defmodule Microprints.MicroprintCache do
  @moduledoc """
  GenServer that caches microprint visualizations in ETS.

  Subscribes to a PubSub topic and invalidates cache entries when files
  change via LiveReload (or any other file watcher).

  ## Configuration

  The PubSub module and topic can be configured via `Application` environment:

      config :microprints,
        pubsub: MyApp.PubSub,
        pubsub_topic: "dev_tools_files"

  Defaults to `Phoenix.PubSub` and `"dev_tools_files"`.
  """

  use GenServer

  alias Microprints.Microprint

  @table_name :microprint_cache
  @live_reload_topic "dev_tools_files"

  # Client API

  @doc """
  Starts the MicroprintCache GenServer.

  ## Options

  * `:pubsub` - The PubSub module to use (defaults to `Phoenix.PubSub`)
  * `:pubsub_topic` - The PubSub topic to subscribe to (defaults to `"dev_tools_files"`)
  """
  def start_link(opts \\ []) do
    pubsub = opts[:pubsub] || Application.get_env(:microprints, :pubsub, Phoenix.PubSub)
    pubsub_topic = opts[:pubsub_topic] || Application.get_env(:microprints, :pubsub_topic, @live_reload_topic)

    GenServer.start_link(__MODULE__, %{pubsub: pubsub, pubsub_topic: pubsub_topic}, name: __MODULE__)
  end

  @doc """
  Returns the microprint for the given file path.

  Returns cached version if available, otherwise generates and caches it.
  """
  @spec get_microprint(String.t()) :: {:ok, Microprint.microprint()} | {:error, term()}
  def get_microprint(path) do
    absolute_path = Path.expand(path)

    case :ets.lookup(@table_name, absolute_path) do
      [{^absolute_path, microprint}] ->
        {:ok, microprint}

      [] ->
        case Microprint.generate(absolute_path) do
          {:ok, microprint} ->
            :ets.insert(@table_name, {absolute_path, microprint})
            {:ok, microprint}

          error ->
            error
        end
    end
  end

  @doc """
  Invalidates the cache entry for the given path.
  """
  @spec invalidate(String.t()) :: true
  def invalidate(path) do
    absolute_path = Path.expand(path)
    :ets.delete(@table_name, absolute_path)
  end

  @doc """
  Clears all cached microprints.
  """
  @spec clear_all() :: true
  def clear_all do
    :ets.delete_all_objects(@table_name)
  end

  # Server callbacks

  @impl true
  def init(%{pubsub: pubsub, pubsub_topic: pubsub_topic}) do
    table = :ets.new(@table_name, [:named_table, :public, :set, read_concurrency: true])
    Phoenix.PubSub.subscribe(pubsub, pubsub_topic)
    {:ok, %{table: table, pubsub_topic: pubsub_topic}}
  end

  @impl true
  def handle_info({:phoenix_live_reload, _topic, path}, state) do
    invalidate(path)
    {:noreply, state}
  end

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