if Code.ensure_loaded?(Req) do
  defmodule DSpace.API.HTTP.Req do
    @moduledoc """
    Default HTTP adapter implementation using Req.

    ## Observability

    Passing a `:plugins` list as part of the adapter options lets the consuming application attach
    custom Req steps for telemetry, logging, etc. that participate in the full request/response
    pipeline:

        %DSpace.API{
          endpoint: "https://example.com/server",
          http_impl: {DSpace.API.HTTP.Req, [plugins: [&MyApp.ReqTelemetry.attach/1]]}
        }
    """

    @behaviour DSpace.API.HTTP

    alias DSpace.API.HTTP

    # Callbacks

    @spec request(keyword()) :: {:ok, HTTP.Response.t()} | {:error, HTTP.Error.t()}
    @impl HTTP
    def request(options) when is_list(options) do
      {plugins, other_options} = Keyword.pop(options, :plugins, [])

      other_options
      |> Req.new()
      |> then(&Enum.reduce(plugins, &1, fn plugin, req -> plugin.(req) end))
      |> Req.request()
      |> transform_response()
    end

    # Private helpers

    defp transform_response({:ok, response}) do
      {:ok, %HTTP.Response{status: response.status, headers: response.headers, body: response.body}}
    end

    defp transform_response({:error, reason}), do: {:error, HTTP.Error.exception(reason: reason)}
  end
end