{-# LANGUAGE DoAndIfThenElse #-}

module Main
where

import Control.Monad (filterM)
import qualified Data.ByteString.UTF8 as BS
import Network.DNS (
  Domain,
  Resolver,
  ResolvConf(..),
  defaultResolvConf,
  makeResolvSeed,
  withResolver)
import Network.DNS.Lookup (lookupMX)
import System.IO (hGetContents, hFlush, hPutStrLn, stdin, stdout)


type Address = BS.ByteString

resolv_conf ::  ResolvConf
resolv_conf = defaultResolvConf { resolvTimeout = 5 * 1000 * 1000 }

validate_mx :: Resolver -> Domain -> IO Bool
validate_mx resolver domain = do
  result <- lookupMX resolver domain
  case result of
      Nothing -> return False
      _       -> return True

validate_syntax :: Address -> IO Bool
validate_syntax address = do
  return True

utf8_split = undefined

validate :: Resolver -> Address -> IO Bool
validate resolver address = do
  valid_syntax <- validate_syntax address
  if valid_syntax then do
    let domain = utf8_split address (BS.fromString "@")
    validate_mx resolver domain
  else do
    return False

empty_string :: BS.ByteString
empty_string = BS.fromString ""

main :: IO ()
main = do
  input <- hGetContents stdin

  let addresses = BS.lines $ BS.fromString input
  let nonempty_addresses = filter (/= empty_string) addresses

  rs <- makeResolvSeed resolv_conf
  withResolver rs $ \resolver -> do
    good_addresses <- filterM (validate resolver) nonempty_addresses
    let good_address_strings = map BS.toString good_addresses
    mapM_ (hPutStrLn stdout) good_address_strings

  hFlush stdout