import Text.Regex.Posix ((=~))
import Data.List

main :: IO ()
main = do
  input <- readFile "input"
  let instructions = findAll "mul\\([0-9]+,[0-9]+\\)|do\\(\\)|don't\\(\\)" input
  let muls = filter (/= "do()") . stripDont $ instructions
  let pairs = map parseMul muls
  let total = sum (map (\(x,y) -> x*y) pairs)
  print total

splitBy :: Eq a => [a] -> a -> ([a], [a])
splitBy xs sep =
  case elemIndex sep xs of
    Nothing -> (xs, [])
    Just i -> (take i xs, drop (i+1) xs)

stripDont :: [String] -> [String]
stripDont xs =
  let (start, temp) = splitBy xs "don't()"
      (_, end) = splitBy temp "do()"
  in if end == [] then start else (start ++ (stripDont end))

parseMul s = read (drop 3 s) :: (Int, Int)

findAll rx s =
  case findFirst rx s of
    Nothing -> []
    Just (match, rest) -> match:(findAll rx rest)

findFirst :: String -> String -> Maybe (String, String)
findFirst rx s =
  -- before, match, after, submatches
  case s =~ rx :: (String, String, String, [String]) of
    (_, "", _ ,_) -> Nothing
    (_, match, rest, _) -> Just (match, rest)