class WorkspaceMerger

class WorkspaceMerger implements WorkspaceMergerInterface {
    
    /**
     * The source workspace entity.
     *
     * @var \Drupal\workspaces\WorkspaceInterface
     */
    protected $sourceWorkspace;
    
    /**
     * The target workspace entity.
     *
     * @var \Drupal\workspaces\WorkspaceInterface
     */
    protected $targetWorkspace;
    
    /**
     * The entity type manager.
     *
     * @var \Drupal\Core\Entity\EntityTypeManagerInterface
     */
    protected $entityTypeManager;
    
    /**
     * The database connection.
     *
     * @var \Drupal\Core\Database\Connection
     */
    protected $database;
    
    /**
     * The workspace association service.
     *
     * @var \Drupal\workspaces\WorkspaceAssociationInterface
     */
    protected $workspaceAssociation;
    
    /**
     * The cache tag invalidator.
     *
     * @var \Drupal\Core\Cache\CacheTagsInvalidatorInterface
     */
    protected $cacheTagsInvalidator;
    
    /**
     * Constructs a new WorkspaceMerger.
     *
     * @param \Drupal\Core\Entity\EntityTypeManagerInterface $entity_type_manager
     *   The entity type manager.
     * @param \Drupal\Core\Database\Connection $database
     *   Database connection.
     * @param \Drupal\workspaces\WorkspaceAssociationInterface $workspace_association
     *   The workspace association service.
     * @param \Drupal\Core\Cache\CacheTagsInvalidatorInterface $cache_tags_invalidator
     *   The cache tags invalidator service.
     * @param \Drupal\workspaces\WorkspaceInterface $source
     *   The source workspace.
     * @param \Drupal\workspaces\WorkspaceInterface $target
     *   The target workspace.
     */
    public function __construct(EntityTypeManagerInterface $entity_type_manager, Connection $database, WorkspaceAssociationInterface $workspace_association, CacheTagsInvalidatorInterface $cache_tags_invalidator, WorkspaceInterface $source, WorkspaceInterface $target) {
        $this->entityTypeManager = $entity_type_manager;
        $this->database = $database;
        $this->workspaceAssociation = $workspace_association;
        $this->cacheTagsInvalidator = $cache_tags_invalidator;
        $this->sourceWorkspace = $source;
        $this->targetWorkspace = $target;
    }
    
    /**
     * {@inheritdoc}
     */
    public function merge() {
        if (!$this->sourceWorkspace
            ->hasParent() || $this->sourceWorkspace->parent->target_id != $this->targetWorkspace
            ->id()) {
            throw new \InvalidArgumentException('The contents of a workspace can only be merged into its parent workspace.');
        }
        if ($this->checkConflictsOnTarget()) {
            throw new WorkspaceConflictException();
        }
        try {
            $transaction = $this->database
                ->startTransaction();
            foreach ($this->getDifferringRevisionIdsOnSource() as $entity_type_id => $revision_difference) {
                $revisions_on_source = $this->entityTypeManager
                    ->getStorage($entity_type_id)
                    ->loadMultipleRevisions(array_keys($revision_difference));
                
                /** @var \Drupal\Core\Entity\ContentEntityInterface $entity */
                foreach ($revisions_on_source as $revision) {
                    // Track all the differing revisions from the source workspace in
                    // the context of the target workspace. This will automatically
                    // update all the descendants of the target workspace as well.
                    $this->workspaceAssociation
                        ->trackEntity($revision, $this->targetWorkspace);
                }
                // Since we're not saving entity objects, we need to invalidate the list
                // cache tags manually.
                $entity_type = $this->entityTypeManager
                    ->getDefinition($entity_type_id);
                $this->cacheTagsInvalidator
                    ->invalidateTags($entity_type->getListCacheTags());
            }
        } catch (\Exception $e) {
            if (isset($transaction)) {
                $transaction->rollBack();
            }
            watchdog_exception('workspaces', $e);
            throw $e;
        }
    }
    
    /**
     * {@inheritdoc}
     */
    public function getSourceLabel() {
        return $this->sourceWorkspace
            ->label();
    }
    
    /**
     * {@inheritdoc}
     */
    public function getTargetLabel() {
        return $this->targetWorkspace
            ->label();
    }
    
    /**
     * {@inheritdoc}
     */
    public function checkConflictsOnTarget() {
        // Nothing to do for now, we can not get to a conflicting state because an
        // entity which is being edited in a workspace can not be edited in any
        // other workspace.
    }
    
    /**
     * {@inheritdoc}
     */
    public function getDifferringRevisionIdsOnTarget() {
        $target_revision_difference = [];
        $tracked_entities_on_source = $this->workspaceAssociation
            ->getTrackedEntities($this->sourceWorkspace
            ->id());
        $tracked_entities_on_target = $this->workspaceAssociation
            ->getTrackedEntities($this->targetWorkspace
            ->id());
        foreach ($tracked_entities_on_target as $entity_type_id => $tracked_revisions) {
            // Now we compare the revision IDs which are tracked by the target
            // workspace to those that are tracked by the source workspace, and the
            // difference between these two arrays gives us all the entities which
            // have a different revision ID on the target.
            if (!isset($tracked_entities_on_source[$entity_type_id])) {
                $target_revision_difference[$entity_type_id] = $tracked_revisions;
            }
            elseif ($revision_difference = array_diff_key($tracked_revisions, $tracked_entities_on_source[$entity_type_id])) {
                $target_revision_difference[$entity_type_id] = $revision_difference;
            }
        }
        return $target_revision_difference;
    }
    
    /**
     * {@inheritdoc}
     */
    public function getDifferringRevisionIdsOnSource() {
        $source_revision_difference = [];
        $tracked_entities_on_source = $this->workspaceAssociation
            ->getTrackedEntities($this->sourceWorkspace
            ->id());
        $tracked_entities_on_target = $this->workspaceAssociation
            ->getTrackedEntities($this->targetWorkspace
            ->id());
        foreach ($tracked_entities_on_source as $entity_type_id => $tracked_revisions) {
            // Now we compare the revision IDs which are tracked by the source
            // workspace to those that are tracked by the target workspace, and the
            // difference between these two arrays gives us all the entities which
            // have a different revision ID on the source.
            if (!isset($tracked_entities_on_target[$entity_type_id])) {
                $source_revision_difference[$entity_type_id] = $tracked_revisions;
            }
            elseif ($revision_difference = array_diff_key($tracked_revisions, $tracked_entities_on_target[$entity_type_id])) {
                $source_revision_difference[$entity_type_id] = $revision_difference;
            }
        }
        return $source_revision_difference;
    }
    
    /**
     * {@inheritdoc}
     */
    public function getNumberOfChangesOnTarget() {
        $total_changes = $this->getDifferringRevisionIdsOnTarget();
        return count($total_changes, COUNT_RECURSIVE) - count($total_changes);
    }
    
    /**
     * {@inheritdoc}
     */
    public function getNumberOfChangesOnSource() {
        $total_changes = $this->getDifferringRevisionIdsOnSource();
        return count($total_changes, COUNT_RECURSIVE) - count($total_changes);
    }

}