defmodule Mix.Tasks.Clje.Nrepl do
  @moduledoc """
  Starts an nREPL server for CljElixir.

  ## Usage

      mix clje.nrepl              # random port
      mix clje.nrepl --port 7888  # specific port

  Writes port to `.nrepl-port` file for editor auto-discovery.
  Cleans up `.nrepl-port` on shutdown.

  If `.nrepl-port` exists and a server is already listening on that port,
  prints the existing port and exits rather than starting a duplicate.
  """

  use Mix.Task

  @shortdoc "Start a CljElixir nREPL server"
  @nrepl_port_file ".nrepl-port"

  @impl Mix.Task
  def run(args) do
    {opts, _, _} = OptionParser.parse(args, strict: [port: :integer])
    requested_port = Keyword.get(opts, :port)

    # Check for existing server via .nrepl-port
    if requested_port == nil and File.exists?(@nrepl_port_file) do
      case check_existing_server() do
        {:ok, existing_port} ->
          IO.puts("nREPL server already running on port #{existing_port}")
          IO.puts("Connect with: nrepl://127.0.0.1:#{existing_port}")
          System.halt(0)

        :stale ->
          File.rm(@nrepl_port_file)
      end
    end

    port = requested_port || 0

    Mix.Task.run("compile")
    Mix.Task.run("app.start")

    {:ok, server} = CljElixir.NRepl.Server.start_link(port: port)
    actual_port = CljElixir.NRepl.Server.port(server)

    File.write!(@nrepl_port_file, Integer.to_string(actual_port))

    # Clean up .nrepl-port on shutdown
    cleanup_on_exit()

    IO.puts("nREPL server started on port #{actual_port} on host 127.0.0.1")
    IO.puts("Port written to #{@nrepl_port_file}")
    IO.puts("Press Ctrl+C to stop\n")

    Process.sleep(:infinity)
  end

  defp check_existing_server do
    with {:ok, content} <- File.read(@nrepl_port_file),
         {port, ""} <- Integer.parse(String.trim(content)),
         true <- port_alive?(port) do
      {:ok, port}
    else
      _ -> :stale
    end
  end

  defp port_alive?(port) do
    case :gen_tcp.connect(~c"127.0.0.1", port, [:binary], 500) do
      {:ok, socket} ->
        :gen_tcp.close(socket)
        true

      {:error, _} ->
        false
    end
  end

  defp cleanup_on_exit do
    # Trap exits so we can clean up the port file
    System.at_exit(fn _status ->
      File.rm(@nrepl_port_file)
    end)
  end
end