defmodule Microwaveprop.Propagation.FreshnessMonitor do
  @moduledoc """
  Monitors propagation score freshness and enqueues grid worker jobs
  when data is stale. Checks every 5 minutes. Covers missed cron ticks,
  slow deploys, worker crashes, and any other gap in hourly scoring.
  """

  use GenServer

  alias Microwaveprop.Propagation
  alias Microwaveprop.Workers.PropagationGridWorker

  require Logger

  @check_interval to_timeout(minute: 5)
  @stale_threshold_minutes 120

  @spec start_link(keyword()) :: GenServer.on_start() | :ignore
  def start_link(_opts) do
    if Application.get_env(:microwaveprop, :start_freshness_monitor, true) do
      GenServer.start_link(__MODULE__, :ok, name: __MODULE__)
    else
      :ignore
    end
  end

  @impl true
  def init(:ok) do
    send(self(), :check)
    {:ok, %{}}
  end

  @impl true
  def handle_info(:check, state) do
    check_freshness()
    Process.send_after(self(), :check, @check_interval)
    {:noreply, state}
  end

  defp check_freshness do
    case Propagation.latest_valid_time() do
      nil ->
        Logger.info("FreshnessMonitor: no scores found, enqueuing grid worker")
        enqueue_if_not_queued()

      latest ->
        age = DateTime.diff(DateTime.utc_now(), latest, :minute)

        if age > @stale_threshold_minutes do
          Logger.info("FreshnessMonitor: scores are #{age}m old, enqueuing grid worker")
          enqueue_if_not_queued()
        end
    end
  end

  defp enqueue_if_not_queued do
    # Oban unique constraint on PropagationGridWorker prevents duplicates —
    # if a job is already available/scheduled/executing/retryable within the
    # last hour, insert is a no-op.
    Oban.insert(PropagationGridWorker.new(%{}))
  end
end