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-- 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.Maybe (isJust)
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 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
Nothing:a:_ -> a
a:_ -> 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
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