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>
    {
        public Guid ID;

        internal readonly HashSet<Type> sendTypes = new HashSet<Type>();
        internal readonly HashSet<Type> receiveTypes = new HashSet<Type>();
        internal readonly Dictionary<Type, int> writePriorities = new Dictionary<Type, int>();

        private EntityManager entityManager;
        private MessageManager messageManager;
        private ComponentManager componentManager;
        private ComponentMessageManager componentMessageManager;

        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)
            {
                sendTypes.UnionWith(activatesAttribute.writePendingTypes);
            }

            var writesAttributes = GetType().GetCustomAttributes<Writes>(false);
            foreach (var writesAttribute in writesAttributes)
            {
                sendTypes.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)
            {
                receiveTypes.UnionWith(readsAttribute.readTypes);
            }

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

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

            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 AssignComponentMessageManager(ComponentMessageManager componentMessageManager)
        {
            this.componentMessageManager = componentMessageManager;
        }

        /// <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(Guid entityID)
        {
            return entityManager.EntityExists(entityID);
        }

        /// <summary>
        /// Returns the Entity with the specified ID.
        /// </summary>
        /// <exception cref="Encompass.Exceptions.EntityNotFoundException">
        /// Thrown when an Entity with the given ID does not exist.
        /// </exception>
        protected Entity GetEntity(Guid entityID)
        {
            return entityManager.GetEntity(entityID);
        }

        /// <summary>
        /// Returns the Entity ID associated with the specified Component Type and ID.
        /// </summary>
        protected Guid GetEntityIDByComponentID<TComponent>(Guid componentID) where TComponent : struct, IComponent
        {
            var pendingRead = receiveTypes.Contains(typeof(PendingComponentMessage<TComponent>));
            var existingRead = receiveTypes.Contains(typeof(ComponentMessage<TComponent>));

            if (!pendingRead && !existingRead)
            {
                throw new IllegalReadException("Engine {0} tried to read undeclared Component {1}", GetType().Name, typeof(TComponent).Name);
            }

            return componentMessageManager.GetEntityIDByComponentID(componentID);
        }

        /// <summary>
        /// Returns the Entity associated with the specified Component Type and ID.
        /// </summary>
        protected Entity GetEntityByComponentID<TComponent>(Guid componentID) where TComponent : struct, IComponent
        {
            return GetEntity(GetEntityIDByComponentID<TComponent>(componentID));
        }

        /// <summary>
        /// Returns an Entity containing the specified Component type. 
        /// </summary>
        protected Entity ReadEntity<TComponent>() where TComponent : struct, IComponent
        {
            var (id, component) = ReadComponent<TComponent>();
            return GetEntityByComponentID<TComponent>(id);
        }

        /// <summary>
        /// Returns all Entities containing the specified Component type. 
        /// </summary>
        protected IEnumerable<Entity> ReadEntities<TComponent>() where TComponent : struct, IComponent
        {
            foreach (var (id, component) in ReadComponents<TComponent>())
            {
                yield return GetEntityByComponentID<TComponent>(id);
            }
        }

        /// <summary>
        /// Returns the Component struct with the specified Component Type and ID.
        /// </summary>
        protected TComponent GetComponentByID<TComponent>(Guid componentID) where TComponent : struct, IComponent
        {
            var pendingRead = receiveTypes.Contains(typeof(PendingComponentMessage<TComponent>));
            var existingRead = receiveTypes.Contains(typeof(ComponentMessage<TComponent>));

            if (!pendingRead && !existingRead)
            {
                throw new IllegalReadException("Engine {0} tried to read undeclared Component {1}", GetType().Name, typeof(TComponent).Name);
            }

            if (componentMessageManager.GetComponentTypeByID(componentID) != typeof(TComponent))
            {
                throw new ComponentTypeMismatchException("Expected Component to be of type {0} but was actually of type {1}", typeof(TComponent).Name, componentMessageManager.GetComponentTypeByID(componentID).Name);
            }

            return (TComponent)componentMessageManager.GetComponentByID(componentID);
        }

        // these next two are for the ComponentMessageEmitter only

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

        internal Entity ReadEntityFromWorld(Guid componentID)
        {
            return GetEntity(componentManager.GetEntityIDByComponentID(componentID));
        }

        /// <summary>
        /// Returns all of the Components with the specified Component Type.
        /// </summary>
        protected IEnumerable<(Guid, TComponent)> ReadComponents<TComponent>() where TComponent : struct, IComponent
        {
            var pendingRead = receiveTypes.Contains(typeof(PendingComponentMessage<TComponent>));
            var existingRead = receiveTypes.Contains(typeof(ComponentMessage<TComponent>));
            if (existingRead && pendingRead)
            {
                return componentMessageManager.ReadExistingAndPendingComponentsByType<TComponent>();
            }
            else if (existingRead)
            {
                return componentMessageManager.ReadExistingComponentsByType<TComponent>();
            }
            else if (pendingRead)
            {
                return componentMessageManager.ReadPendingComponentsByType<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 and ID. If multiples exist, an arbitrary Component is returned. 
        /// </summary>
        protected (Guid, TComponent) ReadComponent<TComponent>() where TComponent : struct, IComponent
        {
            var pendingRead = receiveTypes.Contains(typeof(PendingComponentMessage<TComponent>));
            var existingRead = receiveTypes.Contains(typeof(ComponentMessage<TComponent>));
            if (existingRead && pendingRead)
            {
                return componentMessageManager.ReadFirstExistingOrPendingComponentByType<TComponent>();
            }
            else if (existingRead)
            {
                return componentMessageManager.ReadFirstExistingComponentByType<TComponent>();
            }
            else if (pendingRead)
            {
                return componentMessageManager.ReadFirstPendingComponentByType<TComponent>();
            }
            else
            {
                throw new IllegalReadException("Engine {0} tried to read undeclared Component {1}", GetType().Name, typeof(TComponent).Name);
            }
        }

        /// <summary>
        /// Returns true if any Component with the specified Component Type exists.
        /// </summary>
        protected bool SomeComponent<TComponent>() where TComponent : struct, IComponent
        {
            var pendingRead = receiveTypes.Contains(typeof(PendingComponentMessage<TComponent>));
            var existingRead = receiveTypes.Contains(typeof(ComponentMessage<TComponent>));
            if (existingRead && pendingRead)
            {
                return componentMessageManager.SomeExistingOrPendingComponent<TComponent>();
            }
            else if (existingRead)
            {
                return componentMessageManager.SomeExistingComponent<TComponent>();
            }
            else if (pendingRead)
            {
                return componentMessageManager.SomePendingComponent<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 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 (Guid, TComponent) GetComponent<TComponent>(Entity entity) where TComponent : struct, IComponent
        {
            var pendingRead = receiveTypes.Contains(typeof(PendingComponentMessage<TComponent>));
            var existingRead = receiveTypes.Contains(typeof(ComponentMessage<TComponent>));
            if (existingRead && pendingRead)
            {
                if (componentMessageManager.HasPendingComponent<TComponent>(entity))
                {
                    return componentMessageManager.ReadPendingComponentByEntityAndType<TComponent>(entity);
                }
                else if (componentMessageManager.HasExistingComponent<TComponent>(entity))
                {
                    return componentMessageManager.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 componentMessageManager.ReadExistingComponentByEntityAndType<TComponent>(entity);
            }
            else if (pendingRead)
            {
                return componentMessageManager.ReadPendingComponentByEntityAndType<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<TComponent>(Entity entity) where TComponent : struct, IComponent
        {
            var pendingRead = receiveTypes.Contains(typeof(PendingComponentMessage<TComponent>));
            var existingRead = receiveTypes.Contains(typeof(ComponentMessage<TComponent>));

            if (pendingRead && existingRead)
            {
                return componentMessageManager.HasExistingOrPendingComponent<TComponent>(entity);
            }
            else if (existingRead)
            {
                return componentMessageManager.HasExistingComponent<TComponent>(entity);
            }
            else if (pendingRead)
            {
                return componentMessageManager.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 pending = typeof(PendingComponentMessage<>).MakeGenericType(type);
            var existing = typeof(ComponentMessage<>).MakeGenericType(type);

            var pendingRead = receiveTypes.Contains(pending);
            var existingRead = receiveTypes.Contains(existing);

            if (pendingRead && existingRead)
            {
                return componentMessageManager.HasExistingOrPendingComponent(entity, type);
            }
            else if (existingRead)
            {
                return componentMessageManager.HasExistingComponent(entity, type);
            }
            else if (pendingRead)
            {
                return componentMessageManager.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 Guid SetComponent<TComponent>(Entity entity, TComponent component) where TComponent : struct, IComponent
        {
            var priority = writePriorities.ContainsKey(typeof(TComponent)) ? writePriorities[typeof(TComponent)] : 0;

            var componentID = componentManager.MarkComponentForWrite(entity, component, priority);

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

            if (sendTypes.Contains(typeof(PendingComponentMessage<TComponent>)))
            {
                PendingComponentMessage<TComponent> newComponentMessage;
                newComponentMessage.entity = entity;
                newComponentMessage.componentID = componentID;
                newComponentMessage.component = component;
                newComponentMessage.priority = priority;
                SendPendingComponentMessage(newComponentMessage);
            }

            return componentID;
        }

        /// <summary>
        /// Overwrites Component struct data associated with the specified Component ID. 
        /// </summary>
        protected Guid SetComponent<TComponent>(Guid componentID, TComponent component) where TComponent : struct, IComponent
        {
            return SetComponent(GetEntityByComponentID<TComponent>(componentID), component);
        }

        /// <summary>
        /// Sets Draw Component data for the specified Component Type on the specified Entity. 
        /// This method must be used for the Draw Component to be readable by an OrderedRenderer.
        /// 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 Guid SetDrawComponent<TComponent>(Entity entity, TComponent component, int layer = 0) where TComponent : struct, IComponent, IDrawComponent
        {
            var priority = writePriorities.ContainsKey(typeof(TComponent)) ? writePriorities[typeof(TComponent)] : 0;

            var componentID = componentManager.MarkDrawComponentForWrite(entity, component, priority, layer);

            if (!sendTypes.Contains(typeof(ComponentWriteMessage<TComponent>)))
            {
                throw new IllegalWriteException("Engine {0} tried to write undeclared Component {1}", GetType().Name, typeof(TComponent).Name);
            }

            if (sendTypes.Contains(typeof(PendingComponentMessage<TComponent>)))
            {
                PendingComponentMessage<TComponent> newComponentMessage;
                newComponentMessage.entity = entity;
                newComponentMessage.componentID = componentID;
                newComponentMessage.component = component;
                newComponentMessage.priority = priority;
                SendPendingComponentMessage(newComponentMessage);
            }

            return componentID;
        }

        /// <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.
        /// </summary>
        /// <param name="time">The time in seconds that will elapse before the message is sent.</param>
        protected void SendMessageDelayed<TMessage>(TMessage message, double time) where TMessage : struct, IMessage
        {
            messageManager.AddMessageDelayed(message, time);
        }

        // unparameterized version to enable dynamic dispatch
        protected void SendMessage(IMessage message)
        {
            var type = message.GetType();

            if (!sendTypes.Contains(type) || !type.IsValueType)
            {
                throw new IllegalSendException("Engine {0} tried to send undeclared Message {1}", GetType().Name, type.Name);
            }

            messageManager.AddMessage(message);
        }

        internal void SendExistingComponentMessage<TComponent>(ComponentMessage<TComponent> message) where TComponent : struct, IComponent
        {
            componentMessageManager.AddExistingComponentMessage(message);
        }

        internal void SendPendingComponentMessage<TComponent>(PendingComponentMessage<TComponent> message) where TComponent : struct, IComponent
        {
            componentMessageManager.AddPendingComponentMessage(message);
        }

        /// <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 ReadMessages<TMessage>().First();
        }

        /// <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 ReadMessages<TMessage>().Any();
        }

        /// <summary>
        /// Destroys the Entity with the specified ID. 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(Guid entityID)
        {
            entityManager.MarkForDestroy(entityID);
        }

        /// <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.ID);
        }

        /// <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 the Component with the specified ID from its Entity.
        /// </summary>
        protected void RemoveComponent(Guid componentID)
        {
            componentManager.MarkForRemoval(componentID);
        }
    }
}