defmodule Goth do
  @external_resource "README.md"

  @moduledoc "README.md"
             |> File.read!()
             |> String.split("<!-- MDOC !-->")
             |> Enum.fetch!(1)

  @doc """
  Fetches the token.

  If the token is not in the cache, we immediately request it.

  To fetch the token bypassing the cache, see `Goth.Token.fetch/1`.
  """
  @doc since: "1.3.0"
  defdelegate fetch(server), to: Goth.Server

  @retry_after 1000
  @refresh_before_minutes 5

  @doc """
  Starts the server.

  When the server is started, we attempt to fetch the token and store it in
  internal cache. If we fail, we'll try up to 3 times with #{@retry_after}ms
  cooldown between requests and if we couldn't retrieve it, we crash.

  ## Options

    * `:name` - the name to register the server under.

    * `:source` - the source to retrieve the token from.

      See documentation for the `:source` option in `Goth.Token.fetch/1` for
      more information.

    * `:retry_after` - Time in milliseconds between retrying requests, defaults
      to `#{@retry_after}`.

    * `:refresh_before` - Time in seconds before the token is about to expire
      that it is tried to be automatically refreshed. Defaults to
      `#{@refresh_before_minutes * 60}` (#{@refresh_before_minutes} minutes).

    * `:http_client` - a `{module, opts}` tuple, where `module` implements the
      `Goth.HTTPClient` behaviour and `opts` is a keywords list to initialize the client with.
      Defaults to `{Goth.HTTPClient.Hackney, []}`.

  ## Examples

  Generate a token using a service account credentials file:

      iex> credentials = "credentials.json" |> File.read!() |> Jason.decode!()
      iex> Goth.start_link(name: MyApp.Goth, source: {:service_account, credentials, []})
      iex> Goth.fetch(MyApp.Goth)
      {:ok, %Goth.Token{...}}

  Retrieve the token using a refresh token:

      iex> credentials = "credentials.json" |> File.read!() |> Jason.decode!()
      iex> Goth.start_link(name: MyApp.Goth, source: {:refresh_token, credentials, []})
      iex> Goth.fetch(MyApp.Goth)
      {:ok, %Goth.Token{...}}

  Retrieve the token using the Google metadata server:

      iex> Goth.start_link(name: MyApp.Goth, source: {:metadata, []})
      iex> Goth.fetch(MyApp.Goth)
      {:ok, %Goth.Token{...}}

  """
  @doc since: "1.3.0"
  def start_link(opts) do
    opts |> with_default_opts() |> Goth.Server.start_link()
  end

  @doc """
  Returns a supervision child spec.

  Accepts the same options as `start_link/1`.
  """
  @doc since: "1.3.0"
  def child_spec(opts) do
    opts |> with_default_opts() |> Goth.Server.child_spec()
  end

  defp with_default_opts(opts) do
    opts
    |> Keyword.put_new(:retry_after, @retry_after)
    |> Keyword.put_new(:refresh_before, @refresh_before_minutes * 60)
    |> Keyword.put_new(:http_client, {Goth.HTTPClient.Hackney, []})
  end
end