defmodule Vela.Distributed.RingManager do
  @moduledoc """
  Watches for cluster membership changes and updates the partitioned
  topology's hash ring in persistent_term.

  Subscribes to `Vela.Distributed.Cluster` events (:node_up / :node_down).
  When the cluster changes, rebuilds the ring and atomically swaps it
  into the topology state stored in persistent_term.
  """

  use GenServer

  alias Vela.Distributed.Ring

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

  def server_name(cache_name), do: :"vela_ring_manager_#{cache_name}"

  @impl true
  def init(cache_name) do
    # Subscribe to cluster events via pg
    Vela.Distributed.Cluster.subscribe()

    # Sync the ring immediately with current cluster membership,
    # in case nodes connected before this process started
    sync_ring(cache_name)

    {:ok, %{cache_name: cache_name}}
  end

  @impl true
  def handle_info({:vela_cluster_event, event, changed_node}, state)
      when event in [:node_up, :node_down] do
    update_ring(state.cache_name, event, changed_node)
    {:noreply, state}
  end

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

  defp update_ring(cache_name, event, changed_node) do
    ts = :persistent_term.get({:vela_topology_state, cache_name})

    new_ring =
      case event do
        :node_up -> Ring.add_node(ts.ring, changed_node)
        :node_down -> Ring.remove_node(ts.ring, changed_node)
      end

    :persistent_term.put(
      {:vela_topology_state, cache_name},
      %{ts | ring: new_ring}
    )
  end

  defp sync_ring(cache_name) do
    ts = :persistent_term.get({:vela_topology_state, cache_name})
    current_nodes = [node() | Node.list()]
    new_ring = Ring.new(current_nodes)

    :persistent_term.put(
      {:vela_topology_state, cache_name},
      %{ts | ring: new_ring}
    )
  end
end