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-- requires cabal install --lib megaparsec parser-combinators
module Main (main) where
import Control.Monad (void, when)
import qualified Data.Set as S
import Data.Void (Void)
import Text.Megaparsec
import Text.Megaparsec.Char
import System.Exit (die)
exampleExpectedOutput = 4
data Bag = Bag { name :: String
, contents :: [(Int, String)]
} deriving (Show)
type Parser = Parsec Void String
parseName :: Parser String
parseName = do
adjective <- some letterChar
void (char ' ')
color <- some letterChar
return $ adjective ++ " " ++ color
parseInt :: Parser Int
parseInt = do
i <- some digitChar
return $ read i
parseContent :: Parser (Int, String)
parseContent = do
i <- parseInt
void (char ' ')
name <- parseName
void (string " bag")
void (optional $ char 's')
void (string ", " <|> string ".")
return (i, name)
parseContents :: Parser [(Int, String)]
parseContents = some parseContent <|> (string "no other bags." *> return [])
parseBag :: Parser Bag
parseBag = do
name <- parseName
void $ string " bags contain "
contents <- parseContents
void (char '\n')
return $ Bag { name=name, contents=contents }
parseBags :: Parser [Bag]
parseBags = some parseBag <* eof
parseInput :: String -> IO [Bag]
parseInput filename = do
input <- readFile filename
case runParser parseBags filename input of
Left bundle -> die $ errorBundlePretty bundle
Right bags -> return bags
compute :: [Bag]-> Int
compute bags = (length . compute' $ S.fromList ["shiny gold"]) - 1
where
bagsContaining :: S.Set String -> S.Set String
bagsContaining s = S.union s $ S.fromList . (map name) $ filter (\x -> not . S.null . S.intersection s $ S.fromList . (map snd) . contents $ x) bags
compute' :: S.Set String -> S.Set String
compute' s = if s == s' then s else compute' s'
where
s' = bagsContaining s
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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