gaemstone.ECS/src/gaemstone.ECS/Rule.cs

using System;
using System.Collections;
using System.Collections.Generic;
using gaemstone.Utility;
using static flecs_hub.flecs;

namespace gaemstone.ECS;

public unsafe class Rule
	: IDisposable
{
	public World World { get; }
	public ecs_rule_t* Handle { get; }
	public Filter Filter { get; }

	private VariableCollection? _variables;
	public VariableCollection Variables => _variables ??= new(this);
	public Iterator.Variable? ThisVar => Filter.ThisVar;

	public Rule(World world, FilterDesc desc)
	{
		using var alloc = TempAllocator.Use();
		var flecsDesc = desc.ToFlecs(alloc);
		World  = world;
		Handle = ecs_rule_init(world, &flecsDesc);
		Filter = new(World, ecs_rule_get_filter(Handle));
	}

	public void Dispose()
		=> ecs_rule_fini(this);

	public Iterator Iter()
		=> new(World, IteratorType.Rule, ecs_rule_iter(World, this));

	public override string ToString()
		=> ecs_rule_str(Handle).FlecsToStringAndFree()!;

	public static implicit operator ecs_rule_t*(Rule q) => q.Handle;


	public unsafe class VariableCollection
		: IReadOnlyCollection<Iterator.Variable>
	{
		private readonly List<Iterator.Variable> _variables = new();

		public Rule Rule { get; }

		public int Count => _variables.Count;
		public Iterator.Variable? this[int index]
			=> _variables.Find(v => v.Index == index);
		public Iterator.Variable? this[string name]
			=> _variables.Find(v => string.Equals(
				v.Name, name, StringComparison.OrdinalIgnoreCase));

		internal VariableCollection(Rule rule)
		{
			Rule = rule;

			// Find the $This variable, if the rule has one.
			var thisIndex = ecs_filter_find_this_var(ecs_rule_get_filter(Rule));
			if (thisIndex >= 0) _variables.Add(new(thisIndex, "This"));

			// Find all the other "accessible" variables.
			var count = ecs_rule_var_count(Rule);
			for (var i = 0; i < count; i++) {
				if ((i == thisIndex) || !ecs_rule_var_is_entity(Rule, i)) continue;
				var name = ecs_rule_var_name(Rule, i).FlecsToString()!;
				_variables.Add(new(i, name));
			}
		}

		// IEnumerable implementation
		public IEnumerator<Iterator.Variable> GetEnumerator() => _variables.GetEnumerator();
		IEnumerator IEnumerable.GetEnumerator() => _variables.GetEnumerator();
	}
}