using System;
using System.Collections.Generic;
using System.Runtime.InteropServices;

namespace MoonTools.ECS
{
	internal abstract class ComponentStorage
	{
		public abstract bool Has(int entityID);
		public abstract bool Remove(int entityID);
		public abstract object Debug_Get(int entityID);
		public abstract ComponentStorageState CreateState();
		public abstract void Save(ComponentStorageState state);
		public abstract void Load(ComponentStorageState state);
	}

	internal class ComponentStorage<TComponent> : ComponentStorage where TComponent : unmanaged
	{
		private int nextID;
		private readonly Dictionary<int, int> entityIDToStorageIndex = new Dictionary<int, int>(16);
		private int[] entityIDs = new int[16];
		private TComponent[] components = new TComponent[16];

		public bool Any()
		{
			return nextID > 0;
		}

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

		public ref readonly TComponent Get(int entityID)
		{
			return ref components[entityIDToStorageIndex[entityID]];
		}

		public override object Debug_Get(int entityID)
		{
			return components[entityIDToStorageIndex[entityID]];
		}

		public ref readonly TComponent Get()
		{
#if DEBUG
			if (nextID == 0)
			{
				throw new IndexOutOfRangeException("Component storage is empty!");
			}
#endif
			return ref components[0];
		}

		// Returns true if the entity already had this component.
		public bool Set(int entityID, in TComponent component)
		{
			bool result = true;

			if (!entityIDToStorageIndex.ContainsKey(entityID))
			{
				var index = nextID;
				nextID += 1;

				if (index >= components.Length)
				{
					Array.Resize(ref components, components.Length * 2);
					Array.Resize(ref entityIDs, entityIDs.Length * 2);
				}

				entityIDToStorageIndex[entityID] = index;
				entityIDs[index] = entityID;

				result = false;
			}

			components[entityIDToStorageIndex[entityID]] = component;

			return result;
		}

		// Returns true if the entity had this component.
		public override bool Remove(int entityID)
		{
			if (entityIDToStorageIndex.ContainsKey(entityID))
			{
				var storageIndex = entityIDToStorageIndex[entityID];
				entityIDToStorageIndex.Remove(entityID);

				var lastElementIndex = nextID - 1;

				// move a component into the hole to maintain contiguous memory
				if (lastElementIndex != storageIndex)
				{
					var lastEntityID = entityIDs[lastElementIndex];
					entityIDToStorageIndex[lastEntityID] = storageIndex;
					components[storageIndex] = components[lastElementIndex];
					entityIDs[storageIndex] = lastEntityID;
				}

				nextID -= 1;

				return true;
			}

			return false;
		}

		public void Clear()
		{
			nextID = 0;
			entityIDToStorageIndex.Clear();
		}

		public ReadOnlySpan<TComponent> AllComponents()
		{
			return new ReadOnlySpan<TComponent>(components, 0, nextID);
		}

		public Entity FirstEntity()
		{
#if DEBUG
			if (nextID == 0)
			{
				throw new IndexOutOfRangeException("Component storage is empty!");
			}
#endif
			return new Entity(entityIDs[0]);
		}

		public override ComponentStorageState CreateState()
		{
			return ComponentStorageState.Create<TComponent>(nextID);
		}

		public override void Save(ComponentStorageState state)
		{
			ReadOnlySpan<byte> entityIDBytes = MemoryMarshal.Cast<int, byte>(new ReadOnlySpan<int>(entityIDs, 0, nextID));

			if (entityIDBytes.Length > state.EntityIDs.Length)
			{
				Array.Resize(ref state.EntityIDs, entityIDBytes.Length);
			}
			entityIDBytes.CopyTo(state.EntityIDs);

			ReadOnlySpan<byte> componentBytes = MemoryMarshal.Cast<TComponent, byte>(AllComponents());
			if (componentBytes.Length > state.Components.Length)
			{
				Array.Resize(ref state.Components, componentBytes.Length);
			}
			componentBytes.CopyTo(state.Components);

			state.Count = nextID;
		}

		public override void Load(ComponentStorageState state)
		{
			state.EntityIDs.CopyTo(MemoryMarshal.Cast<int, byte>(entityIDs));
			state.Components.CopyTo(MemoryMarshal.Cast<TComponent, byte>(components));

			entityIDToStorageIndex.Clear();
			for (var i = 0; i < state.Count; i += 1)
			{
				entityIDToStorageIndex[entityIDs[i]] = i;
			}

			nextID = state.Count;
		}
	}
}