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;

[AttributeUsage(AttributeTargets.Class | AttributeTargets.Struct)]
public class EntityAttribute : Attribute {  }

public unsafe readonly struct Entity
{
	public Universe Universe { get; }
	public ecs_entity_t Value { get; }

	public EntityType Type => new(Universe, ecs_get_type(Universe, Value));
	public string Name => ecs_get_name(Universe, Value).ToStringAndFree();
	public string FullPath => ecs_get_path_w_sep(Universe, default, Value, ".", default).ToStringAndFree();

	public bool IsNone => Value.Data == 0;
	public bool IsAlive => ecs_is_alive(Universe, Value);

	public IEnumerable<Entity> Children { get {
		var term = new ecs_term_t { id = Universe.EcsChildOf & this };
		var iter = Iterator.FromTerm(Universe, term);
		while (iter.Next())
			for (var i = 0; i < iter.Count; i++)
				yield return iter.Entity(i);
	} }

	public Entity(Universe universe, ecs_entity_t value)
		{ Universe = universe; Value = value; }

	public void ThrowIfNone() { if (IsNone) throw new InvalidOperationException("Entity isn't valid"); }
	public void ThrowIfDead() { if (!IsAlive) throw new InvalidOperationException("Entity is dead"); }

	public void Delete() => ecs_delete(Universe, Value);


	public Entity Add(ecs_id_t   id) { ecs_add_id(Universe, this, id); return this; }
	public Entity Add(Identifier id) { ecs_add_id(Universe, this, id); return this; }
	public Entity Add(Entity relation, Entity target) => Add(relation & target);

	public Entity Add<T>()
		=> Add(Universe.Lookup<T>());
	public Entity Add<TRelation, TTarget>()
		=> Add(Universe.Lookup<TRelation>(), Universe.Lookup<TTarget>());
	public Entity Add<TRelation>(Entity target)
		=> Add(Universe.Lookup<TRelation>(), target);


	public Entity Override(ecs_id_t   id) { ecs_override_id(Universe, this, id); return this; }
	public Entity Override(Identifier id) { ecs_override_id(Universe, this, id); return this; }
	public Entity Override(Entity relation, Entity target) => Override(relation & target);

	public Entity Override<T>()
		=> Override(Universe.Lookup<T>());
	public Entity Override<TRelation, TTarget>()
		=> Override(Universe.Lookup<TRelation>(), Universe.Lookup<TTarget>());
	public Entity Override<TRelation>(Entity target)
		=> Override(Universe.Lookup<TRelation>(), target);


	public void Remove(ecs_id_t   id) => ecs_remove_id(Universe, this, id);
	public void Remove(Identifier id) => ecs_remove_id(Universe, this, id);
	public void Remove<T>() => Remove(Universe.Lookup<T>());


	public bool Has(ecs_id_t   id) => ecs_has_id(Universe, this, id);
	public bool Has(Identifier id) => ecs_has_id(Universe, this, id);
	public bool Has(Entity relation, Entity target) => Has(relation & target);

	public bool Has<T>()
		=> Has(Universe.Lookup<T>());
	public bool Has<TRelation, TTarget>()
		=> Has(Universe.Lookup<TRelation>(), Universe.Lookup<TTarget>());
	public bool Has<TRelation>(Entity target)
		=> Has(Universe.Lookup<TRelation>(), target);


	/// <summary>
	/// Gets a component value from this entity. If the component is a value
	/// type, this will return a copy. If the component is a reference type,
	/// it will return the reference itself.
	/// When modifying a reference, consider calling <see cref="Modified"/>.
	/// </summary>
	public T Get<T>()
	{
		var comp = Universe.Lookup<T>();
		var ptr  = ecs_get_id(Universe, this, comp);
		if (typeof(T).IsValueType) {
			return Unsafe.Read<T>(ptr);
		} else {
			var handle = (GCHandle)Unsafe.Read<nint>(ptr);
			return (T)handle.Target!;
		}
	}

	/// <summary>
	/// Gets a reference to a component value from this entity. Only works for
	/// value types. When modifying, consider calling <see cref="Modified"/>.
	/// </summary>
	public ref T GetRef<T>()
		where T : unmanaged
	{
		var comp = Universe.Lookup<T>();
		var ptr  = ecs_get_mut_id(Universe, this, comp);
		return ref Unsafe.AsRef<T>(ptr);
	}

	/// <summary>
	/// Marks a component as modified. Do this after getting a reference to
	/// it with <see cref="Get"/> or <see cref="GetRef"/>, making sure change
	/// detection will kick in.
	/// </summary>
	public void Modified<T>()
	{
		var comp = Universe.Lookup<T>();
		ecs_modified_id(Universe, this, comp);
	}


	public Entity Set<T>(in T value)
		where T : unmanaged
	{
		var comp = Universe.Lookup<T>();
		var size = (ulong)Unsafe.SizeOf<T>();
		fixed (T* ptr = &value) ecs_set_id(Universe, this, comp, size, ptr);
		return this;
	}
	public Entity SetOverride<T>(in T value)
		where T : unmanaged
	{
		var comp = Universe.Lookup<T>();
		var size = (ulong)Unsafe.SizeOf<T>();
		ecs_add_id(Universe, this, Universe.ECS_OVERRIDE | comp);
		fixed (T* ptr = &value) ecs_set_id(Universe, this, comp, size, ptr);
		return this;
	}

	public Entity Set(Type type, object obj)
	{
		var comp   = Universe.Lookup(type);
		var handle = (nint)GCHandle.Alloc(obj);
		ecs_set_id(Universe, this, comp, (ulong)sizeof(nint), &handle);
		// FIXME: Handle needs to be freed when component is removed!
		return this;
	}
	public Entity Set<T>(T obj) where T : class
		=> Set(typeof(T), obj);
	public Entity SetOverride<T>(T obj)
		where T : class
	{
		var comp   = Universe.Lookup<T>();
		var handle = (nint)GCHandle.Alloc(obj);
		ecs_add_id(Universe, this, Universe.ECS_OVERRIDE | comp);
		ecs_set_id(Universe, this, comp, (ulong)sizeof(nint), &handle);
		// FIXME: Handle needs to be freed when component is removed!
		return this;
	}


	public static Identifier operator &(Entity       first, Entity second) => Identifier.Pair(first, second);
	public static Identifier operator &(ecs_entity_t first, Entity second) => Identifier.Pair(first, second);
	public static Identifier operator |(ecs_id_t left, Entity right) => new(right.Universe, left | right.Value.Data);

	public static implicit operator ecs_id_t(Entity e) => e.Value.Data;
	public static implicit operator ecs_entity_t(Entity e) => e.Value;
	public static implicit operator Identifier(Entity e) => new(e.Universe, e);
}

public unsafe readonly struct EntityType
	: IEnumerable<Identifier>
{
	public Universe Universe { get; }
	public unsafe ecs_type_t* Handle { get; }

	public int Count => Handle->count;
	public Identifier this[int index] => new(Universe, Handle->array[index]);

	public EntityType(Universe universe, ecs_type_t* handle)
		{ Universe = universe; Handle = handle; }

	public IEnumerator<Identifier> GetEnumerator()
		{ for (var i = 0; i < Count; i++) yield return this[i]; }
	IEnumerator IEnumerable.GetEnumerator()
		=> GetEnumerator();

	public override string ToString()
		=> ecs_type_str(Universe, Handle).ToStringAndFree();
}