A behaviour tree system for Entitas

EntitasBT.cs

public sealed class BTSystem: IExecuteSystem 
{
	IBehaviorNode rootNode;
	public BTSystem(IBehaviorNode rootNode)
	{
		this.rootNode = rootNode;
	}

	void Execute()
	{
		rootNode.Execute();
	}
}

enum NodeStatus
{
	Success, Failure, Running
}

public interface IBehaviorNode
{
	NodeStatus Execute(); 
}

public sealed class Sequence : IBehaviorNode
{
	IBehaviorNode[] nodes;
	int startIndex = 0;

	public Sequence(params IBehaviorNode[] nodes) 
	{
		this.nodes = nodes;
	}

	public NodeStatus Execute()
	{
		var result = NodeStatus.Success;
		for (int i = startIndex; i < nodes.Length; i++)
		{
			result = nodes[i].Execute();
			if (result == NodeStatus.Failure) {
				break;
			}
			if (result == NodeStatus.Running) {
				startIndex = i;
				return result;
			}
		}
		startIndex = 0;
		return result;
	}
}

public sealed class Selector : IBehaviorNode
{
	IBehaviorNode[] nodes;
	int startIndex = 0;

	public Selector(params IBehaviorNode[] nodes) 
	{
		this.nodes = nodes;
	}

	public NodeStatus Execute()
	{
		var result = NodeStatus.Success;
		for (int i = startIndex; i < nodes.Length; i++)
		{
			result = nodes[i].Execute();
			if (result == NodeStatus.Success){
				break;
			}
			if (result == NodeStatus.Running) {
				startIndex = i;
				return result;
			}
		}
		startIndex = 0;
		return result;
	}
}