56 lines
1.7 KiB
Haskell
56 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.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 = 143
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type Rule = (Int, Int)
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type Update = [Int]
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data Input = Input [Rule] [Update] deriving Show
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type Parser = Parsec Void String
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parseNumber :: Parser Int
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parseNumber = read <$> some digitChar
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parseRule :: Parser Rule
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parseRule = (,) <$> parseNumber <* char '|'
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<*> parseNumber <* eol
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parseUpdate :: Parser Update
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parseUpdate = some (parseNumber <* optional (char ',')) <* eol
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parseInput' :: Parser Input
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parseInput' = Input <$> some parseRule <* eol
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<*> some parseUpdate <* 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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compute :: Input -> Int
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compute (Input rules updates) = L.foldl' compute' 0 updates
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where
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compute' acc update | valid update = acc + update !! (length update `div` 2)
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| otherwise = acc
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valid update = all (valid' update) rules
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valid' update (x, y) = case (L.elemIndices x update, L.elemIndices y update) of
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([i], [j]) -> i < j
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_ -> True
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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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