defmodule Codex.IO.Transport.Erlexec do
  @moduledoc "Erlexec-backed subprocess transport implementation."

  use GenServer

  import Kernel, except: [send: 2]

  alias Codex.TaskSupport

  @behaviour Codex.IO.Transport

  @default_max_buffer_size 1_048_576
  @default_max_stderr_buffer_size 262_144
  @default_call_timeout 5_000
  @force_close_timeout 500
  @default_headless_timeout_ms 5_000
  @finalize_delay_ms 25
  @max_lines_per_batch 200

  defstruct [
    :subprocess,
    :status,
    :task_supervisor,
    subscribers: %{},
    stdout_buffer: "",
    pending_lines: :queue.new(),
    drain_scheduled?: false,
    overflowed?: false,
    stderr_buffer: "",
    max_buffer_size: @default_max_buffer_size,
    max_stderr_buffer_size: @default_max_stderr_buffer_size,
    pending_calls: %{},
    finalize_timer_ref: nil,
    headless_timeout_ms: @default_headless_timeout_ms,
    headless_timer_ref: nil
  ]

  @type subscriber_info :: %{
          monitor_ref: reference(),
          tag: Codex.IO.Transport.subscription_tag()
        }

  @impl Codex.IO.Transport
  def start(opts) when is_list(opts) do
    case GenServer.start(__MODULE__, opts) do
      {:ok, pid} -> {:ok, pid}
      {:error, reason} -> transport_error(reason)
    end
  catch
    :exit, reason ->
      transport_error(reason)
  end

  @impl Codex.IO.Transport
  def start_link(opts) when is_list(opts) do
    case GenServer.start_link(__MODULE__, opts) do
      {:ok, pid} -> {:ok, pid}
      {:error, reason} -> transport_error(reason)
    end
  catch
    :exit, reason ->
      transport_error(reason)
  end

  @impl Codex.IO.Transport
  def send(transport, message) when is_pid(transport) do
    case safe_call(transport, {:send, message}) do
      {:ok, result} -> result
      {:error, reason} -> transport_error(reason)
    end
  end

  @impl Codex.IO.Transport
  def subscribe(transport, pid) when is_pid(transport) and is_pid(pid) do
    subscribe(transport, pid, :legacy)
  end

  @impl Codex.IO.Transport
  def subscribe(transport, pid, tag)
      when is_pid(transport) and is_pid(pid) and (tag == :legacy or is_reference(tag)) do
    case safe_call(transport, {:subscribe, pid, tag}) do
      {:ok, result} -> result
      {:error, reason} -> transport_error(reason)
    end
  end

  @impl Codex.IO.Transport
  def close(transport) when is_pid(transport) do
    GenServer.stop(transport, :normal)
  catch
    :exit, {:noproc, _} -> :ok
    :exit, :noproc -> :ok
  end

  @impl Codex.IO.Transport
  def force_close(transport) when is_pid(transport) do
    case safe_call(transport, :force_close, @force_close_timeout) do
      {:ok, :ok} ->
        :ok

      {:error, :not_connected} ->
        :ok

      {:error, reason} ->
        transport_error(reason)
    end
  end

  @impl Codex.IO.Transport
  def status(transport) when is_pid(transport) do
    case safe_call(transport, :status) do
      {:ok, status} when status in [:connected, :disconnected, :error] -> status
      {:ok, _other} -> :error
      {:error, _reason} -> :disconnected
    end
  end

  @impl Codex.IO.Transport
  def end_input(transport) when is_pid(transport) do
    case safe_call(transport, :end_input) do
      {:ok, result} -> result
      {:error, reason} -> transport_error(reason)
    end
  end

  @impl Codex.IO.Transport
  def stderr(transport) when is_pid(transport) do
    case safe_call(transport, :stderr) do
      {:ok, data} when is_binary(data) -> data
      _ -> ""
    end
  end

  @impl GenServer
  def init(opts) do
    command = Keyword.fetch!(opts, :command)
    args = Keyword.get(opts, :args, [])
    cwd = Keyword.get(opts, :cwd)
    env = Keyword.get(opts, :env, [])
    task_supervisor = Keyword.get(opts, :task_supervisor, Codex.TaskSupervisor)
    subscriber = Keyword.get(opts, :subscriber)
    headless_timeout_ms = Keyword.get(opts, :headless_timeout_ms, @default_headless_timeout_ms)
    max_buffer_size = Keyword.get(opts, :max_buffer_size, @default_max_buffer_size)

    max_stderr_buffer_size =
      Keyword.get(opts, :max_stderr_buffer_size, @default_max_stderr_buffer_size)

    case start_subprocess(command, args,
           cwd: cwd,
           env: env,
           task_supervisor: task_supervisor,
           subscriber: subscriber,
           headless_timeout_ms: headless_timeout_ms,
           max_buffer_size: max_buffer_size,
           max_stderr_buffer_size: max_stderr_buffer_size
         ) do
      {:ok, state} -> {:ok, state}
      {:error, reason} -> {:stop, reason}
    end
  end

  @impl GenServer
  def handle_call({:subscribe, pid, tag}, _from, state) do
    state =
      state
      |> put_subscriber(pid, tag)
      |> cancel_headless_timer()

    {:reply, :ok, state}
  end

  def handle_call({:send, message}, from, %{subprocess: {pid, _os_pid}} = state) do
    case start_io_task(state, fn -> send_payload(pid, message) end) do
      {:ok, task} ->
        pending_calls = Map.put(state.pending_calls, task.ref, from)
        {:noreply, %{state | pending_calls: pending_calls}}

      {:error, reason} ->
        {:reply, transport_error(reason), state}
    end
  end

  def handle_call({:send, _message}, _from, %{subprocess: nil} = state) do
    {:reply, transport_error(:not_connected), state}
  end

  def handle_call(:status, _from, state), do: {:reply, state.status, state}

  def handle_call(:end_input, from, %{subprocess: {pid, _os_pid}} = state) do
    case start_io_task(state, fn -> send_eof(pid) end) do
      {:ok, task} ->
        pending_calls = Map.put(state.pending_calls, task.ref, from)
        {:noreply, %{state | pending_calls: pending_calls}}

      {:error, reason} ->
        {:reply, transport_error(reason), state}
    end
  end

  def handle_call(:end_input, _from, %{subprocess: nil} = state) do
    {:reply, transport_error(:not_connected), state}
  end

  def handle_call(:stderr, _from, state), do: {:reply, state.stderr_buffer, state}

  def handle_call(:force_close, _from, state) do
    state = force_stop_subprocess(state)
    {:stop, :normal, :ok, state}
  end

  @impl GenServer
  def handle_info({:stdout, os_pid, chunk}, %{subprocess: {_pid, os_pid}} = state) do
    state =
      state
      |> append_stdout_data(IO.iodata_to_binary(chunk))
      |> maybe_schedule_drain()

    {:noreply, state}
  end

  def handle_info({:stderr, _os_pid, chunk}, state) do
    data = IO.iodata_to_binary(chunk)
    stderr_buffer = append_stderr_data(state.stderr_buffer, data, state.max_stderr_buffer_size)
    {:noreply, %{state | stderr_buffer: stderr_buffer}}
  end

  def handle_info({ref, result}, %{pending_calls: pending_calls} = state)
      when is_reference(ref) do
    case Map.pop(pending_calls, ref) do
      {nil, _} ->
        {:noreply, state}

      {from, rest} ->
        Process.demonitor(ref, [:flush])
        GenServer.reply(from, normalize_call_result(result))
        {:noreply, %{state | pending_calls: rest}}
    end
  end

  def handle_info({:DOWN, os_pid, :process, pid, reason}, %{subprocess: {pid, os_pid}} = state) do
    state = cancel_finalize_timer(state)

    timer_ref =
      Process.send_after(self(), {:finalize_exit, os_pid, pid, reason}, @finalize_delay_ms)

    {:noreply, %{state | finalize_timer_ref: timer_ref}}
  end

  def handle_info({:finalize_exit, os_pid, pid, reason}, %{subprocess: {pid, os_pid}} = state) do
    state =
      state
      |> Map.put(:finalize_timer_ref, nil)
      |> Map.put(:drain_scheduled?, false)
      |> drain_stdout_lines(@max_lines_per_batch)

    if :queue.is_empty(state.pending_lines) do
      state = flush_stdout_fragment(state)

      if state.stderr_buffer != "" do
        send_event(state.subscribers, {:stderr, state.stderr_buffer})
      end

      send_event(state.subscribers, {:exit, reason})
      {:stop, :normal, %{state | status: :disconnected, subprocess: nil}}
    else
      Kernel.send(self(), {:finalize_exit, os_pid, pid, reason})
      {:noreply, state}
    end
  end

  def handle_info({:DOWN, ref, :process, pid, reason}, %{pending_calls: pending_calls} = state)
      when is_reference(ref) do
    case Map.pop(pending_calls, ref) do
      {from, rest} when not is_nil(from) ->
        GenServer.reply(from, transport_error({:send_failed, reason}))
        {:noreply, %{state | pending_calls: rest}}

      {nil, _} ->
        handle_subscriber_down(ref, pid, state)
    end
  end

  def handle_info(:drain_stdout, state) do
    state =
      state
      |> Map.put(:drain_scheduled?, false)
      |> drain_stdout_lines(@max_lines_per_batch)
      |> maybe_schedule_drain()

    {:noreply, state}
  end

  def handle_info(:headless_timeout, state) do
    state = %{state | headless_timer_ref: nil}

    if map_size(state.subscribers) == 0 and not is_nil(state.subprocess) do
      {:stop, :normal, state}
    else
      {:noreply, state}
    end
  end

  def handle_info(_other, state), do: {:noreply, state}

  @impl GenServer
  def terminate(_reason, state) do
    state =
      state
      |> cancel_finalize_timer()
      |> cancel_headless_timer()

    demonitor_subscribers(state.subscribers)
    cleanup_pending_calls(state.pending_calls)
    _ = force_stop_subprocess(state)
    :ok
  catch
    _, _ -> :ok
  end

  defp safe_call(transport, message, timeout \\ @default_call_timeout)

  defp safe_call(transport, message, timeout)
       when is_pid(transport) and is_integer(timeout) and timeout >= 0 do
    case TaskSupport.async_nolink(Codex.TaskSupervisor, fn ->
           try do
             {:ok, GenServer.call(transport, message, :infinity)}
           catch
             :exit, reason -> {:error, normalize_call_exit(reason)}
           end
         end) do
      {:ok, task} ->
        await_task_result(task, timeout)

      {:error, reason} ->
        {:error, normalize_call_task_start_error(reason)}
    end
  end

  defp await_task_result(task, timeout) do
    case Task.yield(task, timeout) || Task.shutdown(task, :brutal_kill) do
      {:ok, result} ->
        result

      {:exit, reason} ->
        {:error, normalize_call_exit(reason)}

      nil ->
        {:error, :timeout}
    end
  end

  defp normalize_call_task_start_error(:noproc), do: :transport_stopped
  defp normalize_call_task_start_error(reason), do: {:call_exit, reason}

  defp normalize_call_exit({:noproc, _}), do: :not_connected
  defp normalize_call_exit(:noproc), do: :not_connected
  defp normalize_call_exit({:normal, _}), do: :not_connected
  defp normalize_call_exit({:shutdown, _}), do: :not_connected
  defp normalize_call_exit({:timeout, _}), do: :timeout
  defp normalize_call_exit(reason), do: {:call_exit, reason}

  defp normalize_call_result(:ok), do: :ok
  defp normalize_call_result({:error, {:transport, _reason}} = error), do: error
  defp normalize_call_result({:error, reason}), do: transport_error(reason)
  defp normalize_call_result(other), do: transport_error({:unexpected_task_result, other})

  defp start_subprocess(command, args, opts) do
    cwd = Keyword.get(opts, :cwd)
    env = Keyword.get(opts, :env, [])
    subscriber = Keyword.get(opts, :subscriber)
    task_supervisor = Keyword.get(opts, :task_supervisor, Codex.TaskSupervisor)
    headless_timeout_ms = Keyword.get(opts, :headless_timeout_ms, @default_headless_timeout_ms)
    max_buffer_size = Keyword.get(opts, :max_buffer_size, @default_max_buffer_size)

    max_stderr_buffer_size =
      Keyword.get(opts, :max_stderr_buffer_size, @default_max_stderr_buffer_size)

    state = %__MODULE__{
      status: :disconnected,
      task_supervisor: task_supervisor,
      max_buffer_size: max_buffer_size,
      max_stderr_buffer_size: max_stderr_buffer_size,
      headless_timeout_ms: headless_timeout_ms
    }

    exec_opts =
      []
      |> maybe_put_cwd(cwd)
      |> maybe_put_env(env)
      |> Kernel.++([:stdin, :stdout, :stderr, :monitor])

    argv = normalize_command_argv(command, args)

    case :exec.run(argv, exec_opts) do
      {:ok, pid, os_pid} ->
        state =
          state
          |> Map.put(:subprocess, {pid, os_pid})
          |> Map.put(:status, :connected)

        with {:ok, state} <- add_bootstrap_subscriber(state, subscriber) do
          {:ok, maybe_schedule_headless_timer(state)}
        end

      {:error, reason} ->
        {:error, reason}
    end
  end

  defp maybe_put_cwd(opts, nil), do: opts
  defp maybe_put_cwd(opts, cwd), do: [{:cd, to_charlist(cwd)} | opts]

  defp maybe_put_env(opts, env) do
    case normalize_env(env) do
      [] -> opts
      value -> [{:env, value} | opts]
    end
  end

  defp normalize_command_argv(command, args) when is_list(command) do
    cond do
      command == [] ->
        Enum.map(List.wrap(args), &to_charlist/1)

      Enum.all?(command, &is_integer/1) ->
        [to_charlist(command) | Enum.map(List.wrap(args), &to_charlist/1)]

      true ->
        Enum.map(command ++ List.wrap(args), &to_charlist/1)
    end
  end

  defp normalize_command_argv(command, args) do
    [command | List.wrap(args)] |> Enum.map(&to_charlist/1)
  end

  defp normalize_env(nil), do: []
  defp normalize_env([]), do: []

  defp normalize_env(env) when is_map(env) do
    Enum.map(env, fn {key, value} -> {to_string(key), to_string(value)} end)
  end

  defp normalize_env(env) when is_list(env) do
    Enum.flat_map(env, fn
      {key, value} -> [{to_string(key), to_string(value)}]
      _other -> []
    end)
  end

  defp normalize_env(_other), do: []

  defp add_bootstrap_subscriber(state, nil), do: {:ok, state}

  defp add_bootstrap_subscriber(state, {pid, tag})
       when is_pid(pid) and (tag == :legacy or is_reference(tag)) do
    {:ok, put_subscriber(state, pid, tag)}
  end

  defp add_bootstrap_subscriber(_state, _subscriber), do: {:error, :invalid_subscriber}

  defp put_subscriber(state, pid, tag) do
    subscribers =
      case Map.fetch(state.subscribers, pid) do
        {:ok, %{monitor_ref: monitor_ref}} ->
          Map.put(state.subscribers, pid, %{monitor_ref: monitor_ref, tag: tag})

        :error ->
          monitor_ref = Process.monitor(pid)
          Map.put(state.subscribers, pid, %{monitor_ref: monitor_ref, tag: tag})
      end

    %{state | subscribers: subscribers}
  end

  defp handle_subscriber_down(ref, pid, state) do
    subscribers =
      case Map.pop(state.subscribers, pid) do
        {%{monitor_ref: ^ref}, rest} -> rest
        {_value, rest} -> rest
      end

    state = %{state | subscribers: subscribers}

    if map_size(subscribers) == 0 do
      {:stop, :normal, state}
    else
      {:noreply, state}
    end
  end

  defp maybe_schedule_headless_timer(%{headless_timer_ref: ref} = state) when not is_nil(ref),
    do: state

  defp maybe_schedule_headless_timer(%{subscribers: subscribers} = state)
       when map_size(subscribers) > 0,
       do: state

  defp maybe_schedule_headless_timer(%{headless_timeout_ms: :infinity} = state), do: state

  defp maybe_schedule_headless_timer(%{headless_timeout_ms: timeout_ms} = state)
       when is_integer(timeout_ms) and timeout_ms > 0 do
    timer_ref = Process.send_after(self(), :headless_timeout, timeout_ms)
    %{state | headless_timer_ref: timer_ref}
  end

  defp maybe_schedule_headless_timer(state), do: state

  defp cancel_headless_timer(%{headless_timer_ref: nil} = state), do: state

  defp cancel_headless_timer(state) do
    _ = Process.cancel_timer(state.headless_timer_ref, async: false, info: false)
    flush_headless_timeout_message()
    %{state | headless_timer_ref: nil}
  end

  defp flush_headless_timeout_message do
    receive do
      :headless_timeout -> :ok
    after
      0 -> :ok
    end
  end

  defp start_io_task(state, fun) when is_function(fun, 0) do
    TaskSupport.async_nolink(state.task_supervisor, fun)
  end

  defp send_payload(pid, message) do
    payload = message |> normalize_payload() |> ensure_newline()
    :exec.send(pid, payload)
    :ok
  catch
    kind, reason ->
      transport_error({:send_failed, {kind, reason}})
  end

  defp send_eof(pid) do
    :exec.send(pid, :eof)
    :ok
  catch
    kind, reason ->
      transport_error({:send_failed, {kind, reason}})
  end

  defp send_event(subscribers, event) do
    Enum.each(subscribers, fn {pid, info} ->
      dispatch_event(pid, info, event)
    end)
  end

  defp dispatch_event(pid, %{tag: :legacy}, {:message, line}),
    do: Kernel.send(pid, {:transport_message, line})

  defp dispatch_event(pid, %{tag: :legacy}, {:error, reason}),
    do: Kernel.send(pid, {:transport_error, reason})

  defp dispatch_event(pid, %{tag: :legacy}, {:stderr, data}),
    do: Kernel.send(pid, {:transport_stderr, data})

  defp dispatch_event(pid, %{tag: :legacy}, {:exit, reason}),
    do: Kernel.send(pid, {:transport_exit, reason})

  defp dispatch_event(pid, %{tag: ref}, event) when is_reference(ref),
    do: Kernel.send(pid, {:codex_io_transport, ref, event})

  defp append_stdout_data(%{overflowed?: true} = state, data) do
    case String.split(data, "\n", parts: 2) do
      [_single] ->
        state

      [_dropped, rest] ->
        state
        |> Map.put(:overflowed?, false)
        |> Map.put(:stdout_buffer, "")
        |> append_stdout_data(rest)
    end
  end

  defp append_stdout_data(state, data) do
    full = state.stdout_buffer <> data
    {complete_lines, remaining} = split_complete_lines(full)

    pending_lines =
      Enum.reduce(complete_lines, state.pending_lines, fn line, queue ->
        :queue.in(line, queue)
      end)

    state = %{state | pending_lines: pending_lines, stdout_buffer: "", overflowed?: false}

    if byte_size(remaining) > state.max_buffer_size do
      send_event(state.subscribers, {:error, {:buffer_overflow, byte_size(remaining)}})
      %{state | stdout_buffer: "", overflowed?: true}
    else
      %{state | stdout_buffer: remaining}
    end
  end

  defp drain_stdout_lines(state, 0), do: state

  defp drain_stdout_lines(state, remaining) when is_integer(remaining) and remaining > 0 do
    case :queue.out(state.pending_lines) do
      {:empty, _queue} ->
        state

      {{:value, line}, queue} ->
        state = %{state | pending_lines: queue}

        if byte_size(line) > state.max_buffer_size do
          send_event(state.subscribers, {:error, {:buffer_overflow, byte_size(line)}})
        else
          send_event(state.subscribers, {:message, line})
        end

        drain_stdout_lines(state, remaining - 1)
    end
  end

  defp maybe_schedule_drain(%{drain_scheduled?: true} = state), do: state

  defp maybe_schedule_drain(state) do
    if :queue.is_empty(state.pending_lines) do
      state
    else
      Kernel.send(self(), :drain_stdout)
      %{state | drain_scheduled?: true}
    end
  end

  defp split_complete_lines(""), do: {[], ""}

  defp split_complete_lines(data) do
    lines = String.split(data, "\n")

    case List.pop_at(lines, -1) do
      {nil, _} -> {[], ""}
      {"", rest} -> {rest, ""}
      {last, rest} -> {rest, last}
    end
  end

  defp flush_stdout_fragment(state) do
    line = String.trim(state.stdout_buffer)

    cond do
      line == "" ->
        %{state | stdout_buffer: "", overflowed?: false, drain_scheduled?: false}

      byte_size(line) > state.max_buffer_size ->
        send_event(state.subscribers, {:error, {:buffer_overflow, byte_size(line)}})
        %{state | stdout_buffer: "", overflowed?: false, drain_scheduled?: false}

      true ->
        send_event(state.subscribers, {:message, line})
        %{state | stdout_buffer: "", overflowed?: false, drain_scheduled?: false}
    end
  end

  defp cancel_finalize_timer(%{finalize_timer_ref: nil} = state), do: state

  defp cancel_finalize_timer(state) do
    _ = Process.cancel_timer(state.finalize_timer_ref, async: false, info: false)
    flush_finalize_message(state.subprocess)
    %{state | finalize_timer_ref: nil}
  end

  defp flush_finalize_message({pid, os_pid}) do
    receive do
      {:finalize_exit, ^os_pid, ^pid, _reason} -> :ok
    after
      0 -> :ok
    end
  end

  defp flush_finalize_message(_), do: :ok

  defp append_stderr_data(_existing, _data, max_size)
       when not is_integer(max_size) or max_size <= 0,
       do: ""

  defp append_stderr_data(existing, data, max_size) do
    combined = existing <> data
    combined_size = byte_size(combined)

    if combined_size <= max_size do
      combined
    else
      :binary.part(combined, combined_size - max_size, max_size)
    end
  end

  defp cleanup_pending_calls(pending_calls) do
    Enum.each(pending_calls, fn {ref, from} ->
      Process.demonitor(ref, [:flush])
      GenServer.reply(from, transport_error(:transport_stopped))
    end)
  end

  defp demonitor_subscribers(subscribers) do
    Enum.each(subscribers, fn {_pid, %{monitor_ref: ref}} ->
      Process.demonitor(ref, [:flush])
    end)
  end

  defp force_stop_subprocess(%{subprocess: {pid, _os_pid}} = state) do
    stop_subprocess(pid)
    %{state | subprocess: nil, status: :disconnected}
  end

  defp force_stop_subprocess(state), do: state

  defp stop_subprocess(pid) when is_pid(pid) do
    :exec.stop(pid)
    _ = :exec.kill(pid, 9)
    :ok
  catch
    _, _ -> :ok
  end

  defp normalize_payload(message) when is_binary(message), do: message
  defp normalize_payload(message) when is_map(message), do: Jason.encode!(message)

  defp normalize_payload(message) when is_list(message) do
    IO.iodata_to_binary(message)
  rescue
    ArgumentError ->
      Jason.encode!(message)
  end

  defp normalize_payload(message), do: to_string(message)

  defp ensure_newline(payload) do
    if String.ends_with?(payload, "\n"), do: payload, else: payload <> "\n"
  end

  defp transport_error(reason), do: {:error, {:transport, reason}}
end