-- 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 = "mxmxvkd,sqjhc,fvjkl"

newtype Ingredient = Ingredient String deriving (Eq, Ord, Read, Show)
newtype Allergen = Allergen String deriving (Eq, Ord, Read, Show)
type Rule = ([Ingredient], [Allergen])
type Input = [Rule]

type Parser = Parsec Void String

parseRule :: Parser Rule
parseRule = do
  ingredients <- some ((Ingredient <$> some letterChar) <* char ' ')
  void $ string "(contains "
  allergens <- some ((Allergen <$> some letterChar) <* optional (string ", "))
  void $ string ")\n"
  return $ (ingredients, allergens)

parseInput' :: Parser Input
parseInput' = some parseRule <* eof

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'

compute :: Input -> String
compute rules = L.intercalate "," . map (\(_, Ingredient i) -> i) $ M.assocs allergensMap
  where
    candidatesMap :: M.Map Allergen [Ingredient]
    candidatesMap = L.foldl' appendIngredientsForRule M.empty rules
    appendIngredientsForRule :: M.Map Allergen [Ingredient] -> Rule -> M.Map Allergen [Ingredient]
    appendIngredientsForRule m (is, as) = L.foldl' (appendIngredientsForAllergen is) m as
    appendIngredientsForAllergen :: [Ingredient] -> M.Map Allergen [Ingredient] -> Allergen -> M.Map Allergen [Ingredient]
    appendIngredientsForAllergen is m a = case M.lookup a m of
      Nothing -> M.insert a is m
      Just is' -> M.insert a (L.intersect is is') m
    allergensMap :: M.Map Allergen Ingredient
    allergensMap = allergensMap' candidatesMap M.empty
    allergensMap' :: M.Map Allergen [Ingredient] -> M.Map Allergen Ingredient -> M.Map Allergen Ingredient
    allergensMap' m acc
      | M.size m == 0 = acc
      | otherwise = allergensMap' (M.map (L.delete (head oneI)) $ M.delete oneK m) $ M.insert oneK (head oneI) acc
      where
        one = M.filter (\is -> length is == 1) m
        (oneK, oneI) = head $ M.assocs one

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