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-- requires cabal install --lib megaparsec parser-combinators heap vector
module Main (main) where
import Control.Monad (void, when)
import qualified Data.List as L
import Data.Void (Void)
import Text.Megaparsec
import Text.Megaparsec.Char
exampleExpectedOutput = 123
type Rule = (Int, Int)
type Update = [Int]
data Input = Input [Rule] [Update] deriving Show
type Parser = Parsec Void String
parseNumber :: Parser Int
parseNumber = read <$> some digitChar
parseRule :: Parser Rule
parseRule = (,) <$> parseNumber <* char '|'
<*> parseNumber <* eol
parseUpdate :: Parser Update
parseUpdate = some (parseNumber <* optional (char ',')) <* eol
parseInput' :: Parser Input
parseInput' = Input <$> some parseRule <* eol
<*> some parseUpdate <* eof
parseInput :: String -> IO Input
parseInput filename = do
input <- readFile filename
case runParser parseInput' filename input of
Left bundle -> error $ errorBundlePretty bundle
Right input' -> return input'
compute :: Input -> Int
compute (Input rules updates) = sum $ map compute' invalids
where
invalids = L.filter (not . valid) updates
valid update = all (valid' update) rules
valid' update (x, y) = case (L.elemIndices x update, L.elemIndices y update) of
([i], [j]) -> i < j
_ -> True
compute' update = (L.sortBy sortByRule update) !! (length update `div` 2)
sortByRule :: Int -> Int -> Ordering
sortByRule x y = if L.elem (x, y) rules then LT else GT
main :: IO ()
main = do
example <- parseInput "example"
let exampleOutput = compute example
when (exampleOutput /= exampleExpectedOutput) (error $ "example failed: got " ++ show exampleOutput ++ " instead of " ++ show exampleExpectedOutput)
input <- parseInput "input"
print $ compute input
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