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

module Main
where

import Control.Concurrent.ParallelIO.Global (parallel, stopGlobalPool)
import Control.Monad (unless)
import qualified Data.ByteString as BS
import qualified Data.ByteString.UTF8 as BSU
import Network.DNS (
  Domain,
  Resolver,
  ResolvConf(..),
  defaultResolvConf,
  makeResolvSeed,
  withResolver)
import Network.DNS.Lookup (lookupMX)
import System.Directory (doesFileExist)
import System.Exit (exitWith, ExitCode(..))
import System.IO (
  Handle,
  IOMode( WriteMode ),
  hClose,
  hFlush,
  openFile,
  stdin,
  stdout)


import CommandLine (Args(..), apply_args)
import EmailAddress
import ExitCodes (exit_input_file_doesnt_exist)


-- | Resolver parameters. We increase the default timeout from 3 to 5
--   seconds.
resolv_conf ::  ResolvConf
resolv_conf = defaultResolvConf { resolvTimeout = 5 * 1000 * 1000 }

-- | A list of common domains, there's no need to waste MX lookups
--   on these.
common_domains :: [Domain]
common_domains = map BSU.fromString [ "aol.com",
                                      "comcast.net",
                                      "gmail.com",
                                      "msn.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
        Nothing -> return False
        _       -> return True



-- | 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 -> Address -> IO (Address, Bool)
validate resolver address = do
  let valid_syntax = validate_syntax address
  if valid_syntax then do
    let (_,domain) = parts address
    mx_result <- validate_mx resolver domain
    return (address, mx_result)
  else
    return (address, False)


-- | Append a ByteString to a file Handle, followed by a newline.
append_handle_with_newline :: Handle -> BS.ByteString -> IO ()
append_handle_with_newline h bs = do
  BS.hPutStr h bs
  BS.hPutStr h newline
  where
    newline = BSU.fromString "\n"


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

  -- Get the input from either stdin, or the file given on the command
  -- line.
  input <- case input_file of
             Nothing   -> BS.hGetContents stdin
             Just path -> do
               is_file <- doesFileExist path
               unless is_file $
                 exitWith (ExitFailure exit_input_file_doesnt_exist)
               BS.readFile path

  -- Do the same for the output handle and stdout.
  output_handle <- case output_file of
                     Nothing -> return stdout
                     Just path -> openFile path WriteMode

  -- Split the input into lines.
  let addresses = BSU.lines input

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

  rs <- makeResolvSeed resolv_conf
  withResolver rs $ \resolver -> do
    -- Construst a list of [IO (Address, Bool)]
    let actions = map (validate resolver) nonempty_addresses
    -- And compute them in parallel.
    results <- parallel actions
    stopGlobalPool
    -- Find the pairs with a True in the second position.
    let good_pairs = filter snd results
    -- And output the results.
    mapM_ ((append_handle_with_newline output_handle) . fst) good_pairs

  -- Clean up. It's safe to try to close stdout.
  hFlush output_handle
  hClose output_handle