defmodule CljElixir.NRepl.SessionManager do
  @moduledoc "Manages nREPL sessions with independent REPL state."

  use GenServer

  def start_link(opts \\ []) do
    GenServer.start_link(__MODULE__, %{}, opts)
  end

  @impl true
  def init(_) do
    {:ok, %{sessions: %{}}}
  end

  def create_session(manager) do
    GenServer.call(manager, :create_session)
  end

  def close_session(manager, session_id) do
    GenServer.call(manager, {:close_session, session_id})
  end

  def list_sessions(manager) do
    GenServer.call(manager, :list_sessions)
  end

  def eval(manager, session_id, code) do
    GenServer.call(manager, {:eval, session_id, code}, :infinity)
  end

  @doc "Eval with stdout capture. Returns {output, result} where result is {:ok, value} | {:error, error}."
  def eval_with_capture(manager, session_id, code) do
    GenServer.call(manager, {:eval_with_capture, session_id, code}, :infinity)
  end

  # --- Callbacks ---

  @impl true
  def handle_call(:create_session, _from, state) do
    id = generate_uuid()
    {:ok, pid} = Agent.start_link(fn -> CljElixir.REPL.new() end)
    new_sessions = Map.put(state.sessions, id, pid)
    {:reply, id, %{state | sessions: new_sessions}}
  end

  @impl true
  def handle_call({:close_session, id}, _from, state) do
    case Map.pop(state.sessions, id) do
      {nil, _} ->
        {:reply, :not_found, state}

      {pid, new_sessions} ->
        Agent.stop(pid)
        {:reply, :ok, %{state | sessions: new_sessions}}
    end
  end

  @impl true
  def handle_call(:list_sessions, _from, state) do
    {:reply, Map.keys(state.sessions), state}
  end

  @impl true
  def handle_call({:eval, id, code}, _from, state) do
    case Map.get(state.sessions, id) do
      nil ->
        {:reply, {:error, "unknown session"}, state}

      pid ->
        result =
          Agent.get_and_update(
            pid,
            fn repl_state ->
              case CljElixir.REPL.eval(code, repl_state) do
                {:ok, value, new_state} -> {{:ok, value}, new_state}
                {:error, error, new_state} -> {{:error, error}, new_state}
              end
            end,
            :infinity
          )

        {:reply, result, state}
    end
  end

  @impl true
  def handle_call({:eval_with_capture, id, code}, _from, state) do
    case Map.get(state.sessions, id) do
      nil ->
        {:reply, {"", {:error, "unknown session"}}, state}

      pid ->
        {output, result} =
          Agent.get_and_update(
            pid,
            fn repl_state ->
              {output, eval_result, new_state} = eval_capturing_output(code, repl_state)

              {{output, eval_result}, new_state}
            end,
            :infinity
          )

        {:reply, {output, result}, state}
    end
  end

  # Evaluate code while capturing stdout within the current process
  defp eval_capturing_output(code, repl_state) do
    {:ok, string_io} = StringIO.open("")
    old_gl = Process.group_leader()
    Process.group_leader(self(), string_io)

    try do
      case CljElixir.REPL.eval(code, repl_state) do
        {:ok, value, new_state} ->
          Process.group_leader(self(), old_gl)
          {_input, output} = StringIO.contents(string_io)
          StringIO.close(string_io)
          {output, {:ok, value}, new_state}

        {:error, error, new_state} ->
          Process.group_leader(self(), old_gl)
          {_input, output} = StringIO.contents(string_io)
          StringIO.close(string_io)
          {output, {:error, error}, new_state}
      end
    rescue
      e ->
        Process.group_leader(self(), old_gl)
        StringIO.close(string_io)
        {"", {:error, Exception.message(e)}, repl_state}
    end
  end

  # Simple UUID v4 generation using :crypto (OTP built-in, no deps)
  defp generate_uuid do
    <<a::32, b::16, c::16, d::16, e::48>> = :crypto.strong_rand_bytes(16)

    :io_lib.format("~8.16.0b-~4.16.0b-4~3.16.0b-~4.16.0b-~12.16.0b", [
      a,
      b,
      Bitwise.band(c, 0x0FFF),
      Bitwise.bor(Bitwise.band(d, 0x3FFF), 0x8000),
      e
    ])
    |> IO.iodata_to_binary()
  end
end