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Daven
Daven

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Making a Cards Against Humanity with Elixir & Phoenix

When the pandemic started me and other friends are looking for a game to play online. One of the games we found was https://pyx-1.pretendyoure.xyz/zy/game.jsp which is a Cards Against Humanity clone written in Java. They did a really good job of implementing features like being able to pick card packs you want & in-game chat feature. The problem is that it's not mobile friendly so you'll have to play it through the desktop view.

I've been using this programming language called Elixir at my job and figured that it might be a good idea to write a CAH clone using it. So i created Leafblower

Cards against mobile

It has the following features

  • In-game chat
  • It's mobile friendly so you can play it on the go
  • It currently uses the base CAH card pack

Show me the code!

It all starts by generating a game code. We call GameStatem.generate_game_code/1 to find an unused game code first. Then we call GameSupervisor.new_game to start a game

leafblower_web/controllers/game_splash_live.ex

  def handle_event("new_game", %{"user" => params}, socket) do
    {:ok, code} = Leafblower.GameStatem.generate_game_code()

    data =
      cast_user(params)
      |> Ecto.Changeset.apply_changes()

    {:ok, game} =
      Leafblower.GameSupervisor.new_game(id: code, countdown_duration: 120, min_player_count: 2)

    Leafblower.GameStatem.join_player(game, socket.assigns.user_id, data.name)

    {:noreply,
     socket
     |> push_redirect(to: Routes.live_path(socket, LeafblowerWeb.GameLive, code), replace: true)}
  end
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When the GameSupervisor.new_game/1 is called it will start 2 process:

  1. GameTicker - The process that handles countdown timers and publishes a tick message everytime it ticks
  2. GameStatem - This stores the current state of the game as well as additional data like player names and ids
 def new_game(arg) do
    {:ok, _} =
      Horde.DynamicSupervisor.start_child(
        __MODULE__,
        {GameTicker, Keyword.take(arg, [:id])}
      )

    Horde.DynamicSupervisor.start_child(
      __MODULE__,
      {GameStatem, arg}
    )
  end
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The GameStatem is where most of the magic happens. If you look at the file, you'll see the status and data typesecs

defmodule Leafblower.GameStatem do
  @type player_id :: binary()
  @type player_info :: %{player_id() => %{name: binary()}}
  @type status :: :waiting_for_players | :round_started_waiting_for_response | :round_ended
  @type data :: %{
          id: binary(),
          active_players: MapSet.t(player_id()),
          player_info: player_info(),
          round_number: non_neg_integer(),
          round_player_answers: %{player_id() => list(binary())},
          required_white_cards_count: non_neg_integer | nil,
          leader_player_id: player_id() | nil,
          winner_player_id: player_id() | nil,
      ....
        }
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When the game is started, the default status will be :waiting_for_players. This will change as the game progresses. The controller sends messages to the GameStatem process and it will broadcast any changes to its state and data through an internal handle_event callback

  def handle_event(:internal, :broadcast, status, data) do
    Phoenix.PubSub.broadcast(
      Leafblower.PubSub,
      topic(data.id),
      {:game_state_changed, status, Map.drop(data, [:deck])}
    )

    :keep_state_and_data
  end
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Closing thoughts

I had extreme fun building this game. Getting feedback from my friends who played was also very helpful. I hope you enjoy as well!

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