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

namespace gaemstone.ECS;

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

	public bool Completed { get; private set; }
	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, Term term)
	{
		using var alloc = TempAllocator.Use();
		var flecsTerm = term.ToFlecs(alloc);
		var flecsIter = ecs_term_iter(universe, &flecsTerm);
		return new(universe, IteratorType.Term, flecsIter);
	}

	public void Dispose()
	{
		GC.SuppressFinalize(this);
		// NOTE: When an iterator is iterated until completion, resources are automatically freed.
		if (!Completed)
			fixed (ecs_iter_t* ptr = &Value)
				ecs_iter_fini(ptr);
	}

	public void SetThis(Entity entity)
	{
		fixed (ecs_iter_t* ptr = &Value)
			ecs_iter_set_var(ptr, 0, entity);
	}

	public bool Next()
	{
		fixed (ecs_iter_t* ptr = &Value) {
			var result = 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),
			};
			Completed = !result;
			return result;
		}
	}

	public EntityRef Entity(int index)
		=> new(Universe, new(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 isSelf  = ecs_field_is_self(ptr, index);
			var pointer = ecs_field_w_size(ptr, size, index);
			return new(pointer, isSelf ? Count : 1);
		}
	}

	public Span<T> MaybeField<T>(int index)
		where T : unmanaged => FieldIsSet(index) ? Field<T>(index) : default;

	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)
	{
		fixed (ecs_iter_t* ptr = &Value) {
			var id   = ecs_field_id(ptr, index);
			var comp = Universe.LookupOrThrow<T>();
			return id == comp.Entity.Value.Data;
		}
	}

	public override string ToString()
	{
		fixed (ecs_iter_t* ptr = &Value)
			return ecs_iter_str(ptr).FlecsToStringAndFree()!;
	}

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

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