defmodule ReqServerSentEvents do
  @moduledoc """
  Req plugin for Server-Sent Events (SSE).

  Attach to any `%Req.Request{}` via `attach/2`. The plugin intercepts
  Req's three streaming hooks and transparently decodes raw SSE byte chunks
  into `%ReqServerSentEvents.Frame{}` structs.

  ## Usage

      # into: fun — frames delivered as {:sse_event, %Frame{}} arguments
      Req.get!(url, into: fn {:sse_event, frame}, {req, resp} ->
        IO.inspect(frame)
        {:cont, {req, resp}}
      end)
      |> ReqServerSentEvents.attach()

      # into: :self — frames sent as messages to the calling process
      task = Task.async(fn -> Req.get!(url, into: :self) |> ReqServerSentEvents.attach() end)
      resp = Task.await(task)
      sse_ref = ReqServerSentEvents.ref(resp)
      receive do
        {^sse_ref, {:sse_event, frame}} -> IO.inspect(frame)
        {^sse_ref, :sse_done}           -> :done
      end

      # into: collectable — frames collected into any Collectable
      {:ok, resp} = Req.get(url, into: []) |> ReqServerSentEvents.attach()
      frames = resp.body  # [%ReqServerSentEvents.Frame{}, ...]
  """

  @doc """
  Attach the SSE decoder to a `%Req.Request{}`.

  Registers two Req steps:
  - A request step (`sse_rewrite`) that rewrites the `into:` field to
    decode SSE frames before delivering them to the user's handler.
  - A response step (`sse_done`) that sends a `{ref, :sse_done}` sentinel
    when `into: :self` is used (the `into: fun` callback has no `:done` signal).
  """
  @spec attach(Req.Request.t(), keyword()) :: Req.Request.t()
  def attach(%Req.Request{} = req, _opts \\ []) do
    req
    |> sse_rewrite()
    |> Req.Request.append_response_steps(sse_done: &send_sse_done/1)
  end

  @doc """
  Return the SSE ref for a `into: :self` request.

  Accepts either the final `%Req.Request{}` or `%Req.Response{}` — Req's
  high-level functions (`Req.get!/2` etc.) return only the response, while
  `Req.request/2` returns `{request, response}`.
  """
  @spec ref(Req.Request.t() | Req.Response.t()) :: reference() | nil
  def ref(%Req.Request{} = req), do: req.private[:sse_ref]
  def ref(%Req.Response{} = resp), do: resp.private[:sse_ref]

  # ---------------------------------------------------------------------------
  # Request step — rewrite req.into before the HTTP adapter runs
  # ---------------------------------------------------------------------------

  defp sse_rewrite(%Req.Request{into: nil} = req), do: req
  defp sse_rewrite(%Req.Request{into: :self} = req), do: wrap_self(req)
  defp sse_rewrite(%Req.Request{into: f} = req) when is_function(f, 2), do: wrap_fun(req, f)
  defp sse_rewrite(%Req.Request{into: c} = req), do: wrap_collectable(req, c)

  # ---------------------------------------------------------------------------
  # Response step — send :sse_done sentinel for the :self path
  # ---------------------------------------------------------------------------

  defp send_sse_done({req, resp}) do
    with caller when not is_nil(caller) <- req.private[:sse_caller],
         sse_ref when not is_nil(sse_ref) <- req.private[:sse_ref] do
      send(caller, {sse_ref, :sse_done})
    end

    {req, resp}
  end

  # ---------------------------------------------------------------------------
  # into: fun — buffer lives in resp.private[:sse_buf]
  # ---------------------------------------------------------------------------

  defp wrap_fun(%Req.Request{} = req, user_fun) do
    wrapped = fn {:data, chunk}, {req, resp} ->
      buf = (resp.private[:sse_buf] || "") <> chunk
      {frames, leftover} = ReqServerSentEvents.Frame.split(buf)
      resp = put_in(resp.private[:sse_buf], leftover)

      Enum.reduce_while(frames, {:cont, {req, resp}}, fn raw, {:cont, {req, resp}} ->
        frame = ReqServerSentEvents.Frame.parse(raw)

        case user_fun.({:sse_event, frame}, {req, resp}) do
          {:cont, acc} -> {:cont, {:cont, acc}}
          {:halt, acc} -> {:halt, {:halt, acc}}
        end
      end)
    end

    %{req | into: wrapped}
  end

  # ---------------------------------------------------------------------------
  # into: :self — rewrite to into: fun that sends messages
  # ---------------------------------------------------------------------------

  defp wrap_self(%Req.Request{} = req) do
    caller = self()
    sse_ref = make_ref()

    wrapped = fn {:data, chunk}, {req, resp} ->
      buf = (resp.private[:sse_buf] || "") <> chunk
      {frames, leftover} = ReqServerSentEvents.Frame.split(buf)

      resp =
        resp
        |> put_in([Access.key(:private), :sse_buf], leftover)
        |> put_in([Access.key(:private), :sse_ref], sse_ref)

      Enum.each(frames, fn raw ->
        send(caller, {sse_ref, {:sse_event, ReqServerSentEvents.Frame.parse(raw)}})
      end)

      {:cont, {req, resp}}
    end

    req
    |> Req.Request.put_private(:sse_ref, sse_ref)
    |> Req.Request.put_private(:sse_caller, caller)
    |> Map.replace!(:into, wrapped)
  end

  # ---------------------------------------------------------------------------
  # into: collectable — wrap in CollectableWrapper
  # ---------------------------------------------------------------------------

  defp wrap_collectable(%Req.Request{} = req, collectable) do
    %{req | into: %ReqServerSentEvents.CollectableWrapper{inner: collectable}}
  end
end