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+-- requires cabal install --lib megaparsec parser-combinators heap vector
+module Main (main) where
+
+import Control.Monad (void, when)
+import Data.Functor
+import qualified Data.List as L
+import Data.Maybe
+import Data.Void (Void)
+import Text.Megaparsec
+import Text.Megaparsec.Char
+
+exampleExpectedOutput = 48
+
+data Op = Mul Int Int | Do | Dont | Nop deriving (Eq, Show)
+type Input = [Op]
+
+type Parser = Parsec Void String
+
+parseNumber :: Parser Int
+parseNumber = read <$> some digitChar
+
+parseMul :: Parser Op
+parseMul = Mul <$> (string "mul(" *> parseNumber)
+ <*> (char ',' *> parseNumber <* char ')')
+
+parseOp :: Parser Op
+parseOp = try parseMul
+ <|> string "do()" $> Do
+ <|> string "don't()" $> Dont
+ <|> anySingle $> Nop
+
+parseInput' :: Parser Input
+parseInput' = filter (/= Nop) <$> some parseOp <* eof
+
+parseInput :: String -> IO Input
+parseInput filename = do
+ input <- readFile filename
+ case runParser parseInput' filename input of
+ Left bundle -> error $ errorBundlePretty bundle
+ Right input' -> return input'
+
+compute :: Input -> Int
+compute = fst . L.foldl' compute' (0, True)
+ where
+ compute' :: (Int, Bool) -> Op -> (Int, Bool)
+ compute' (acc, False) Do = (acc, True)
+ compute' (acc, True) Dont = (acc, False)
+ compute' (acc, True) (Mul x y) = (acc + (x * y), True)
+ compute' acc _ = acc
+
+main :: IO ()
+main = do
+ example <- parseInput "example2"
+ let exampleOutput = compute example
+ when (exampleOutput /= exampleExpectedOutput) (error $ "example failed: got " ++ show exampleOutput ++ " instead of " ++ show exampleExpectedOutput)
+ input <- parseInput "input"
+ print $ compute input