using System;
using System.Collections.Generic;
using System.Runtime.CompilerServices;

namespace Encompass
{
    internal abstract class TypedComponentStore
    {
        public abstract int Count { get; }
        public abstract bool Has(int entity);
        public abstract bool Remove(int entity, int priority);
        public abstract void ForceRemove(int entity);
        public abstract void Clear();
        public abstract void ClearPriorities();
    }

    internal class TypedComponentStore<TComponent> : TypedComponentStore where TComponent : struct
    {
        private int _nextID = 0;
        private readonly Dictionary<int, int> _entityIDToStorageIndex = new Dictionary<int, int>(128);
        private readonly Dictionary<int, int> _priorities = new Dictionary<int, int>(128);
        private Entity[] _storageIndexToEntities = new Entity[128];
        private TComponent[] _components = new TComponent[128];

        public override int Count { get => _entityIDToStorageIndex.Count; }

        public ref TComponent Get(int entityID)
        {
            if (!_entityIDToStorageIndex.ContainsKey(entityID)) { throw new Exceptions.NoComponentOfTypeOnEntityException("No component of type {0} exists on Entity with ID {1}", typeof(TComponent), entityID); }
            return ref _components[_entityIDToStorageIndex[entityID]];
        }

        public ref TComponent Singular()
        {
            return ref _components[0];
        }

        public ref Entity SingularEntity()
        {
            return ref _storageIndexToEntities[0];
        }

        public void Set(int entityID, in TComponent component)
        {
            InternalSet(entityID, component);
        }

        public bool Set(int entityID, in TComponent component, int priority)
        {
            if (!_priorities.ContainsKey(entityID) || priority <= _priorities[entityID]) // if priorities are equal that means it's the same engine
            {
                InternalSet(entityID, component);
                _priorities[entityID] = priority;
                return true;
            }

            return false;
        }

        private void InternalSet(int entityID, in TComponent component)
        {
            if (!_entityIDToStorageIndex.ContainsKey(entityID))
            {
                var index = _nextID++;
                if (index >= _components.Length)
                {
                    System.Array.Resize(ref _components, _components.Length * 2);
                    System.Array.Resize(ref _storageIndexToEntities, _storageIndexToEntities.Length * 2);
                }
                _entityIDToStorageIndex[entityID] = index;
                _storageIndexToEntities[index] = new Entity(entityID);
            }

            _components[_entityIDToStorageIndex[entityID]] = component;
        }

        public override bool Remove(int entityID, int priority)
        {
            if (!_priorities.ContainsKey(entityID) || priority <= _priorities[entityID]) // if priorities are equal that means it's the same engine
            {
                _priorities[entityID] = priority;
                ForceRemove(entityID);
                return true;
            }

            return false;
        }

        public override void ForceRemove(int entityID)
        {
            if (_entityIDToStorageIndex.ContainsKey(entityID))
            {
                var storageIndex = _entityIDToStorageIndex[entityID];
                _entityIDToStorageIndex.Remove(entityID);
                _priorities.Remove(entityID);

                // move a component into the hole to maintain contiguous memory
                if (_nextID > 1 && storageIndex != _nextID - 1)
                {
                    var lastStorageIndex = _nextID - 1;
                    ref readonly var lastEntity = ref _storageIndexToEntities[lastStorageIndex];

                    _entityIDToStorageIndex[lastEntity.ID] = storageIndex;
                    _storageIndexToEntities[storageIndex] = lastEntity;
                    _components[storageIndex] = _components[lastStorageIndex];

                }

                _nextID--;
            }
        }

        public override bool Has(int entityID)
        {
            return _entityIDToStorageIndex.ContainsKey(entityID);
        }

        public override void Clear()
        {
            _nextID = 0;
            _entityIDToStorageIndex.Clear();
            _priorities.Clear();
        }

        public override void ClearPriorities()
        {
            _priorities.Clear();
        }

        public ReadOnlySpan<Entity> AllEntities()
        {
            return new ReadOnlySpan<Entity>(_storageIndexToEntities, 0, _nextID);
        }

        public IEnumerable<Entity> AllEntitiesAsEnumerable()
        {
            return new ArraySegment<Entity>(_storageIndexToEntities, 0, _nextID);
        }

        public ReadOnlySpan<TComponent> AllComponents()
        {
            return new ReadOnlySpan<TComponent>(_components, 0, _nextID);
        }

        public IEnumerable<TComponent> AllComponentsAsEnumerable()
        {
            return new ArraySegment<TComponent>(_components, 0, _nextID);
        }
    }
}