defmodule Argos.Parallel.Worker do
  @moduledoc """
  GenServer que ejecuta una lista de tareas en secuencia y notifica al Líder.

  Cada tarea puede ser `fun/0` o `{mod, fun, args}`. Emite mensajes de
  `:started`, `{:progress, ...}`, `{:result, ...}` y `:error`.
  """

  use GenServer
  require Logger

  @type task_spec :: (-> any()) | {module(), atom(), [any()]}

  @start_delay 0

  @spec start_link(%{leader: pid, id: any, tasks: list}) :: GenServer.on_start()
  def start_link(%{leader: leader, id: id, tasks: tasks}) when is_list(tasks) do
    GenServer.start_link(__MODULE__, %{leader: leader, id: id, tasks: tasks})
  end

  @spec start_link(term) :: {:error, :invalid_args}
  def start_link(_other) do
    {:error, :invalid_args}
  end

  @impl true
  def init(%{leader: leader, id: id, tasks: tasks}) do
    Process.send_after(self(), :start_work, @start_delay)
    {:ok, %{leader: leader, id: id, tasks: tasks, total: length(tasks)}}
  end

  @impl true
  def handle_info(:start_work, %{leader: leader, id: id} = state) do
    safe_cast(leader, {:worker_msg, id, :started})

    state =
      Enum.with_index(state.tasks, 1)
      |> Enum.reduce_while(state, fn {task, idx}, acc ->
        if Map.get(acc, :errored) do
          {:halt, acc}
        else
          total = acc.total

          try do
            result = execute_task(task)
            percent = percent(idx, total)

            safe_cast(leader, {:worker_msg, id, {:progress, idx, total, percent}})
            safe_cast(leader, {:worker_msg, id, {:result, idx, result}})

            new_acc = Map.put(acc, :last_result, result) |> Map.put(:current_task_index, idx)
            {:cont, new_acc}
          rescue
            e ->
              reason = {e, __STACKTRACE__}
              safe_cast(leader, {:worker_msg, id, {:error, idx, reason}})

              errored_acc =
                Map.put(acc, :last_error, reason)
                |> Map.put(:errored, true)
                |> Map.put(:current_task_index, idx)

              {:halt, errored_acc}
          catch
            kind, value ->
              reason = {kind, value}
              safe_cast(leader, {:worker_msg, id, {:error, idx, reason}})

              errored_acc =
                Map.put(acc, :last_error, reason)
                |> Map.put(:errored, true)
                |> Map.put(:current_task_index, idx)

              {:halt, errored_acc}
          end
        end
      end)

    if Map.get(state, :errored) do
      :telemetry.execute(
        [:argos, :parallel, :worker, :finished_with_error],
        %{tasks_completed: Map.get(state, :current_task_index, 0)},
        %{worker_id: id, error: Map.get(state, :last_error)}
      )
    else
      safe_cast(leader, {:worker_msg, id, :finished})

      :telemetry.execute(
        [:argos, :parallel, :worker, :finished],
        %{tasks_completed: state.total},
        %{worker_id: id}
      )
    end

    {:stop, :normal, state}
  end

  @doc """
  Ejecuta una tarea individual.
  """
  @spec execute_task((-> any) | {module, atom, [any]}) :: any
  def execute_task(fun) when is_function(fun, 0) do
    fun.()
  end

  def execute_task({mod, fun, args}) when is_atom(mod) and is_atom(fun) and is_list(args) do
    apply(mod, fun, args)
  end

  def execute_task(other) do
    raise ArgumentError, "Invalid task spec: #{inspect(other)}"
  end

  defp percent(idx, total) when total > 0 do
    idx * 100 / total
  end

  defp percent(_idx, _total), do: 0

  defp safe_cast(leader, msg) do
    GenServer.cast(leader, msg)
  end
end