I’m trying to implement the war game in Elixir using a Queue

This is the brief game description:

The game starts with a shuffled deck of cards. The deck will be passed into your program already shuffled (details below). The cards are dealt in an alternating fashion to each player, so that each player has 26 cards.
In each round, both players reveal the top card of their pile. The player with the higher card (by rank) wins both cards, placing them at the bottom of their pile. Aces are considered high, meaning the card ranks in ascending order are 2-10, Jack, Queen, King, Ace.
If the revealed cards are tied, there is war! Each player turns up one card face down followed by one card face up. The player with the higher face-up card takes both piles (six cards – the two original cards that were tied, plus the four cards from the war). If the turned-up cards are again the same rank, each player places another card face down and turns another card face up. The player with the higher card takes all 10 cards, and so on.
When one player runs out of cards, they are the loser, and the other the winner. If, during a war, a player runs out of cards, this counts as a loss as well.

This is my Queue implementation:

defmodule Queue do
  use GenServer

  @intial_state %{
    size: 0,
    queue: [],
  }

  # Client - Public and high level API

  def start_link do
    GenServer.start_link(__MODULE__, @intial_state)
  end

  def enqueue(pid, elems) do
    GenServer.cast(pid, {:enqueue, elems})
  end

  def dequeue(pid) do
    GenServer.call(pid, :dequeue)
  end

  def dequeue(pid, many) do
    GenServer.call(pid, {:dequeue, many})
  end

  def size(pid) do
    GenServer.call(pid, :size)
  end

  def front(pid) do
    GenServer.call(pid, :front)
  end

  def rear(pid) do
    GenServer.call(pid, :rear)
  end

  def flush(pid) do
    GenServer.call(pid, :flush)
  end

  # Server - Public but internal API
  # handle_cast - handle the demand asynchronously
  # handle_call - handle the demand eagerly

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

  @impl true
  def handle_call(:size, _from, state) do
    {:reply, state.size, state}
  end

  def handle_call(:front, _from, state) do
    %{queue: xs} = state

    case xs do
      [] -> {:reply, nil, state}
      [x | _] -> {:reply, x, state}
    end
  end

  def handle_call(:rear, _from, state) do
    %{queue: xs} = state

    case xs do
      [] -> {:reply, nil, state}
      xs -> {:reply, List.last(xs), state}
    end
  end

  def handle_call(:flush, _from, state) do
    {elems, state} = deq_many(state, state.size)

    {:reply, elems, state}
  end

  def handle_call(:dequeue, _from, state) do
    {elem, state} = deq(state)

    {:reply, elem, state}
  end

  def handle_call({:dequeue, many}, _from, state) do
    {elems, state} = deq_many(state, many)

    {:reply, Enum.reverse(elems), state}
  end

  defp deq_many(state, n) do
    Enum.reduce(1..n, {[], state}, fn
      _, {elems, state} ->
        {elem, state} = deq(state)

        {[elem | elems], state}
    end)
  end

  defp deq(state) do
    %{queue: xs, size: size} = state

    case xs do
      [] -> {nil, state}
      [x | xs] -> {x, %{state | queue: xs, size: size - 1}}
    end
  end

  @impl true
  def handle_cast({:enqueue, elems}, state) when is_list(elems) do
    %{queue: xs, size: x} = state

    xs = List.foldr(elems, xs, &[&1 | &2])

    {:noreply,
      %{state | size: x + length(elems), queue: xs}}
  end

  def handle_cast({:enqueue, elem}, state) do
    %{queue: xs, size: x} = state

    {:noreply, %{state | size: x + 1, queue: Enum.reverse([elem | xs])}}
  end
end

This is my current implementation so far of war game (it fails with timeout):

defmodule War do
  @doc """
  The main module to the challenge.
  This module exposes a deal/1 function to play the game.

  You can run all tests executing `elixir war.ex`.
  """
  require Integer

  # prefers to use a weight as we don't represent the cards
  @ace_weight 14

  def deal(deck) do
    {deck_1, deck_2} = deal_deck(deck)

    {:ok, player_1} = Queue.start_link()
    {:ok, player_2} = Queue.start_link()

    :ok = Queue.enqueue(player_1, deck_1)
    :ok = Queue.enqueue(player_2, deck_2)

    winner = play_game(player_1, player_2)

    Queue.size(winner)
    |> then(&Queue.dequeue(winner, &1))
    |> Enum.map(&remove_ace_weight/1)
  end

  defp deal_deck(deck) do
    List.foldr(deck, {[], []}, &deal_player/2)
  end

  defp play_game(p1, p2) do
    if winner = maybe_get_winner(p1, p2) do
      winner
    else
      case play_turn(p1, p2) do
        {winner, []} ->
          winner

        {turn_winner, cards} ->
          push_cards(turn_winner, cards)
          play_game(p1 ,p2)
      end
    end
  end

  defp play_turn(p1, p2, x \\ nil, y \\ nil, tied \\ []) do
    x = x || Queue.dequeue(p1)
    y = y || Queue.dequeue(p2)
    cards = [x, y]

    cond do
      x > y -> {p1, cards ++ tied}
      x < y -> {p2, cards ++ tied}
      x == y -> war(p1, p2, cards ++ tied)
    end
  end

  defp war(p1, p2, tied) do
    [x, y] = Enum.take(tied, 2)
    tied = Enum.drop(tied, 2)

    cond do
      !able_to_war?(p1) ->
        cards = tied ++ Queue.flush(p1)
        push_cards(p2, cards)
        {p2, []}


      !able_to_war?(p2) ->
        cards = tied ++ Queue.flush(p2)
        push_cards(p1, cards)
        {p1, []}

      true ->
        {turn_winner, cards} = play_turn(p1, p2, x, y, tied)
        push_cards(turn_winner, cards)
        play_game(p1, p2)
    end
  end

  defp deal_player(card, {p1, p2}) do
    if length(p1) == length(p2) do
      {[apply_ace_weight(card) | p1], p2}
    else
      {p1, [apply_ace_weight(card) | p2]}
    end
  end

  defp able_to_war?(player) do
    Queue.size(player) > 3
  end

  # The game ends when a player losses all their cards
  # so their Stack is empty
  defp maybe_get_winner(player_1, player_2) do
    cond do
      Queue.size(player_1) == 0 -> player_2
      Queue.size(player_2) == 0 -> player_1
      true -> nil
    end
  end

  defp apply_ace_weight(card) do
    (card == 1 && @ace_weight) || card
  end

  defp remove_ace_weight(card) do
    (card == @ace_weight && 1) || card
  end

  # Cards won from a war needs to be pushed in descending order
  defp push_cards(player, cards) do
    cards = Enum.sort(cards, :desc)

    Queue.enqueue(player, cards)
  end
end

And these are the tests cases:

defmodule WarTest do
  use ExUnit.Case

  describe "War" do
    test "deal_1" do
      t1 = [1,1,1,1,13,13,13,13,11,11,11,11,12,12,12,12,10,10,10,10,9,9,9,9,7,7,7,7,8,8,8,8,6,6,6,6,5,5,5,5,4,4,4,4,3,3,3,3,2,2,2,2]
      r1 = [1,1,1,1,13,13,13,13,12,12,12,12,11,11,11,11,10,10,10,10,9,9,9,9,8,8,8,8,7,7,7,7,6,6,6,6,5,5,5,5,4,4,4,4,3,3,3,3,2,2,2,2]
      assert War.deal(t1) == r1
    end

    test "deal_2" do
      t2 = [1,13,1,13,1,13,1,13,12,11,12,11,12,11,12,11,10,9,10,9,10,9,10,9,8,7,8,7,8,7,8,7,6,5,6,5,6,5,6,5,4,3,4,3,4,3,4,3,2,2,2,2]
      r2 = [4,3,2,2,2,2,4,3,4,3,4,3,6,5,6,5,6,5,6,5,8,7,8,7,8,7,8,7,10,9,10,9,10,9,10,9,12,11,12,11,12,11,12,11,1,13,1,13,1,13,1,13]
      assert War.deal(t2) == r2
    end

    test "deal_3" do
      t3 = [13,1,13,1,13,1,13,1,11,12,11,12,11,12,11,12,9,10,9,10,9,10,9,10,7,8,7,8,7,8,7,8,5,6,5,6,5,6,5,6,3,4,3,4,3,4,3,4,2,2,2,2]
      r3 = [4,3,2,2,2,2,4,3,4,3,4,3,6,5,6,5,6,5,6,5,8,7,8,7,8,7,8,7,10,9,10,9,10,9,10,9,12,11,12,11,12,11,12,11,1,13,1,13,1,13,1,13]
      assert War.deal(t3) == r3
    end

    test "deal_4" do
      t4 = [10,11,12,13,1,2,3,4,5,6,7,8,9,10,11,12,13,1,2,3,4,5,6,7,8,9,10,11,12,13,1,2,3,4,5,6,7,8,9,10,11,12,13,1,2,3,4,5,6,7,8,9]
      r4 = [1,1,13,12,9,5,11,4,9,3,8,7,7,2,13,10,12,5,10,4,9,6,8,3,1,1,13,12,7,5,11,4,9,3,8,6,7,2,13,10,12,5,11,11,10,8,6,4,6,3,2,2]
      assert War.deal(t4) == r4
    end

    test "deal_5" do
      t5 = [1,2,3,4,5,6,7,8,9,10,11,12,13,1,2,3,4,5,6,7,8,9,10,11,12,13,1,2,3,4,5,6,7,8,9,10,11,12,13,1,2,3,4,5,6,7,8,9,10,11,12,13]
      r5 = [1,10,13,8,11,9,8,7,11,8,13,7,13,6,12,6,9,5,8,5,7,4,7,4,11,6,12,10,6,3,2,2,12,5,9,3,10,4,9,2,10,3,5,2,1,1,1,13,12,11,4,3]
      assert War.deal(t5) == r5
    end

    defp create_deck do
      deck =
        for n <- 1..13, _ <- 1..4 do
          n
        end

      Enum.shuffle(deck)
    end

    test "should return the same number of cards after deal" do
      deck = create_deck()

      assert length(deck) == 52
      assert length(War.deal(deck)) == 52
    end

    test "should remove the ace weight after a win" do
      deck = create_deck()

      assert Enum.all?(War.deal(deck), &(&1 != 14))
    end
  end
end

I also made a public repo of this challenge: GitHub - zoedsoupe/war.ex: War card game implemented in Elixir · GitHub

I’m trying to understand why my code fails and how I could improve it to achieve the desired result.