gaemstone/src/gaemstone/ECS/Filter.cs

using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
using System.Runtime.CompilerServices;
using gaemstone.Utility;
using gaemstone.Utility.IL;
using static flecs_hub.flecs;

namespace gaemstone.ECS;

public unsafe sealed class Filter
	: IEnumerable<Iterator>
	, IDisposable
{
	public Universe Universe { get; }
	public ecs_filter_t* Handle { get; }

	public Filter(Universe universe, FilterDesc desc)
	{
		using var alloc = TempAllocator.Lock();
		var flecsDesc = desc.ToFlecs(alloc);
		Universe = universe;
		Handle   = ecs_filter_init(universe, &flecsDesc);
	}

	public static void RunOnce(Universe universe, Delegate action)
	{
		var gen  = IterActionGenerator.GetOrBuild(universe, action.Method);
		var desc = new FilterDesc(gen.Terms.ToArray()) { Name = action.Method.Name };
		using var filter = new Filter(universe, desc);
		foreach (var iter in filter) gen.RunWithTryCatch(action.Target, iter);
	}

	public void Dispose()
		=> ecs_filter_fini(Handle);

	public override string ToString()
		=> ecs_filter_str(Universe, Handle).FlecsToStringAndFree()!;

	public static implicit operator ecs_filter_t*(Filter q) => q.Handle;

	// IEnumerable implementation
	public Iterator Iter() => new(Universe, IteratorType.Filter, ecs_filter_iter(Universe, this));
	public IEnumerator<Iterator> GetEnumerator() => Iter().GetEnumerator();
	IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();
}

public class FilterDesc
{
	public IReadOnlyList<Term> Terms { get; }

	public string? Expression { get; }

	/// <summary>
	/// Optional name of filter, used for debugging. If a filter is created
	/// for a system, the provided name should match the system name.
	/// </summary>
	public string? Name { get; set; }

	/// <summary>
	/// When true, terms returned by an iterator may either contain 1 or N
	/// elements, where terms with N elements are owned, and terms with 1
	/// element are shared, for example from a parent or base entity. When
	/// false, the iterator will at most return 1 element when the result
	/// contains both owned and shared terms.
	/// </summary>
	public bool Instanced { get; set; }

	public FilterDesc(params Term[] terms)
		=> Terms = terms;
	public FilterDesc(string expression) : this()
		=> Expression = expression;

	public unsafe ecs_filter_desc_t ToFlecs(IAllocator allocator)
	{
		var desc = new ecs_filter_desc_t {
			name = allocator.AllocateCString(Name),
			expr = allocator.AllocateCString(Expression),
			instanced = Instanced,
		};
		var span = desc.terms;
		if (Terms.Count > span.Length) {
			span = allocator.Allocate<ecs_term_t>(Terms.Count);
			desc.terms_buffer = (ecs_term_t*)Unsafe.AsPointer(ref span[0]);
			desc.terms_buffer_count = Terms.Count;
		}
		for (var i = 0; i < Terms.Count; i++)
			span[i] = Terms[i].ToFlecs(allocator);
		return desc;
	}
}