defmodule Mv.Membership.SettingsCache do
  @moduledoc """
  Process-based cache for global settings to avoid repeated DB reads on hot paths
  (e.g. RegistrationEnabled validation, Layouts.public_page, Plugs).

  Uses a short TTL (default 60 seconds). Cache is invalidated on every settings
  update so that changes take effect quickly. If no settings process exists
  (e.g. in tests), get/1 falls back to direct read.
  """
  use GenServer

  @default_ttl_seconds 60

  def start_link(opts \\ []) do
    GenServer.start_link(__MODULE__, opts, name: __MODULE__)
  end

  @doc """
  Returns cached settings or fetches and caches them. Uses TTL; invalidate on update.
  """
  def get do
    case Process.whereis(__MODULE__) do
      nil ->
        # No cache process (e.g. test) – read directly
        do_fetch()

      _pid ->
        GenServer.call(__MODULE__, :get, 10_000)
    end
  end

  @doc """
  Invalidates the cache so the next get/0 will refetch from the database.
  Call after update_settings and any other path that mutates settings.
  """
  def invalidate do
    case Process.whereis(__MODULE__) do
      nil -> :ok
      _pid -> GenServer.cast(__MODULE__, :invalidate)
    end
  end

  @impl true
  def init(opts) do
    ttl = Keyword.get(opts, :ttl_seconds, @default_ttl_seconds)
    state = %{ttl_seconds: ttl, cached: nil, expires_at: nil}
    {:ok, state}
  end

  @impl true
  def handle_call(:get, _from, state) do
    now = System.monotonic_time(:second)
    expired? = state.expires_at == nil or state.expires_at <= now

    {result, new_state} =
      if expired? do
        fetch_and_cache(now, state)
      else
        {{:ok, state.cached}, state}
      end

    {:reply, result, new_state}
  end

  defp fetch_and_cache(now, state) do
    case do_fetch() do
      {:ok, settings} = ok ->
        expires = now + state.ttl_seconds
        {ok, %{state | cached: settings, expires_at: expires}}

      err ->
        result = if state.cached, do: {:ok, state.cached}, else: err
        {result, state}
    end
  end

  @impl true
  def handle_cast(:invalidate, state) do
    {:noreply, %{state | cached: nil, expires_at: nil}}
  end

  defp do_fetch do
    Mv.Membership.get_settings_uncached()
  end
end