90 lines
2.8 KiB
Haskell
90 lines
2.8 KiB
Haskell
-- requires cabal install --lib megaparsec parser-combinators
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module Main (main) where
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import Control.Monad (void, when)
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import Data.Either
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import Data.List (any, foldl')
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import Data.Maybe (catMaybes)
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import Data.Map qualified as M
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import Data.Void (Void)
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import Text.Megaparsec
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import Text.Megaparsec.Char
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import System.Exit (die)
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exampleExpectedOutput = 12
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type Match = Char
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type RuleID = Int
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type Action = [RuleID]
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type Rule = Either Match [Action]
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type Message = String
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data Input = Input { rules :: M.Map RuleID Rule
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, messages :: [Message]
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} deriving (Show)
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type Parser = Parsec Void String
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parseInt :: Parser Int
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parseInt = do
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n <- some digitChar
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void $ optional (char ' ')
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return $ read n
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parseMatch :: Parser Match
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parseMatch = do
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void $ char '"'
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c <- letterChar
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void $ char '"'
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return c
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parseAction :: Parser Action
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parseAction = some (parseInt <* optional (char ' '))
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parseRule :: Parser (RuleID, Rule)
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parseRule = do
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id <- some digitChar
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void $ string ": "
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rule <- (Left <$> parseMatch) <|> (Right <$> some (parseAction <* optional (string "| ")))
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void $ char '\n'
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return $ (read id, rule)
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parseInput' :: Parser Input
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parseInput' = do
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rules <- M.fromList <$> some parseRule
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void $ char '\n'
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messages <- some (some letterChar <* (optional $ char '\n'))
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void eof
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let rules' = M.insert 8 (Right $ reverse [take i (repeat 42)|i<-[1..5]]) rules
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rules'' = M.insert 11 (Right $ reverse [take i (repeat 42) ++ take i (repeat 31)|i<-[1..5]]) rules'
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return $ Input rules'' messages
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parseInput :: String -> IO Input
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parseInput filename = do
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input <- readFile filename
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case runParser parseInput' filename input of
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Left bundle -> die $ errorBundlePretty bundle
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Right input' -> return input'
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compute :: Input -> Int
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compute (Input rules messages) = length . filter id $ map isValid messages
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where
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isValid :: Message -> Bool
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isValid msg = any (== "") $ matchAll 0 [msg]
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matchAll :: RuleID -> [Message] -> [Message]
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matchAll ruleId msgs = concat $ map (matches ruleId) msgs
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matches :: RuleID -> Message -> [Message]
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matches _ [] = []
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matches ruleId msg = case rules M.! ruleId of
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Left c -> if head msg == c then [tail msg] else []
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Right actions -> concat $ map (processAction msg) actions
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processAction :: Message -> Action -> [Message]
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processAction msg action = foldl' step [msg] action
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step :: [Message] -> RuleID -> [Message]
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step msgs ruleID = matchAll ruleID msgs
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main :: IO ()
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main = do
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example <- parseInput "example2"
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let exampleOutput = compute example
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when (exampleOutput /= exampleExpectedOutput) (die $ "example failed: got " ++ show exampleOutput ++ " instead of " ++ show exampleExpectedOutput)
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input <- parseInput "input"
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print $ compute input
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