using System;
using System.Collections;
using System.Collections.Generic;
using System.Runtime.CompilerServices;
using System.Runtime.InteropServices;
using static flecs_hub.flecs;

namespace gaemstone.ECS;

public unsafe class Iterator
	: IEnumerable<Iterator>
{
	public Universe Universe { get; }
	public IteratorType? Type { get; }
	public ecs_iter_t Value;

	public int Count => Value.count;
    public TimeSpan DeltaTime       => TimeSpan.FromSeconds(Value.delta_time);
    public TimeSpan DeltaSystemTime => TimeSpan.FromSeconds(Value.delta_system_time);

	public Iterator(Universe universe, IteratorType? type, ecs_iter_t value)
		{ Universe = universe; Type = type; Value = value; }

	public static Iterator FromTerm(Universe universe, in ecs_term_t term)
	{
		fixed (ecs_term_t* ptr = &term)
			return new(universe, IteratorType.Term, ecs_term_iter(universe, ptr));
	}

	public bool Next()
	{
		fixed (ecs_iter_t* ptr = &Value)
			return Type switch {
				IteratorType.Term   => ecs_term_next(ptr),
				IteratorType.Filter => ecs_filter_next(ptr),
				IteratorType.Query  => ecs_query_next(ptr),
				IteratorType.Rule   => ecs_rule_next(ptr),
				_ => ecs_iter_next(ptr),
			};
	}

	public Entity Entity(int index)
		=> new(Universe, Value.entities[index]);

	public Span<T> Field<T>(int index)
		where T : unmanaged
	{
		fixed (ecs_iter_t* ptr = &Value) {
			var size    = (ulong)Unsafe.SizeOf<T>();
			var pointer = ecs_field_w_size(ptr, size, index);
			return new Span<T>(pointer, Count);
		}
	}

	public SpanToRef<T> FieldRef<T>(int index)
		where T : class => new(Field<nint>(index));

	public bool FieldIsSet(int index)
	{
		fixed (ecs_iter_t* ptr = &Value)
			return ecs_field_is_set(ptr, index);
	}

	public bool FieldIs<T>(int index)
		where T : unmanaged
	{
		fixed (ecs_iter_t* ptr = &Value) {
			var id   = ecs_field_id(ptr, index);
			var comp = Universe.Lookup<T>();
			return id == comp.Value.Data;
		}
	}

	public readonly ref struct SpanToRef<T>
	{
		private readonly Span<nint> _span;
		internal SpanToRef(Span<nint> span) => _span = span;
		public int Length => _span.Length;
		public T this[int index] => (T)((GCHandle)_span[index]).Target!;
	}

	// IEnumerable implementation
	public IEnumerator<Iterator> GetEnumerator() { while (Next()) yield return this; }
	IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();
}

public enum IteratorType
{
	Term,
	Filter,
	Query,
	Rule,
}