-- requires cabal install --lib megaparsec parser-combinators
module Main (main) where
import Control.Monad (void, when)
import Data.Either
import Data.List (foldl')
import Data.Map qualified as M
import Data.Void (Void)
import Text.Megaparsec
import Text.Megaparsec.Char
import System.Exit (die)

exampleExpectedOutput = 2

type Match = Char
type RuleID = Int
type Action = [RuleID]
type Rule = Either Match [Action]
type Message = String
data Input = Input { rules :: M.Map RuleID Rule
                   , messages :: [Message]
                   } deriving (Show)

type Parser = Parsec Void String

parseInt :: Parser Int
parseInt = do
  n <- some digitChar
  void $ optional (char ' ')
  return $ read n

parseMatch :: Parser Match
parseMatch = do
  void $ char '"'
  c <- letterChar
  void $ char '"'
  return c

parseAction :: Parser Action
parseAction = some (parseInt <* optional (char ' '))

parseRule :: Parser (RuleID, Rule)
parseRule = do
  id <- some digitChar
  void $ string ": "
  rule <- (Left <$> parseMatch) <|> (Right <$> some (parseAction <* optional (string "| ")))
  void $ char '\n'
  return $ (read id, rule)

parseInput' :: Parser Input
parseInput' = do
  rules <- M.fromList <$> some parseRule
  void $ char '\n'
  messages <- some (some letterChar <* (optional $ char '\n'))
  void eof
  return $ Input rules messages

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 -> Int
compute (Input rules messages) = length . filter id $ map isValid messages
  where
    isValid :: Message -> Bool
    isValid msg = case matches 0 msg of
      Just "" -> True
      otherwise -> False
    matches :: RuleID -> Message -> Maybe Message
    matches _ [] = Nothing
    matches ruleId msg = case rules M.! ruleId of
      Left c -> if head msg == c then (Just $ tail msg) else Nothing
      Right actions -> case map (processAction msg) actions of
        Just a:_ -> Just a
        Nothing:Just a:_ -> Just a
        otherwise -> Nothing
    processAction :: Message -> Action -> Maybe Message
    processAction msg action = foldl' step (Just msg) action
    step :: Maybe Message -> RuleID -> Maybe Message
    step Nothing _ = Nothing
    step (Just msg) ruleID = matches ruleID msg

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