using System.Diagnostics;
using R3;

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;
    private readonly ChangeBuffer _changes;
    private readonly Dictionary<Type, Entity> _singletonEntities = new();
    private bool _disposed;

    // Deferred structural mutation support: when iterating entities,
    // AddComponent, RemoveComponent, and DestroyEntity are buffered and
    // applied after the iteration completes.
    private int _iterationDepth;
    private readonly List<PendingMutation> _pendingMutations = new();

    private enum PendingMutationKind { AddComponent, RemoveComponent, DestroyEntity }

    private struct PendingMutation
    {
        public PendingMutationKind Kind;
        public Entity Entity;
        public object? Component; // boxed struct for AddComponent
        public Type ComponentType;
    }

    // Auto-dirty-marking: during iteration, component accesses via
    // GetComponent<T> are tracked. When the iteration scope ends,
    // all accessed components are automatically marked as modified.
    // Outside of iteration, MarkModified<T> must be called explicitly.
    private readonly HashSet<(Entity Entity, Type ComponentType)> _accessedComponents = new();
    private readonly HashSet<(Entity Entity, Type ComponentType)> _markedModified = new();

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

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

    /// <summary>
    /// Creates a new entity and returns its handle.
    /// Automatically marks an <see cref="ChangeKind.EntityAdded"/> change.
    /// </summary>
    public Entity CreateEntity()
    {
        var entity = _allocator.Allocate();
        _changes.Pending.MarkEntityAdded(entity);
        return entity;
    }

    /// <summary>
    /// Creates an entity with a specific handle. Used during deserialization
    /// to restore entities with their original IDs and versions.
    /// Does NOT mark an EntityAdded change (caller is responsible for
    /// posting changes after the full load).
    /// </summary>
    internal Entity CreateEntity(Entity handle)
    {
        _allocator.Reserve(handle);
        return handle;
    }

    /// <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 (_iterationDepth > 0)
        {
            _pendingMutations.Add(new PendingMutation
            {
                Kind = PendingMutationKind.DestroyEntity,
                Entity = entity
            });
            return;
        }

        DestroyEntityImpl(entity);
    }

    private void DestroyEntityImpl(Entity entity)
    {
        if (!_allocator.IsAlive(entity))
            return;

        // Remove all relationships where this entity is the target.
        // Materialize to avoid collection-modified-during-enumeration when
        // OnRemoved mutates the same HashSet we're iterating.
        var incoming = _relationships.GetIncomingRelationships(entity)
            .Select(r => (r.RelationshipType, r.Sources.ToList()))
            .ToList();
        foreach (var (relType, sources) in incoming)
        {
            foreach (var source in sources)
            {
                _relationships.OnRemoved(relType, source, entity);
                _components.Remove(source, relType);
                _changes.Pending.MarkComponentRemoved(source, relType);
            }
        }

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

        // Mark component removals for each component type before removing them.
        foreach (var componentType in _components.ComponentTypes.ToList())
        {
            if (_components.GetSet(componentType)?.Contains(entity) == true)
            {
                _changes.Pending.MarkComponentRemoved(entity, componentType);
            }
        }

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

        _changes.Pending.MarkEntityRemoved(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
    {
        if (_iterationDepth > 0)
        {
            _pendingMutations.Add(new PendingMutation
            {
                Kind = PendingMutationKind.AddComponent,
                Entity = entity,
                Component = component,
                ComponentType = typeof(T)
            });
            return;
        }

        AddComponentImpl(entity, component);
    }

    /// <summary>
    /// Adds a boxed component to the entity. Used by <see cref="WorldSerializer"/>
    /// during deserialization after relationship Source fixup.
    /// Calls <see cref="AddComponentImpl{T}"/> via reflection to avoid
    /// duplicating the relationship index logic.
    /// </summary>
    internal void AddComponentBoxed(Entity entity, object component, Type componentType)
    {
        var method = s_addComponentImpl.MakeGenericMethod(componentType);
        method.Invoke(this, [entity, component]);
    }

    private static readonly System.Reflection.MethodInfo s_addComponentImpl =
        typeof(World).GetMethod(
            nameof(AddComponentImpl),
            System.Reflection.BindingFlags.NonPublic | System.Reflection.BindingFlags.Instance)!;

    private void AddComponentImpl<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)
        {
            if (_components.TryGet<T>(entity, out var old))
            {
                var oldRel = (IRelationship)(object)old;
                _relationships.OnRemoved(typeof(T), entity, oldRel.Target);
            }
        }

        bool isNew = !_components.Has<T>(entity);
        _components.Add(entity, component);

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

        // Mark change.
        if (isNew)
            _changes.Pending.MarkComponentAdded(entity, typeof(T));
    }

    /// <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
    {
        if (_iterationDepth > 0)
        {
            _pendingMutations.Add(new PendingMutation
            {
                Kind = PendingMutationKind.RemoveComponent,
                Entity = entity,
                ComponentType = typeof(T)
            });
            return;
        }

        RemoveComponentImpl<T>(entity);
    }

    private void RemoveComponentImpl<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);
            }
        }

        bool existed = _components.Has<T>(entity);
        _components.Remove<T>(entity);

        if (existed)
            _changes.Pending.MarkComponentRemoved(entity, typeof(T));
    }

    /// <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.
    ///
    /// During iteration, this access is auto-marked as modified since it
    /// returns a mutable ref. Use <see cref="ReadComponent{T}"/> if you
    /// only need to read the value.
    /// </summary>
    public ref T GetComponent<T>(Entity entity) where T : struct
    {
        if (_iterationDepth > 0)
            _accessedComponents.Add((entity, typeof(T)));
        return ref _components.Get<T>(entity);
    }

    /// <summary>
    /// Returns a copy of the component of type <typeparamref name="T"/>
    /// for the given entity. Never auto-marks as modified — use this when
    /// you only need to read the value.
    /// </summary>
    public T ReadComponent<T>(Entity entity) where T : struct
    {
        return _components.Get<T>(entity);
    }

    /// <summary>
    /// Tries to get the component of type <typeparamref name="T"/> for the
    /// given entity. Returns true and copies the value to
    /// <paramref name="value"/> if the component exists; otherwise returns false.
    /// Never auto-marks as modified.
    /// </summary>
    public bool TryGetComponent<T>(Entity entity, out T value) where T : struct
    {
        return _components.TryGet<T>(entity, out value);
    }

    /// <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);
    }

    // ── Change Tracking ──────────────────────────────────────────────

    /// <summary>
    /// Marks a component of type <typeparamref name="T"/> on the given entity
    /// as modified. Only needed outside of iteration scopes — during a
    /// <c>Select</c> loop, components accessed via <see cref="GetComponent{T}"/>
    /// are auto-marked.
    /// </summary>
    public void MarkModified<T>(Entity entity) where T : struct
    {
        _changes.Pending.MarkComponentModified(entity, typeof(T));
        _markedModified.Add((entity, typeof(T)));
    }

    /// <summary>
    /// Posts all pending changes to R3 subscribers, then clears the pending set.
    /// Called automatically by <see cref="SystemGroup"/> after each system
    /// and after the full tick.
    /// </summary>
    public void PostChanges()
    {
        _changes.Post();
    }

    /// <summary>
    /// Returns an observable that emits all entity changes (adds, removes,
    /// component adds, component removes, component modifications).
    /// </summary>
    public Observable<EntityChange> ObserveEntityChanges()
    {
        return _changes.ObserveEntityChanges();
    }

    /// <summary>
    /// Returns an observable that emits changes for a specific component type
    /// <typeparamref name="T"/> (added, removed, or modified).
    /// </summary>
    public Observable<EntityChange> ObserveComponentChanges<T>() where T : struct
    {
        return _changes.ObserveComponentChanges(typeof(T));
    }

    /// <summary>
    /// Returns an observable that emits component-level changes matching
    /// the given query's With types and excluding its Without types.
    /// </summary>
    public Observable<EntityChange> ObserveQuery<T1>(Query<T1> query = default)
        where T1 : struct
    {
        return _changes.ObserveQuery(query);
    }

    /// <summary>
    /// Returns an observable that emits component-level changes matching
    /// the given query.
    /// </summary>
    public Observable<EntityChange> ObserveQuery<T1, T2>(Query<T1, T2> query = default)
        where T1 : struct where T2 : struct
    {
        return _changes.ObserveQuery(query);
    }

    // ── Singletons ───────────────────────────────────────────────────

    /// <summary>
    /// Sets (adds or replaces) a singleton component of type <typeparamref name="T"/>.
    /// Each singleton component type gets its own dedicated entity.
    /// </summary>
    public void SetSingleton<T>(T component) where T : struct
    {
        if (!_singletonEntities.TryGetValue(typeof(T), out var entity))
        {
            entity = _allocator.Allocate();
            _singletonEntities[typeof(T)] = entity;
        }
        AddComponent(entity, component);
    }

    /// <summary>
    /// Returns a reference to the singleton component of type <typeparamref name="T"/>.
    /// Throws if the singleton has not been set. Auto-marks as modified during iteration.
    /// </summary>
    public ref T GetSingleton<T>() where T : struct
    {
        return ref GetComponent<T>(GetSingletonEntity<T>());
    }

    /// <summary>
    /// Returns a copy of the singleton component of type <typeparamref name="T"/>.
    /// Never auto-marks as modified — use this when you only need to read the singleton.
    /// </summary>
    public T ReadSingleton<T>() where T : struct
    {
        return ReadComponent<T>(GetSingletonEntity<T>());
    }

    /// <summary>
    /// Returns true if a singleton component of type <typeparamref name="T"/> exists.
    /// </summary>
    public bool HasSingleton<T>() where T : struct
    {
        return _singletonEntities.TryGetValue(typeof(T), out var entity)
            && HasComponent<T>(entity);
    }

    /// <summary>
    /// Removes the singleton component of type <typeparamref name="T"/>.
    /// No-op if the singleton does not have the component.
    /// </summary>
    public void RemoveSingleton<T>() where T : struct
    {
        if (_singletonEntities.TryGetValue(typeof(T), out var entity))
            RemoveComponent<T>(entity);
    }

    /// <summary>
    /// Gets the entity backing the singleton of type <typeparamref name="T"/>,
    /// creating it if it doesn't exist yet.
    /// </summary>
    private Entity GetSingletonEntity<T>() where T : struct
    {
        if (!_singletonEntities.TryGetValue(typeof(T), out var entity))
        {
            entity = _allocator.Allocate();
            _singletonEntities[typeof(T)] = entity;
        }
        return 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);
    }

    // ── Iteration Lifecycle ──────────────────────────────────────────

    /// <summary>
    /// Begins an iteration scope. Structural mutations are buffered
    /// until <see cref="EndIteration"/> is called.
    /// </summary>
    internal void BeginIteration()
    {
        _iterationDepth++;
    }

    /// <summary>
    /// Ends an iteration scope. When the outermost scope ends, all
    /// buffered structural mutations are applied and auto-tracked
    /// component modifications are posted.
    /// </summary>
    internal void EndIteration()
    {
        _iterationDepth--;
        if (_iterationDepth == 0)
        {
            // Auto-mark all components accessed via GetComponent<T>
            // during the iteration as modified.
            foreach (var (entity, componentType) in _accessedComponents)
            {
                if (!_markedModified.Contains((entity, componentType)))
                {
                    _changes.Pending.MarkComponentModified(entity, componentType);
                }
            }
            _accessedComponents.Clear();
            _markedModified.Clear();

            FlushPendingMutations();
        }
    }

    private void FlushPendingMutations()
    {
        if (_pendingMutations.Count == 0)
            return;

        foreach (var m in _pendingMutations)
        {
            switch (m.Kind)
            {
                case PendingMutationKind.AddComponent:
                    // Use reflection to call AddComponentImpl<T>.
                    var addMethod = typeof(World).GetMethod(
                        nameof(AddComponentImpl),
                        System.Reflection.BindingFlags.NonPublic | System.Reflection.BindingFlags.Instance)!;
                    var genericAdd = addMethod.MakeGenericMethod(m.ComponentType);
                    genericAdd.Invoke(this, [m.Entity, m.Component]);
                    break;

                case PendingMutationKind.RemoveComponent:
                    var removeMethod = typeof(World).GetMethod(
                        nameof(RemoveComponentImpl),
                        System.Reflection.BindingFlags.NonPublic | System.Reflection.BindingFlags.Instance)!;
                    var genericRemove = removeMethod.MakeGenericMethod(m.ComponentType);
                    genericRemove.Invoke(this, [m.Entity]);
                    break;

                case PendingMutationKind.DestroyEntity:
                    DestroyEntityImpl(m.Entity);
                    break;
            }
        }

        _pendingMutations.Clear();
    }

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

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

    /// <summary>
    /// Gets the entity that backs the singleton of type <typeparamref name="T"/>,
    /// or <see cref="Entity.Null"/> if the singleton has not been set.
    /// </summary>
    internal Entity GetSingletonEntityOrNull<T>() where T : struct
    {
        _singletonEntities.TryGetValue(typeof(T), out var entity);
        return entity;
    }

    /// <summary>
    /// Registers an existing entity as the backing entity for a singleton component.
    /// Used during deserialization to restore singleton state.
    /// </summary>
    internal void RegisterSingletonEntity(Type componentType, Entity entity)
    {
        _singletonEntities[componentType] = entity;
    }

    /// <summary>
    /// Returns true if the given entity is a singleton entity (backs any singleton component).
    /// </summary>
    internal bool IsSingletonEntity(Entity entity)
    {
        return _singletonEntities.ContainsValue(entity);
    }

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

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

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

    public void Dispose()
    {
        if (_disposed)
            return;
        _disposed = true;
        _changes.Dispose();
    }
}