oecs-sharp/Game.Blackjack/Commands/HitCommand.cs

using MessagePack;
using OECS;

namespace Game.Blackjack;

/// <summary>
/// Player hits: draws one card from the deck to the player's hand.
/// </summary>
[MessagePackObject]
public struct HitCommand : ICommand
{
    public void Execute(World world)
    {
        ref var state = ref world.GetSingleton<GameState>();

        if (state.Phase != GamePhase.PlayerTurn)
            return;

        DrawCard<PlayerHand>(world);
    }

    /// <summary>
    /// Draws the top card from the deck and adds it to the given hand.
    /// </summary>
    internal static void DrawCard<THand>(World world)
        where THand : struct
    {
        var singletonEntity = World.SingletonEntity;
        var deckEntity = FindEntity<Deck>(world, singletonEntity);
        var handEntity = FindEntity<THand>(world, singletonEntity);

        if (deckEntity == Entity.Null || handEntity == Entity.Null)
            return;

        // Find a card still in the deck.
        var cardsInDeck = world.GetSources<InDeck>(deckEntity);
        if (cardsInDeck.Count == 0)
            return;

        var cardEntity = cardsInDeck.First();

        // Remove from deck, add to hand.
        world.RemoveComponent<InDeck>(cardEntity);
        world.AddComponent(cardEntity, new Holds { Target = handEntity });
    }

    internal static Entity FindEntity<T>(World world, Entity singletonEntity)
        where T : struct
    {
        using var iter = world.Select<T>();
        while (iter.MoveNext())
        {
            if (iter.CurrentEntity != singletonEntity)
                return iter.CurrentEntity;
        }
        return Entity.Null;
    }
}