defmodule Vela.TTL.Manager do
  @moduledoc """
  Periodically sweeps the cache and removes expired entries.

  Uses the TTL index table (an ordered_set keyed by {expires_at, key})
  to find expired entries in O(expired) time instead of scanning everything.
  """

  use GenServer

  # ---- Public API ----

  def start_link(opts) do
    name = Keyword.fetch!(opts, :name)
    GenServer.start_link(__MODULE__, opts, name: manager_name(name))
  end

  def manager_name(cache_name), do: :"vela_ttl_manager_#{cache_name}"

  # ---- GenServer Callbacks ----

  @impl true
  def init(opts) do
    config = Keyword.fetch!(opts, :config)

    # Schedule the first sweep
    ref = schedule_sweep(config.sweep_interval)

    state = %{
      config: config,
      sweep_ref: ref
    }

    {:ok, state}
  end

  @impl true
  def handle_info(:sweep, state) do
    now = System.monotonic_time(:millisecond)

    backend = state.config.backend
    backend_state = get_backend_state(state.config.name)

    count =
      case backend.flush_expired(backend_state, now) do
        {:ok, count, _new_bs} -> count
        _ -> 0
      end

    :telemetry.execute(
      state.config.telemetry_prefix ++ [:ttl_sweep, :stop],
      %{evicted: count},
      %{cache: state.config.name}
    )

    # Schedule the next sweep
    ref = schedule_sweep(state.config.sweep_interval)

    {:noreply, %{state | sweep_ref: ref}}
  end

  defp schedule_sweep(interval) do
    Process.send_after(self(), :sweep, interval)
  end

  defp get_backend_state(cache_name) do
    ts = :persistent_term.get({:vela_topology_state, cache_name})
    ts.backend_state
  end
end