using System;
using System.Reflection;
using System.Collections.Generic;
using System.Linq;
using Encompass.Exceptions;

namespace Encompass
{
    /// <summary>
    /// Engines are the Encompass notion of an ECS System.
    /// They are responsible for reading the World state, reading messages, emitting messages, and creating or mutating Entities and Components.
    /// Engines run once per World Update.
    /// </summary>
    public abstract class Engine : IEquatable<Engine>
    {
        internal Guid ID;

        internal readonly HashSet<Type> readTypes = new HashSet<Type>();
        internal readonly HashSet<Type> readPendingTypes = new HashSet<Type>();
        internal readonly HashSet<Type> sendTypes = new HashSet<Type>();
        internal readonly HashSet<Type> receiveTypes = new HashSet<Type>();
        internal readonly HashSet<Type> writeTypes = new HashSet<Type>();
        internal readonly HashSet<Type> writePendingTypes = new HashSet<Type>();
        internal readonly Dictionary<Type, int> writePriorities = new Dictionary<Type, int>();
        internal readonly int defaultWritePriority = 0;

        /// <summary>
        /// If false, the Engine will ignore time dilation.
        /// </summary>
        internal bool usesTimeDilation = true;
        public bool TimeDilationActive { get => usesTimeDilation && timeManager.TimeDilationActive; }

        private EntityManager entityManager;
        private MessageManager messageManager;
        private ComponentManager componentManager;
        private ComponentUpdateManager componentUpdateManager;
        private TimeManager timeManager;

        protected Engine()
        {
            ID = Guid.NewGuid();

            var sendsAttribute = GetType().GetCustomAttribute<Sends>(false);
            if (sendsAttribute != null)
            {
                sendTypes = sendsAttribute.sendTypes;
            }

            var activatesAttribute = GetType().GetCustomAttribute<WritesPending>(false);
            if (activatesAttribute != null)
            {
                writePendingTypes = activatesAttribute.writePendingTypes;
            }

            var defaultWritePriorityAttribute = GetType().GetCustomAttribute<DefaultWritePriority>(false);

            if (defaultWritePriorityAttribute != null)
            {
                defaultWritePriority = defaultWritePriorityAttribute.writePriority;
            }

            foreach (var writesAttribute in GetType().GetCustomAttributes<Writes>(false))
            {
                writeTypes.UnionWith(writesAttribute.writeTypes);
                writePriorities = new Dictionary<Type, int>[2] { writePriorities, writesAttribute.priorities }.SelectMany(dict => dict).ToDictionary(pair => pair.Key, pair => pair.Value);
            }

            var receivesAttribute = GetType().GetCustomAttribute<Receives>(false);
            if (receivesAttribute != null)
            {
                receiveTypes = receivesAttribute.receiveTypes;
            }

            var readsAttribute = GetType().GetCustomAttribute<Reads>(false);
            if (readsAttribute != null)
            {
                readTypes = readsAttribute.readTypes;
            }

            var readsPendingAttribute = GetType().GetCustomAttribute<ReadsPending>(false);
            if (readsPendingAttribute != null)
            {
                readPendingTypes = readsPendingAttribute.readPendingTypes;
            }
        }

        public override bool Equals(object obj)
        {
            if (obj is Engine engine)
            {
                return Equals(engine);
            }

            return false;
        }

        public bool Equals(Engine other)
        {
            return other.ID == ID;
        }

        public override int GetHashCode()
        {
            return ID.GetHashCode();
        }

        internal void AssignEntityManager(EntityManager entityManager)
        {
            this.entityManager = entityManager;
        }

        internal void AssignComponentManager(ComponentManager componentManager)
        {
            this.componentManager = componentManager;
        }

        internal void AssignMessageManager(MessageManager messageManager)
        {
            this.messageManager = messageManager;
        }

        internal void AssignComponentUpdateManager(ComponentUpdateManager componentUpdateManager)
        {
            this.componentUpdateManager = componentUpdateManager;
        }

        internal void AssignTimeManager(TimeManager timeManager)
        {
            this.timeManager = timeManager;
        }

        /// <summary>
        /// Runs once per World update with the calculated delta-time.
        /// </summary>
        /// <param name="dt">The time in seconds that has elapsed since the previous frame.</param>
        public abstract void Update(double dt);

        /// <summary>
        /// Creates and returns a new empty Entity.
        /// </summary>
        protected Entity CreateEntity()
        {
            return entityManager.CreateEntity();
        }

        /// <summary>
        /// Returns true if an Entity with the specified ID exists.
        /// </summary>
        protected bool EntityExists(Entity entity)
        {
            return entityManager.EntityExists(entity.ID);
        }

        /// <summary>
        /// Returns an Entity containing the specified Component type.
        /// </summary>
        protected Entity ReadEntity<TComponent>() where TComponent : struct, IComponent
        {
            return ReadComponentHelper<TComponent>().Item1;
        }

        /// <summary>
        /// Returns all Entities containing the specified Component type.
        /// </summary>
        protected IEnumerable<Entity> ReadEntities<TComponent>() where TComponent : struct, IComponent
        {
            return ReadComponentsHelper<TComponent>().Select(pair => pair.Item1);
        }

        // these next two are for the ComponentMessageEmitter only

        internal IEnumerable<TComponent> ReadComponentsFromWorld<TComponent>() where TComponent : struct, IComponent
        {
            return componentManager.GetComponentsByType<TComponent>();
        }

        private IEnumerable<(Entity, TComponent)> ReadComponentsHelper<TComponent>() where TComponent : struct, IComponent
        {
            var pendingRead = readPendingTypes.Contains(typeof(TComponent));
            var existingRead = readTypes.Contains(typeof(TComponent));
            if (existingRead && pendingRead)
            {
                return componentUpdateManager.ReadExistingAndPendingComponentsByType<TComponent>();
            }
            else if (existingRead)
            {
                return componentUpdateManager.ReadExistingComponentsByType<TComponent>();
            }
            else if (pendingRead)
            {
                return componentUpdateManager.ReadPendingComponentsByType<TComponent>();
            }
            else
            {
                throw new IllegalReadException("Engine {0} tried to read undeclared Component {1}", GetType().Name, typeof(TComponent).Name);
            }
        }

        /// <summary>
        /// Returns all of the Components with the specified Component Type.
        /// </summary>
        protected IEnumerable<TComponent> ReadComponents<TComponent>() where TComponent : struct, IComponent
        {
            return ReadComponentsHelper<TComponent>().Select(tuple => tuple.Item2);
        }

        /// <summary>
        /// Returns all of the components of the specified type including an Entity reference for each Component.
        /// </summary>
        protected IEnumerable<(TComponent, Entity)> ReadComponentsIncludingEntity<TComponent>() where TComponent : struct, IComponent
        {
            return ReadComponentsHelper<TComponent>().Select((tuple) => (tuple.Item2, tuple.Item1));
        }

        internal IEnumerable<(TComponent, Entity)> InternalRead<TComponent>() where TComponent : struct, IComponent
        {
            return componentManager.GetComponentsIncludingEntity<TComponent>();
        }

        private (Entity, TComponent) ReadComponentHelper<TComponent>() where TComponent : struct, IComponent
        {
            var pendingRead = readPendingTypes.Contains(typeof(TComponent));
            var existingRead = readTypes.Contains(typeof(TComponent));
            if (existingRead && pendingRead)
            {
                return componentUpdateManager.ReadFirstExistingOrPendingComponentByType<TComponent>();
            }
            else if (existingRead)
            {
                return componentUpdateManager.ReadFirstExistingComponentByType<TComponent>();
            }
            else if (pendingRead)
            {
                return componentUpdateManager.ReadFirstPendingComponentByType<TComponent>();
            }
            else
            {
                throw new IllegalReadException("Engine {0} tried to read undeclared Component {1}", GetType().Name, typeof(TComponent).Name);
            }
        }

        /// <summary>
        /// Returns a Component with the specified Component Type. If multiples exist, an arbitrary Component is returned.
        /// </summary>
        protected TComponent ReadComponent<TComponent>() where TComponent : struct, IComponent
        {
            return ReadComponentHelper<TComponent>().Item2;
        }

        /// <summary>
        /// Returns a component of the specified type including its Entity reference. If multiples exist, an arbitrary Component is returned.
        /// </summary>
        protected (TComponent, Entity) ReadComponentIncludingEntity<TComponent>() where TComponent : struct, IComponent
        {
            var (entity, component) = ReadComponentHelper<TComponent>();
            return (component, entity);
        }

        /// <summary>
        /// Returns true if any Component with the specified Component Type exists.
        /// </summary>
        protected bool SomeComponent<TComponent>() where TComponent : struct, IComponent
        {
            var pendingRead = readPendingTypes.Contains(typeof(TComponent));
            var existingRead = readTypes.Contains(typeof(TComponent));
            if (existingRead && pendingRead)
            {
                return componentUpdateManager.SomeExistingOrPendingComponent<TComponent>();
            }
            else if (existingRead)
            {
                return componentUpdateManager.SomeExistingComponent<TComponent>();
            }
            else if (pendingRead)
            {
                return componentUpdateManager.SomePendingComponent<TComponent>();
            }
            else
            {
                throw new IllegalReadException("Engine {0} tried to read undeclared Component {1}", GetType().Name, typeof(TComponent).Name);
            }
        }

        private TComponent GetComponentHelper<TComponent>(Entity entity) where TComponent : struct, IComponent
        {
            var pendingRead = readPendingTypes.Contains(typeof(TComponent));
            var existingRead = readTypes.Contains(typeof(TComponent));
            if (existingRead && pendingRead)
            {
                if (componentUpdateManager.HasPendingComponent<TComponent>(entity))
                {
                    return componentUpdateManager.ReadPendingComponentByEntityAndType<TComponent>(entity);
                }
                else if (componentUpdateManager.HasExistingComponent<TComponent>(entity))
                {
                    return componentUpdateManager.ReadExistingComponentByEntityAndType<TComponent>(entity);
                }
                else
                {
                    throw new NoComponentOfTypeOnEntityException("No Component of type {0} exists on Entity {1}", typeof(TComponent).Name, entity.ID);
                }
            }
            else if (existingRead)
            {
                return componentUpdateManager.ReadExistingComponentByEntityAndType<TComponent>(entity);
            }
            else if (pendingRead)
            {
                return componentUpdateManager.ReadPendingComponentByEntityAndType<TComponent>(entity);
            }
            else
            {
                throw new IllegalReadException("Engine {0} tried to read undeclared Component {1}", GetType().Name, typeof(TComponent).Name);
            }
        }

        /// <summary>
        /// Returns a Component with the specified Type that exists on the Entity.
        /// </summary>
        /// <exception cref="Encompass.Exceptions.NoComponentOfTypeOnEntityException">
        /// Thrown when the Entity does not have a Component of the specified Type
        /// </exception>
        /// <exception cref="Encompass.Exceptions.IllegalReadException">
        /// Thrown when the Engine does not declare that it reads the given Component Type.
        /// </exception>
        protected TComponent GetComponent<TComponent>(Entity entity) where TComponent : struct, IComponent
        {
            return GetComponentHelper<TComponent>(entity);
        }

        /// <summary>
        /// Returns true if the Entity has a Component of the given Type.
        /// </summary>
        /// <exception cref="Encompass.Exceptions.IllegalReadException">
        /// Thrown when the Engine does not declare that is Reads the given Component Type.
        /// </exception>
        protected bool HasComponent<TComponent>(Entity entity) where TComponent : struct, IComponent
        {
            var pendingRead = readPendingTypes.Contains(typeof(TComponent));
            var existingRead = readTypes.Contains(typeof(TComponent));

            if (pendingRead && existingRead)
            {
                return componentUpdateManager.HasExistingOrPendingComponent<TComponent>(entity);
            }
            else if (existingRead)
            {
                return componentUpdateManager.HasExistingComponent<TComponent>(entity);
            }
            else if (pendingRead)
            {
                return componentUpdateManager.HasPendingComponent<TComponent>(entity);
            }
            else
            {
                throw new IllegalReadException("Engine {0} tried to read undeclared Component {1}", GetType().Name, typeof(TComponent).Name);
            }
        }

        /// <summary>
        /// Returns true if the Entity has a Component of the given Type.
        /// </summary>
        /// <exception cref="Encompass.Exceptions.IllegalReadException">
        /// Thrown when the Engine does not declare that is Reads the given Component Type.
        /// </exception>
        protected bool HasComponent(Entity entity, Type type)
        {
            var pendingRead = readPendingTypes.Contains(type);
            var existingRead = readTypes.Contains(type);

            if (pendingRead && existingRead)
            {
                return componentUpdateManager.HasExistingOrPendingComponent(entity, type);
            }
            else if (existingRead)
            {
                return componentUpdateManager.HasExistingComponent(entity, type);
            }
            else if (pendingRead)
            {
                return componentUpdateManager.HasPendingComponent(entity, type);
            }
            else
            {
                throw new IllegalReadException("Engine {0} tried to read undeclared Component {1}", GetType().Name, type.Name);
            }
        }

        /// <summary>
        /// Sets Component data for the specified Component Type on the specified Entity. If Component data for this Type already existed on the Entity, the component data is overwritten.
        /// </summary>
        /// <exception cref="Encompass.Exceptions.IllegalWriteException">
        /// Thrown when the Engine does not declare that it Writes the given Component Type.
        /// </exception>
        protected void SetComponent<TComponent>(Entity entity, TComponent component) where TComponent : struct, IComponent
        {
            var priority = writePriorities.ContainsKey(typeof(TComponent)) ? writePriorities[typeof(TComponent)] : defaultWritePriority;

            if (!writeTypes.Contains(typeof(TComponent)))
            {
                throw new IllegalWriteException("Engine {0} tried to update undeclared Component {1}", GetType().Name, typeof(TComponent).Name);
            }

            bool written;
            if (writePendingTypes.Contains(typeof(TComponent)))
            {
                written = AddPendingComponent(entity, component, priority);
            }
            else
            {
                written = componentUpdateManager.UpdateComponent(entity, component, priority);
            }

            if (written && component is IDrawableComponent drawableComponent)
            {
                componentManager.RegisterDrawableComponent(entity, component, drawableComponent.Layer);
            }
        }

        /// <summary>
        /// Sends a Message.
        /// </summary>
        /// <exception cref="Encompass.Exceptions.IllegalSendException">
        /// Thrown when the Engine does not declare that it Sends the Message Type.
        /// </exception>
        protected void SendMessage<TMessage>(TMessage message) where TMessage : struct, IMessage
        {
            if (!sendTypes.Contains(typeof(TMessage)))
            {
                throw new IllegalSendException("Engine {0} tried to send undeclared Message {1}", GetType().Name, typeof(TMessage).Name);
            }

            messageManager.AddMessage(message);
        }

        /// <summary>
        /// Sends a message after the specified number of seconds, respecting time dilation.
        /// </summary>
        /// <param name="time">The time in seconds that will elapse before the message is sent.</param>
        protected void SendMessage<TMessage>(TMessage message, double time) where TMessage : struct, IMessage
        {
            messageManager.AddMessage(message, time);
        }

        /// <summary>
        /// Sends a message after the specified number of seconds, ignoring time dilation.
        /// </summary>
        /// <param name="time">The time in seconds that will elapse before the message is sent.</param>
        protected void SendMessageIgnoringTimeDilation<TMessage>(TMessage message, double time) where TMessage : struct, IMessage
        {
            messageManager.AddMessageIgnoringTimeDilation(message, time);
        }

        internal void AddExistingComponent<TComponent>(Entity entity, TComponent component) where TComponent : struct, IComponent
        {
            componentUpdateManager.AddExistingComponent(entity, component);
        }

        internal bool AddPendingComponent<TComponent>(Entity entity, TComponent component, int priority) where TComponent : struct, IComponent
        {
            return componentUpdateManager.AddPendingComponent(entity, component, priority);
        }

        /// <summary>
        /// Reads all messages of the specified Type.
        /// </summary>
        /// <exception cref="Encompass.Exceptions.IllegalReadException">
        /// Thrown when the Engine does not declare that it Receives the specified Message Type.
        /// </exception>
        protected IEnumerable<TMessage> ReadMessages<TMessage>() where TMessage : struct, IMessage
        {
            if (!receiveTypes.Contains(typeof(TMessage)))
            {
                throw new IllegalReadException("Engine {0} tried to read undeclared Message {1}", this.GetType().Name, typeof(TMessage).Name);
            }

            return messageManager.GetMessagesByType<TMessage>();
        }

        /// <summary>
        /// Reads an arbitrary message of the specified Type.
        /// </summary>
        /// <exception cref="Encompass.Exceptions.IllegalReadException">
        /// Thrown when the Engine does not declare that it Receives the specified Message Type.
        /// </exception>
        protected TMessage ReadMessage<TMessage>() where TMessage : struct, IMessage
        {
            return messageManager.First<TMessage>();
        }

        /// <summary>
        /// Returns true if a Message of the specified Type has been sent this frame.
        /// </summary>
        /// <exception cref="Encompass.Exceptions.IllegalReadException">
        /// Thrown when the Engine does not declare that it Receives the specified Message Type.
        /// </exception>
        protected bool SomeMessage<TMessage>() where TMessage : struct, IMessage
        {
            if (!receiveTypes.Contains(typeof(TMessage)))
            {
                throw new IllegalReadException("Engine {0} tried to read undeclared Message {1}", GetType().Name, typeof(TMessage).Name);
            }

            return messageManager.Any<TMessage>();
        }

        /// <summary>
        /// Destroys the specified Entity. This also removes all of the Components associated with the Entity.
        /// Entity destruction takes place after all the Engines have been processed by World Update.
        /// </summary>
        protected void Destroy(Entity entity)
        {
            entityManager.MarkForDestroy(entity);
        }

        /// <summary>
        /// Destroys an arbitrary Entity containing a Component of the specified Type.
        /// Entity destruction takes place after all the Engines have been processed by World Update.
        /// </summary>
        protected void DestroyWith<TComponent>() where TComponent : struct, IComponent
        {
            Destroy(ReadEntity<TComponent>());
        }

        /// <summary>
        /// Destroys all Entities containing a Component of the specified Type.
        /// Entity destruction takes place after all the Engines have been processed by World Update.
        /// </summary>
        protected void DestroyAllWith<TComponent>() where TComponent : struct, IComponent
        {
            foreach (var entity in ReadEntities<TComponent>())
            {
                Destroy(entity);
            }
        }

        /// <summary>
        /// Removes a Component with the specified type from the given Entity.
        /// Note that the Engine must Read the Component type that is being removed.
        /// If a Component with the specified type does not exist on the Entity, returns false and does not mutate the Entity.
        /// </summary>
        protected bool RemoveComponent<TComponent>(Entity entity) where TComponent : struct, IComponent
        {
            if (!HasComponent<TComponent>(entity)) { return false; }

            componentManager.Remove<TComponent>(entity);
            return true;
        }

        /// <summary>
        /// Activates the Encompass time dilation system.
        /// Engines that have the IgnoresTimeDilation property will ignore all time dilation.
        /// If multiple time dilations are active they will be averaged.
        /// </summary>
        /// <param name="factor">The time dilation factor, which is multiplied by real delta time.</param>
        /// <param name="easeInTime">The time that will elapse before time is fully dilated, in real time.</param>
        /// <param name="activeTime">The length of real time that time will be fully dilated.</param>
        /// <param name="easeOutTime">The time that will elapse before time is fully undilated.</param>
        public void ActivateTimeDilation(double factor, double easeInTime, double activeTime, double easeOutTime)
        {
            timeManager.ActivateTimeDilation(factor, easeInTime, activeTime, easeOutTime);
        }

        /// <summary>
        /// Activates the Encompass time dilation system.
        /// Engines that have the IgnoresTimeDilation property will ignore all time dilation.
        /// If multiple time dilations are active they will be averaged.
        /// </summary>
        /// <param name="factor">The time dilation factor, which is multiplied by real delta time.</param>
        /// <param name="easeInTime">The time that will elapse before time is fully dilated, in real time.</param>
        /// <param name="easeInFunction">An easing function for the easing in of time dilation.</param>
        /// <param name="activeTime">The length of real time that time will be fully dilated.</param>
        /// <param name="easeOutTime">The time that will elapse before time is fully undilated.</param>
        public void ActivateTimeDilation(double factor, double easeInTime, System.Func<double, double, double, double, double> easeInFunction, double activeTime, double easeOutTime)
        {
            timeManager.ActivateTimeDilation(factor, easeInTime, easeInFunction, activeTime, easeOutTime);
        }

        /// <summary>
        /// Activates the Encompass time dilation system.
        /// Engines that have the IgnoresTimeDilation property will ignore all time dilation.
        /// If multiple time dilations are active they will be averaged.
        /// </summary>
        /// <param name="factor">The time dilation factor, which is multiplied by real delta time.</param>
        /// <param name="easeInTime">The time that will elapse before time is fully dilated, in real time.</param>
        /// <param name="activeTime">The length of real time that time will be fully dilated.</param>
        /// <param name="easeOutTime">The time that will elapse before time is fully undilated.</param>
        /// <param name="easeOutFunction">An easing function for the easing out of time dilation.</param>
        public void ActivateTimeDilation(double factor, double easeInTime, double activeTime, double easeOutTime, System.Func<double, double, double, double, double> easeOutFunction)
        {
            timeManager.ActivateTimeDilation(factor, easeInTime, activeTime, easeOutTime, easeOutFunction);
        }

        /// <summary>
        /// Activates the Encompass time dilation system.
        /// Engines that have the IgnoresTimeDilation property will ignore all time dilation.
        /// If multiple time dilations are active they will be averaged.
        /// </summary>
        /// <param name="factor">The time dilation factor, which is multiplied by real delta time.</param>
        /// <param name="easeInTime">The time that will elapse before time is fully dilated, in real time.</param>
        /// <param name="easeInFunction">An easing function for the easing in of time dilation.</param>
        /// <param name="activeTime">The length of real time that time will be fully dilated.</param>
        /// <param name="easeOutTime">The time that will elapse before time is fully undilated.</param>
        /// <param name="easeOutFunction">An easing function for the easing out of time dilation.</param>
        public void ActivateTimeDilation(double factor, double easeInTime, System.Func<double, double, double, double, double> easeInFunction, double activeTime, double easeOutTime, System.Func<double, double, double, double, double> easeOutFunction)
        {
            timeManager.ActivateTimeDilation(factor, easeInTime, easeInFunction, activeTime, easeOutTime, easeOutFunction);
        }
    }
}