type Parser = Parsec Void String
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parseElt :: Parser Bool
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parseElt = do
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c <- (char '#') <|> (char '.')
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return $ c == '#'
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parseLine :: Parser Line
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parseLine = some parseElt <* char '\n'
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parseField :: Parser Field
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parseField = some parseLine <* eof
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parseInput :: String -> IO Field
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parseInput filename = do
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input <- readFile filename
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case runParser parseField filename input of
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Left bundle -> die $ errorBundlePretty bundle
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Right field -> return field
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compute :: Field -> Int
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compute field = snd $ foldl' tree (0, 0) field
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where
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tree :: (Int, Int) -> Line -> (Int, Int)
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tree (index, trees) l = (next index, trees + if l !! index then 1 else 0)
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next n = (n + 3) `mod` fieldLen
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fieldLen = length $ field !! 0
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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) (die $ "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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