VectorOfMotionStates& PhysicalEntitySimulation::getObjectsToAdd() {
    _tempVector.clear();
    SetOfEntities::iterator entityItr = _pendingAdds.begin();
    while (entityItr != _pendingAdds.end()) {
        EntityItemPointer entity = *entityItr;
        assert(!entity->getPhysicsInfo());
        if (!entity->shouldBePhysical()) {
            // this entity should no longer be on the internal _pendingAdds
            entityItr = _pendingAdds.erase(entityItr);
            if (entity->isMoving()) {
                _simpleKinematicEntities.insert(entity);
            }
        } else if (entity->isReadyToComputeShape()) {
            ShapeInfo shapeInfo;
            entity->computeShapeInfo(shapeInfo);
            btCollisionShape* shape = ObjectMotionState::getShapeManager()->getShape(shapeInfo);
            if (shape) {
                EntityMotionState* motionState = new EntityMotionState(shape, entity);
                entity->setPhysicsInfo(static_cast<void*>(motionState));
                _physicalObjects.insert(motionState);
                _tempVector.push_back(motionState);
                entityItr = _pendingAdds.erase(entityItr);
            } else {
                //qDebug() << "Warning!  Failed to generate new shape for entity." << entity->getName();
                ++entityItr;
            }
        } else {
            ++entityItr;
        }
    }
    return _tempVector;
}
void PhysicalEntitySimulation::getObjectsToAddToPhysics(VectorOfMotionStates& result) {
    result.clear();
    QMutexLocker lock(&_mutex);
    SetOfEntities::iterator entityItr = _entitiesToAddToPhysics.begin();
    while (entityItr != _entitiesToAddToPhysics.end()) {
        EntityItemPointer entity = (*entityItr);
        assert(!entity->getPhysicsInfo());
        if (entity->isDead()) {
            prepareEntityForDelete(entity);
        } else if (!entity->shouldBePhysical()) {
            // this entity should no longer be on the internal _entitiesToAddToPhysics
            entityItr = _entitiesToAddToPhysics.erase(entityItr);
            if (entity->isMoving()) {
                _simpleKinematicEntities.insert(entity);
            }
        } else if (entity->isReadyToComputeShape()) {
            ShapeInfo shapeInfo;
            entity->computeShapeInfo(shapeInfo);
            btCollisionShape* shape = ObjectMotionState::getShapeManager()->getShape(shapeInfo);
            if (shape) {
                EntityMotionState* motionState = new EntityMotionState(shape, entity);
                entity->setPhysicsInfo(static_cast<void*>(motionState));
                _physicalObjects.insert(motionState);
                result.push_back(motionState);
                entityItr = _entitiesToAddToPhysics.erase(entityItr);
            } else {
                //qDebug() << "Warning!  Failed to generate new shape for entity." << entity->getName();
                ++entityItr;
            }
        } else {
            ++entityItr;
        }
    }
}
void PhysicalEntitySimulation::removeEntityInternal(EntityItemPointer entity) {
    EntityMotionState* motionState = static_cast<EntityMotionState*>(entity->getPhysicsInfo());
    if (motionState) {
        motionState->clearObjectBackPointer();
        entity->setPhysicsInfo(nullptr);
        _pendingRemoves.insert(motionState);
        _outgoingChanges.remove(motionState);
    }
    _pendingAdds.remove(entity);
}
VectorOfMotionStates& PhysicalEntitySimulation::getObjectsToDelete() {
    _tempVector.clear();
    for (auto stateItr : _pendingRemoves) {
        EntityMotionState* motionState = &(*stateItr);
        _pendingChanges.remove(motionState);
        _physicalObjects.remove(motionState);

        EntityItemPointer entity = motionState->getEntity();
        if (entity) {
            _pendingAdds.remove(entity);
            entity->setPhysicsInfo(nullptr);
            motionState->clearObjectBackPointer();
        }
        _tempVector.push_back(motionState);
    }
    _pendingRemoves.clear();
    return _tempVector;
}
void PhysicalEntitySimulation::clearEntitiesInternal() {
    // TODO: we should probably wait to lock the _physicsEngine so we don't mess up data structures
    // while it is in the middle of a simulation step.  As it is, we're probably in shutdown mode
    // anyway, so maybe the simulation was already properly shutdown?  Cross our fingers...

    // first disconnect each MotionStates from its Entity
    for (auto stateItr : _physicalObjects) {
        EntityMotionState* motionState = static_cast<EntityMotionState*>(&(*stateItr));
        EntityItemPointer entity = motionState->getEntity();
        if (entity) {
            entity->setPhysicsInfo(nullptr);
        }
        motionState->clearObjectBackPointer();
    }

    // then delete the objects (aka MotionStates)
    _physicsEngine->deleteObjects(_physicalObjects);

    // finally clear all lists (which now have only dangling pointers)
    _physicalObjects.clear();
    _pendingRemoves.clear();
    _pendingAdds.clear();
    _pendingChanges.clear();
}