namespace OECS;

/// <summary>
/// The central container for all ECS state.
///
/// Manages entity lifecycle, component storage, and provides the primary
/// API for adding, removing, and querying components.
/// </summary>
public class World : IDisposable
{
    private readonly EntityAllocator _allocator;
    private readonly ComponentStore _components;
    private readonly CommandQueue _commands;
    private readonly RelationshipIndex _relationships;

    public World()
    {
        _allocator = new EntityAllocator();
        _components = new ComponentStore();
        _commands = new CommandQueue();
        _relationships = new RelationshipIndex();
    }

    // ── Entity Management ────────────────────────────────────────────

    /// <summary>
    /// Creates a new entity and returns its handle.
    /// </summary>
    public Entity CreateEntity()
    {
        return _allocator.Allocate();
    }

    /// <summary>
    /// Destroys an entity, removing all its components and recycling its ID.
    ///
    /// Cascade behavior:
    /// - All relationships where this entity is the <b>source</b> are removed.
    /// - All relationships where this entity is the <b>target</b> are removed
    ///   from the source entities.
    /// </summary>
    public void DestroyEntity(Entity entity)
    {
        if (!_allocator.IsAlive(entity))
            return;

        // Singleton entity is never destroyed.
        if (entity.Id == 1)
            return;

        // Remove all relationships where this entity is the target.
        foreach (var (relType, sources) in _relationships.GetIncomingRelationships(entity))
        {
            foreach (var source in sources)
            {
                _relationships.OnRemoved(relType, source, entity);
                _components.Remove(source, relType);
            }
        }

        // Remove all relationships where this entity is the source.
        _relationships.RemoveAllSourcesForEntity(entity);

        _components.RemoveAll(entity);
        _allocator.Free(entity);
    }

    /// <summary>
    /// Returns true if the entity is currently alive (not destroyed).
    /// </summary>
    public bool IsAlive(Entity entity)
    {
        return _allocator.IsAlive(entity);
    }

    // ── Component Management ─────────────────────────────────────────

    /// <summary>
    /// Adds a component to the entity. Replaces the existing component if
    /// the entity already has one of type <typeparamref name="T"/>.
    ///
    /// If <typeparamref name="T"/> implements <see cref="IRelationship"/>,
    /// the reverse index is updated automatically.
    /// </summary>
    public void AddComponent<T>(Entity entity, T component) where T : struct
    {
        ThrowIfNotAlive(entity);

        // If replacing an existing relationship, remove the old index entry first.
        if (component is IRelationship newRel)
        {
            if (_components.TryGet<T>(entity, out var old))
            {
                var oldRel = (IRelationship)(object)old;
                _relationships.OnRemoved(typeof(T), entity, oldRel.Target);
            }
        }

        _components.Add(entity, component);

        // Update relationship index.
        if (component is IRelationship rel)
        {
            _relationships.OnAdded(typeof(T), entity, rel.Target);
        }
    }

    /// <summary>
    /// Removes the component of type <typeparamref name="T"/> from the entity.
    /// No-op if the entity does not have the component.
    ///
    /// If <typeparamref name="T"/> implements <see cref="IRelationship"/>,
    /// the reverse index is updated automatically.
    /// </summary>
    public void RemoveComponent<T>(Entity entity) where T : struct
    {
        // Update relationship index before removal.
        if (typeof(IRelationship).IsAssignableFrom(typeof(T)))
        {
            if (_components.TryGet<T>(entity, out var existing))
            {
                var rel = (IRelationship)(object)existing;
                _relationships.OnRemoved(typeof(T), entity, rel.Target);
            }
        }

        _components.Remove<T>(entity);
    }

    /// <summary>
    /// Returns a reference to the component of type <typeparamref name="T"/>
    /// for the given entity. Throws if the entity does not have the component.
    /// </summary>
    public ref T GetComponent<T>(Entity entity) where T : struct
    {
        return ref _components.Get<T>(entity);
    }

    /// <summary>
    /// Returns true if the entity has a component of type <typeparamref name="T"/>.
    /// </summary>
    public bool HasComponent<T>(Entity entity) where T : struct
    {
        return _components.Has<T>(entity);
    }

    // ── Relationships ────────────────────────────────────────────────

    /// <summary>
    /// Returns all source entities that have a relationship of type
    /// <typeparamref name="T"/> pointing to the given target entity.
    /// </summary>
    public IReadOnlyCollection<Entity> GetSources<T>(Entity target)
        where T : struct, IRelationship
    {
        return _relationships.GetSources<T>(target);
    }

    // ── Commands ──────────────────────────────────────────────────────

    /// <summary>
    /// The command queue for deferred, serializable commands.
    /// Systems enqueue commands via <c>world.Commands.Enqueue(...)</c>.
    /// </summary>
    public CommandQueue Commands => _commands;

    /// <summary>
    /// Manually drains the command queue, executing all pending commands.
    /// Called automatically by <see cref="SystemGroup"/> after each system
    /// and after the full tick.
    /// </summary>
    public void ExecuteCommands()
    {
        _commands.ExecuteAll(this);
    }

    // ── Queries ──────────────────────────────────────────────────────

    /// <summary>
    /// Returns a new <see cref="QueryBuilder"/> for constructing queries.
    /// </summary>
    public QueryBuilder Query() => new();

    /// <summary>
    /// Iterates all entities matching the query, providing ref access to
    /// one component type.
    /// </summary>
    public void ForEach<T1>(
        QueryDescriptor query,
        ForEachAction<T1> action)
        where T1 : struct
    {
        QueryExecutor.ForEach(_components, query, action);
    }

    /// <summary>
    /// Iterates all entities matching the query, providing ref access to
    /// two component types.
    /// </summary>
    public void ForEach<T1, T2>(
        QueryDescriptor query,
        ForEachAction<T1, T2> action)
        where T1 : struct where T2 : struct
    {
        QueryExecutor.ForEach(_components, query, action);
    }

    /// <summary>
    /// Iterates all entities matching the query, providing ref access to
    /// three component types.
    /// </summary>
    public void ForEach<T1, T2, T3>(
        QueryDescriptor query,
        ForEachAction<T1, T2, T3> action)
        where T1 : struct where T2 : struct where T3 : struct
    {
        QueryExecutor.ForEach(_components, query, action);
    }

    /// <summary>
    /// Iterates all entities matching the query, providing ref access to
    /// four component types.
    /// </summary>
    public void ForEach<T1, T2, T3, T4>(
        QueryDescriptor query,
        ForEachAction<T1, T2, T3, T4> action)
        where T1 : struct where T2 : struct where T3 : struct where T4 : struct
    {
        QueryExecutor.ForEach(_components, query, action);
    }

    /// <summary>
    /// Iterates all entities matching the query, providing ref access to
    /// five component types.
    /// </summary>
    public void ForEach<T1, T2, T3, T4, T5>(
        QueryDescriptor query,
        ForEachAction<T1, T2, T3, T4, T5> action)
        where T1 : struct where T2 : struct where T3 : struct where T4 : struct where T5 : struct
    {
        QueryExecutor.ForEach(_components, query, action);
    }

    /// <summary>
    /// Iterates all entities matching the query, providing ref access to
    /// six component types.
    /// </summary>
    public void ForEach<T1, T2, T3, T4, T5, T6>(
        QueryDescriptor query,
        ForEachAction<T1, T2, T3, T4, T5, T6> action)
        where T1 : struct where T2 : struct where T3 : struct where T4 : struct where T5 : struct where T6 : struct
    {
        QueryExecutor.ForEach(_components, query, action);
    }

    // ── Internal Access ───────────────────────────────────────────────

    /// <summary>
    /// The component store. Exposed internally for query execution and
    /// system infrastructure.
    /// </summary>
    internal ComponentStore Components => _components;

    // ── Helpers ───────────────────────────────────────────────────────

    private void ThrowIfNotAlive(Entity entity)
    {
        if (!_allocator.IsAlive(entity))
            throw new InvalidOperationException($"Entity {entity} is not alive.");
    }

    // ── Cleanup ───────────────────────────────────────────────────────

    public void Dispose()
    {
        // Future: dispose R3 subscriptions, etc.
    }
}