defmodule Google.Pubsub.GRPC do
  @moduledoc """
  Google.Pubsub.GRPC is a thin wrapper around gRPC-based Google Pubsub Service.
  It uses [grpc package](https://hex.pm/packages/grpc) for grpc communication, and 
  doesn't attempt to add much more to the code generated by `protoc` (mostly adding
  support for the pubsub emulator, token authorization, as well as some examples).

  ## Installation

  Add the following to your `mix.exs`:

  ```elixir
  def deps do
    [
      {:goth, "~> 1.2.0"},
      {:cowlib, "~> 2.9.0", override: true},
      {:google_protos, "~> 0.1.0"},
      {:grpc, github: "elixir-grpc/grpc", override: true},
      {:google_pubsub_grpc, "~> 0.1.0"}
    ]
  end
  ```
  """

  require Logger

  @emulator_host_env "PUBSUB_EMULATOR_HOST"
  @googleapis_endpoint "pubsub.googleapis.com:443"
  @oath_scope "https://www.googleapis.com/auth/pubsub"

  @doc """
  Obtains a connection (channel) with gRPC server.

  Authorization tokens are by default generated by Goth, but can be overriden,
  if so desired.

  ## Examples
    Google.Pubsub.GRPC.channel()
    Google.Pubsub.GRPC.channel(interceptors: [GRPC.Logger.Client])
    Google.Pubsub.GRPC.channel(interceptors: [GRPC.Logger.Client, level: :info])
    Google.Pubsub.GRPC.channel(accepted_compressors: [GRPC.Compressor.Gzip])
    Google.Pubsub.GRPC.channel(headers: ["authorization": "my custom token"])
  """

  @spec channel(opts :: Keyword.t()) ::
          {:ok, %GRPC.Channel{}} | {:error, %GRPC.RPCError{}}
  def channel(opts \\ []) do
    opts = Keyword.merge(connect_defaults(), opts)

    case GRPC.Stub.connect(endpoint(), opts) do
      {:ok, channel} ->
        {:ok, channel}

      err ->
        Logger.error("Failed to connect to endpoint #{endpoint()}")
        err
    end
  end

  def full_project_id do
    "projects/#{project_id()}"
  end

  def full_topic_name(name) do
    "projects/#{project_id()}/topics/#{name}"
  end

  def full_subscription_name(name) do
    "projects/#{project_id()}/subscriptions/#{name}"
  end

  def stripped_name(full_name) do
    String.replace(full_name, ~r(^.*/), "")
  end

  defp using_pubsub_emulator? do
    !is_nil(System.get_env(@emulator_host_env))
  end

  defp endpoint do
    case using_pubsub_emulator?() do
      true -> System.get_env(@emulator_host_env)
      false -> @googleapis_endpoint
    end
  end

  defp connect_defaults do
    case using_pubsub_emulator?() do
      true ->
        [headers: [authorization: "Dummy token"]]

      false ->
        {:ok, token} = Goth.Token.for_scope(@oath_scope)
        [headers: [authorization: "#{token.type} #{token.token}"], cred: %{ssl: []}]
    end
  end

  defp project_id do
    # Goth will raise on start-up if can't determine the project, so this should always work
    Goth.Config.get(:project_id) |> elem(1)
  end
end