defmodule RNS.PathRequestStore do
  @moduledoc """
  Stores the tags of all sent/received path requests.
  This is so path requests are only answered to once, and so a message can be sent to a process with the answer to the path request.
  """

  use GenServer

  @spec start_link(any()) :: {:ok, pid()}
  def start_link(opts) do
    GenServer.start_link(__MODULE__, opts, name: __MODULE__)
  end

  @doc "Adds a path request tag to the store."
  @spec add(RNS.Packet.PathRequest.tag()) :: :ok | {:error, :tag_exists}
  def add(tag) do
    GenServer.cast(__MODULE__, {:add, tag})
  end

  @doc "Checks if a tag exists in the tag list."
  @spec exists?(RNS.Packet.PathRequest.tag()) :: boolean()
  def exists?(tag) do
    GenServer.call(__MODULE__, {:exists?, tag})
  end

  @impl true
  def init(_opts) do
    {:ok, {[], []}}
  end

  @impl true
  def handle_cast({:add, tag}, {handlers, tags}) do
    new_tags =
      if tag in tags do
        tags
      else
        new_tags = [tag | tags]

        if length(new_tags) > 64 do
          List.delete_at(new_tags, -1)
        else
          new_tags
        end
      end

    new_state = {handlers, new_tags}
    {:noreply, new_state}
  end

  @impl true
  def handle_call({:exists?, tag}, _from, {_, tags} = state) do
    {:reply, tag in tags, state}
  end
end