email_checker

defmodule EmailChecker.Tools do
  @moduledoc false

  @email_regex ~r/^(?<user>[^\s]+)@(?<domain>[^\s]+\.[^\s]+)$/

  @spec domain_name(String.t) :: Sting.t | nil
  def domain_name(email) do
    case Regex.named_captures(email_regex(), email) do
      %{"domain" => domain} ->
        domain
      _ ->
        nil
    end
  end

  def email_regex do
    @email_regex
  end

  @spec lookup(String.t | nil) :: String.t | nil
  def lookup(nil), do: nil
  def lookup(domain_name) do
    domain_name
    |> lookup_all
    |> take_lowest_mx_record
  end

  defp lookup_all(domain_name) do
    domain_name
    |> String.to_char_list
    |> :inet_res.lookup(:in, :mx, [], max_timeout())
    |> normalize_mx_records_to_string
  end

  defp normalize_mx_records_to_string(nil) do
    []
  end
  defp normalize_mx_records_to_string(domains) do
    normalize_mx_records_to_string(domains, [])
  end
  defp normalize_mx_records_to_string([], normalized_domains) do
    normalized_domains
  end
  defp normalize_mx_records_to_string([{priority, domain} | domains], normalized_domains) do
    normalize_mx_records_to_string(domains, [{priority, to_string(domain)} | normalized_domains])
  end

  defp sort_mx_records_by_priority(nil), do: []
  defp sort_mx_records_by_priority(domains) do
    Enum.sort(domains, fn({priority, _domain}, {other_priority, _other_domain}) ->
      priority < other_priority
    end)
  end

  defp take_lowest_mx_record(mx_records) do
    case mx_records |> sort_mx_records_by_priority do
      [{_lower_priority, domain}|_rest] ->
        domain
      _ ->
        nil
    end
  end

  defp max_timeout, do: Application.get_env(:email_checker, :timeout_milliseconds, :infinity)
end