using System;
using System.Linq;
using Svelto.DataStructures;
using Svelto.ECS;
using TechbloxModdingAPI.Common;

namespace TechbloxModdingAPI.Utility
{
    public ref struct OptionalRef<T> where T : struct, IBaseEntityComponent
    {
        private readonly EGID entityId;
        private readonly State state;
        private readonly uint index;
        private NB<T> array;
        private MB<T> managedArray;
        private readonly EntityInitializer initializer;
        //The possible fields are: (index && (array || managedArray)) || initializer
        private readonly EcsObjectBase obj;

        public OptionalRef(NB<T> array, uint index, EGID entityId = default)
        {
            state = State.Native;
            this.array = array;
            this.index = index;
            this.entityId = entityId;
            initializer = default;
            managedArray = default;
            obj = default;
        }

        public OptionalRef(MB<T> array, uint index, EGID entityId = default)
        {
            state = State.Managed;
            managedArray = array;
            this.index = index;
            this.entityId = entityId;
            initializer = default;
            this.array = default;
            obj = default;
        }

        /// <summary>
        /// Wraps the initializer data, if present.
        /// </summary>
        /// <param name="obj">The object with the initializer</param>
        /// <param name="unmanaged">Whether the struct is unmanaged</param>
        public OptionalRef(EcsObjectBase obj, bool unmanaged, EGID entityId = default)
        {
            this.entityId = entityId;
            if (obj.InitData.Valid)
            {
                initializer = obj.InitData.Initializer(obj.Id);
                state = (unmanaged ? State.Native : State.Managed) | State.Initializer;
            }
            else
            {
                initializer = default;
                state = State.Empty;
            }
            array = default;
            index = default;
            managedArray = default;
            this.obj = obj;
        }

        /// <summary>
        /// Returns the value or a default value if empty. Supports objects that are being initialized.
        /// </summary>
        /// <returns>The value or the default value</returns>
        public ref T Get()
        {
            CompRefCache.Default = default; //The default value can be changed by mods
            if (state == State.Empty) return ref CompRefCache.Default;
            // If initializing the entity, check if the component is allowed by the descriptor, otherwise it could cause
            // issues in the game with Add() calls running unexpectedly
            if ((state & State.Initializer) != State.Empty && obj.AllowedEntityComponents.Contains(typeof(T)))
                return ref initializer.GetOrAdd<T>();
            if ((state & State.Native) != State.Empty) return ref array[index];
            return ref managedArray[index];
        }

        /// <summary>
        /// A non-by-ref view that allows getting and setting the value.
        /// </summary>
        public T Value
        {
            get => Get();
            set => Get() = value;
        }

        /// <summary>
        /// The ID of the entity this component belongs to or default if it doesn't exist.
        /// </summary>
        public EGID EGID => entityId;

        public bool Exists => state != State.Empty;
        public T? Nullable() => this ? Get() : default;

        public static implicit operator T(OptionalRef<T> opt) => opt.Get();

        public static implicit operator bool(OptionalRef<T> opt) => opt.state != State.Empty;

        public static implicit operator EGID(OptionalRef<T> opt) => opt.entityId;

        /// <summary>
        /// Creates an instance of a struct T that can be referenced.
        /// </summary>
        private struct CompRefCache
        {
            public static T Default;
        }
        
        /// <summary>
        /// A byte that holds state in its bits.
        /// </summary>
        [Flags]
        private enum State : byte
        {
            Empty,
            Native,
            Managed,
            Initializer = 4
        }
    }
}