namespace OECS;

/// <summary>
/// Describes a single change that occurred in the ECS world.
/// </summary>
public readonly struct EntityChange : IEquatable<EntityChange>
{
    /// <summary>
    /// The entity that was affected.
    /// </summary>
    public Entity Entity { get; }

    /// <summary>
    /// The kind of change.
    /// </summary>
    public ChangeKind Kind { get; }

    /// <summary>
    /// The component type involved, or null for entity-level changes
    /// (<see cref="ChangeKind.EntityAdded"/> / <see cref="ChangeKind.EntityRemoved"/>).
    /// </summary>
    public Type? ComponentType { get; }

    internal EntityChange(Entity entity, ChangeKind kind, Type? componentType = null)
    {
        Entity = entity;
        Kind = kind;
        ComponentType = componentType;
    }

    public bool Equals(EntityChange other)
    {
        return Entity.Equals(other.Entity)
            && Kind == other.Kind
            && ComponentType == other.ComponentType;
    }

    public override bool Equals(object? obj) => obj is EntityChange other && Equals(other);

    public override int GetHashCode() => HashCode.Combine(Entity, Kind, ComponentType);

    public override string ToString() => ComponentType == null
        ? $"{Kind} {Entity}"
        : $"{Kind} {ComponentType.Name} on {Entity}";
}