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

namespace Encompass
{
    internal class EntityManager
    {
        private readonly int entityCapacity;
        private readonly IDManager idManager = new IDManager();
        private readonly HashSet<int> IDs = new HashSet<int>();

        private readonly HashSet<int> entitiesMarkedForDestroy = new HashSet<int>();

        private readonly ComponentManager componentManager;

        public IEnumerable<int> EntityIDs
        {
            get { return IDs; }
        }

        public EntityManager(ComponentManager componentManager, int entityCapacity)
        {
            this.componentManager = componentManager;
            this.entityCapacity = entityCapacity;
        }

        private int NextID()
        {
            return idManager.NextID();
        }

        public Entity CreateEntity()
        {
            if (IDs.Count < entityCapacity)
            {
                var id = NextID();
                var entity = new Entity(id);
                IDs.Add(id);
                return entity;
            }
            else
            {
                throw new EntityOverflowException("The number of entities has exceeded the entity capacity of {0}", entityCapacity);
            }
        }

        public bool EntityExists(int id)
        {
            return IDs.Contains(id);
        }

        public Entity GetEntity(int id)
        {
            if (!EntityExists(id))
            {
                throw new Encompass.Exceptions.EntityNotFoundException("Entity with id {0} does not exist.", id);
            }

            return new Entity(id);
        }

        public void MarkForDestroy(int entityID)
        {
            entitiesMarkedForDestroy.Add(entityID);
        }

        public void DestroyMarkedEntities(IEnumerable<Engine> engines)
        {
            foreach (var entityID in entitiesMarkedForDestroy)
            {
                foreach (var engine in engines) { engine.RegisterDestroyedEntity(entityID); }
                componentManager.MarkAllComponentsOnEntityForRemoval(entityID);
                IDs.Remove(entityID);
                idManager.Free(entityID);
            }

            entitiesMarkedForDestroy.Clear();
        }

        // NOTE: this is very suboptimal
        public void PruneEmptyEntities()
        {
            foreach (var id in EntityIDs)
            {
                if (componentManager.UpToDateEntityIsEmpty(id))
                {
                    MarkForDestroy(id);
                }
            }
        }
    }
}