{-# LANGUAGE DoAndIfThenElse #-}
{-# LANGUAGE RecordWildCards #-}

module Main (main)
where

import Control.Concurrent.ParallelIO.Global (
   parallelInterleaved,
   stopGlobalPool )
import qualified Data.ByteString.Char8 as BS (
  hGetContents,
  hPutStrLn,
  lines,
  null,
  pack )
import Network.DNS (
  Domain,
  Resolver,
  ResolvConf( resolvTimeout ),
  defaultResolvConf,
  makeResolvSeed,
  withResolver )
import Network.DNS.Lookup ( lookupA, lookupMX )
import System.IO (
  hFlush,
  stdin,
  stdout )


import CommandLine (
  Args( Args, accept_a, rfc5322 ),
  get_args )
import EmailAddress(
  Address,
  parts,
  validate_syntax )


-- | Resolver parameters. We increase the default timeout from 3 to 10
--   seconds.
resolv_conf ::  ResolvConf
resolv_conf = defaultResolvConf { resolvTimeout = 10 * 1000 * 1000 }

-- | A list of common domains, there's no need to waste MX lookups on
--   these. This is a very limited list; I don't want to be in the
--   business of monitoring a million domains for MX record updates.
common_domains :: [Domain]
common_domains = map BS.pack [ "aol.com",
                               "comcast.net",
                               "cox.net",
                               "gmail.com",
                               "gmx.de",
                               "googlemail.com",
                               "hotmail.com",
                               "icloud.com",
                               "live.com",
                               "me.com",
                               "msn.com",
                               "outlook.com",
                               "proton.me",
                               "protonmail.ch",
                               "protonmail.com",
                               "yahoo.com",
                               "verizon.net" ]


-- | Check whether the given domain has a valid MX record.
validate_mx :: Resolver -> Domain -> IO Bool
validate_mx resolver domain
  | domain `elem` common_domains = return True
  | otherwise = do
      result <- lookupMX resolver domain
      case result of
        -- A list of one or more elements?
        Right (_:_) -> return True
        _           -> return False


-- | Check whether the given domain has a valid A record.
validate_a :: Resolver -> Domain -> IO Bool
validate_a resolver domain
  | domain `elem` common_domains = return True
  | otherwise = do
      result <- lookupA resolver domain
      case result of
        Right (_:_) -> return True
        _           -> return False


-- | Validate an email address by doing some simple syntax checks and
--   (if those fail) an MX lookup. We don't count an A record as a mail
--   exchanger.
validate :: Resolver -> Bool -> Bool -> Address -> IO (Address, Bool)
validate resolver accept_a rfc5322 address = do
  let valid_syntax = validate_syntax rfc5322 address
  if valid_syntax then do
    let (_,domain) = parts address
    mx_result <- validate_mx resolver domain
    if mx_result
    then return (address, True)
    else
      if accept_a
      then do
        a_result <- validate_a resolver domain
        return (address, a_result)
      else
        return (address, False)
  else
    return (address, False)



main :: IO ()
main = do
  Args{..} <- get_args

  -- Split stdin into lines, which should result in a list of addresses.
  input <- BS.hGetContents stdin
  let addresses = BS.lines input

  -- And remove the empty ones.
  let nonempty_addresses = filter (not . BS.null) addresses

  rs <- makeResolvSeed resolv_conf
  let validate' addr = withResolver rs $ \resolver ->
                         validate resolver accept_a rfc5322 addr

  -- Construct a list of [IO (Address, Bool)]. The withResolver calls
  -- are the ones that should be run in parallel.
  let actions = map validate' nonempty_addresses

  -- Run the lookup actions in parallel.
  results <- parallelInterleaved actions

  -- Filter the bad ones.
  let valid_results = filter snd results

  -- Output the results.
  let valid_addresses = map fst valid_results
  mapM_ (BS.hPutStrLn stdout) valid_addresses

  stopGlobalPool
  hFlush stdout