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

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

namespace gaemstone.ECS;

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

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

	public Rule(World<TContext> 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 RuleIterator<TContext> Iter()
		=> new(ecs_rule_iter(World, this));

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

	public static implicit operator ecs_rule_t*(Rule<TContext> rule) => rule.Handle;


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

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

		internal VariableCollection(ecs_rule_t* handle)
		{
			// Find the $this variable, if the rule has one.
			var thisIndex = ecs_filter_find_this_var(ecs_rule_get_filter(handle));
			if (thisIndex >= 0) _variables.Add(new(thisIndex, "this"));

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

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

public unsafe class RuleIterator<TContext>
	: Iterator<TContext>
{
	internal RuleIterator(ecs_iter_t value)
		: base(value) {  }
	public override bool Next()
		=> ecs_rule_next(Handle);
}