extools

defmodule Extools.Ecto do
  @moduledoc false

  @doc """
  Auto-detect the Ecto Repo module used by the host application.

  Strategy (in order):
    1. Explicit config: `config :extools, repo: MyApp.Repo`
    2. `config :extools, otp_app: :my_app` then scan that app's modules
    3. Scan `Application.loaded_applications()` for a module whose
       behaviour list includes `Ecto.Repo`

  Returns `{:ok, module}` or `{:error, reason}`.
  """
  @spec detect_repo() :: {:ok, module} | {:error, term()}
  def detect_repo do
    with {:error, _} <- from_config(:repo),
         {:error, _} <- from_otp_app_config(),
         {:error, _} <- scan_all_apps() do
      {:error,
       "No Ecto.Repo module found. Set `config :extools, repo: MyApp.Repo` or `config :extools, otp_app: :my_app`."}
    end
  end

  defp from_config(:repo) do
    case Application.get_env(:extools, :repo) do
      nil -> {:error, :not_configured}
      repo when is_atom(repo) -> {:ok, repo}
      repo when is_binary(repo) -> {:ok, String.to_atom("Elixir." <> repo)}
    end
  end

  defp from_otp_app_config do
    case Application.get_env(:extools, :otp_app) do
      nil -> {:error, :not_configured}
      app -> scan_app(app)
    end
  end

  defp scan_all_apps do
    loaded =
      Application.loaded_applications()
      |> Enum.map(fn {app, _, _} -> app end)

    Enum.find_value(loaded, {:error, :not_found}, fn app ->
      case scan_app(app) do
        {:ok, repo} -> {:ok, repo}
        {:error, _} -> nil
      end
    end)
  end

  defp scan_app(app) do
    modules = Application.spec(app, :modules) || []

    Enum.find_value(modules, {:error, :not_found}, fn mod ->
      if repo_behaviour?(mod), do: {:ok, mod}, else: nil
    end)
  end

  defp repo_behaviour?(mod) do
    Code.ensure_loaded?(mod) and function_exported?(mod, :__adapter__, 0) and
      is_ecto_repo?(mod)
  rescue
    _ -> false
  catch
    _, _ -> false
  end

  defp is_ecto_repo?(mod) do
    behaviours =
      mod.module_info(:attributes)
      |> Keyword.get_values(:behaviour)
      |> List.flatten()

    Ecto.Repo in behaviours
  end
end