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-- requires cabal install --lib megaparsec parser-combinators
module Main (main) where
import Control.Monad (void, when)
import Data.List qualified as L
import Data.Map qualified as M
import Data.Maybe (catMaybes)
import Data.Set qualified as S
import Data.Void (Void)
import Text.Megaparsec
import Text.Megaparsec.Char
import System.Exit (die)
exampleExpectedOutput = 306
type Card = Int
type Deck = [Card]
type Input = (Deck, Deck)
type Parser = Parsec Void String
parseCard :: Parser Card
parseCard = do
n <- some digitChar
void $ optional (char '\n')
return $ read n
parseDeck :: Parser Deck
parseDeck = string "Player " *> digitChar *> string ":\n" *> some parseCard
parseInput' :: Parser Input
parseInput' = do
p1 <- parseDeck
void $ char '\n'
p2 <- parseDeck
void $ eof
return (p1, p2)
parseInput :: String -> IO Input
parseInput filename = do
input <- readFile filename
case runParser parseInput' filename input of
Left bundle -> die $ errorBundlePretty bundle
Right input' -> return input'
computeScore :: Deck -> Int
computeScore d = sum $ zipWith (*) d (reverse $ take (length d) [1..])
compute :: Input -> Int
compute ([], d) = computeScore d
compute (d, []) = computeScore d
compute (a:as, b:bs)
| a > b = compute (as ++ [a, b], bs)
| otherwise = compute (as, bs ++ [b, a])
main :: IO ()
main = do
example <- parseInput "example"
let exampleOutput = compute example
when (exampleOutput /= exampleExpectedOutput) (die $ "example failed: got " ++ show exampleOutput ++ " instead of " ++ show exampleExpectedOutput)
input <- parseInput "input"
print $ compute input
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