func tagReferenceToTagEvent(stream *api.ImageStream, tagRef api.TagReference, tagOrID string) (*api.TagEvent, error) {
	switch tagRef.From.Kind {
	case "DockerImage":
		return &api.TagEvent{
			Created:              unversioned.Now(),
			DockerImageReference: tagRef.From.Name,
		}, nil

	case "ImageStreamImage":
		ref, err := api.DockerImageReferenceForStream(stream)
		if err != nil {
			return nil, err
		}

		resolvedIDs := api.ResolveImageID(stream, tagOrID)
		switch len(resolvedIDs) {
		case 1:
			ref.ID = resolvedIDs.List()[0]
			return &api.TagEvent{
				Created:              unversioned.Now(),
				DockerImageReference: ref.String(),
				Image:                ref.ID,
			}, nil
		case 0:
			return nil, fmt.Errorf("no images match the prefix %q", tagOrID)
		default:
			return nil, fmt.Errorf("multiple images match the prefix %q: %s", tagOrID, strings.Join(resolvedIDs.List(), ", "))
		}
	case "ImageStreamTag":
		return api.LatestTaggedImage(stream, tagOrID), nil
	default:
		return nil, fmt.Errorf("invalid from.kind %q: it must be ImageStreamImage or ImageStreamTag", tagRef.From.Kind)
	}
}

// resolveImageStreamReference looks up the ImageStream[Tag/Image] and converts it to a
// docker pull spec that can be used in an Image field.
func (g *BuildGenerator) resolveImageStreamReference(ctx kapi.Context, from kapi.ObjectReference, defaultNamespace string) (string, error) {
	var namespace string
	if len(from.Namespace) != 0 {
		namespace = from.Namespace
	} else {
		namespace = defaultNamespace
	}

	glog.V(4).Infof("Resolving ImageStreamReference %s of Kind %s in namespace %s", from.Name, from.Kind, namespace)
	switch from.Kind {
	case "ImageStream":
		// NOTE: The 'ImageStream' reference should be used only for the 'output' image
		is, err := g.Client.GetImageStream(kapi.WithNamespace(ctx, namespace), from.Name)
		if err != nil {
			glog.V(2).Infof("Error getting ImageStream %s/%s: %v", namespace, from.Name, err)
			return "", err
		}
		image, err := imageapi.DockerImageReferenceForStream(is)
		if err != nil {
			glog.V(2).Infof("Error resolving Docker image reference for %s/%s: %v", namespace, from.Name, err)
			return "", err
		}
		return image.String(), nil
	case "ImageStreamImage":
		imageStreamImage, err := g.Client.GetImageStreamImage(kapi.WithNamespace(ctx, namespace), from.Name)
		if err != nil {
			glog.V(2).Infof("Error ImageStreamReference %s in namespace %s: %v", from.Name, namespace, err)
			if errors.IsNotFound(err) {
				return "", err
			}
			return "", fatalError{err}
		}
		image := imageStreamImage.Image
		glog.V(4).Infof("Resolved ImageStreamReference %s to image %s with reference %s in namespace %s", from.Name, image.Name, image.DockerImageReference, namespace)
		return image.DockerImageReference, nil
	case "ImageStreamTag":
		imageStreamTag, err := g.Client.GetImageStreamTag(kapi.WithNamespace(ctx, namespace), from.Name)
		if err != nil {
			glog.V(2).Infof("Error resolving ImageStreamTag reference %s in namespace %s: %v", from.Name, namespace, err)
			if errors.IsNotFound(err) {
				return "", err
			}
			return "", fatalError{err}
		}
		image := imageStreamTag.Image
		glog.V(4).Infof("Resolved ImageStreamTag %s to image %s with reference %s in namespace %s", from.Name, image.Name, image.DockerImageReference, namespace)
		return image.DockerImageReference, nil
	case "DockerImage":
		return from.Name, nil
	default:
		return "", fatalError{fmt.Errorf("Unknown From Kind %s", from.Kind)}
	}
}

// pruneImages invokes imagePruneFunc with each image that is prunable, along
// with the image streams that reference the image. After imagePruneFunc is
// invoked, the image node is removed from the graph, so that layers eligible
// for pruning may later be identified.
func pruneImages(g graph.Graph, imageNodes []*graph.ImageNode, pruneImage ImagePruneFunc, pruneStream ImageStreamPruneFunc, pruneManifest ManifestPruneFunc) {
	for _, imageNode := range imageNodes {
		glog.V(4).Infof("Examining image %q", imageNode.Image.Name)

		if !imageIsPrunable(g, imageNode) {
			glog.V(4).Infof("Image has strong references - not pruning")
			continue
		}

		glog.V(4).Infof("Image has only weak references - pruning")

		if err := pruneImage(imageNode.Image); err != nil {
			util.HandleError(fmt.Errorf("error pruning image %q: %v", imageNode.Image.Name, err))
		}

		for _, n := range g.Predecessors(imageNode) {
			if streamNode, ok := n.(*graph.ImageStreamNode); ok {
				stream := streamNode.ImageStream
				repoName := fmt.Sprintf("%s/%s", stream.Namespace, stream.Name)

				glog.V(4).Infof("Pruning image from stream %s", repoName)
				updatedStream, err := pruneStream(stream, imageNode.Image)
				if err != nil {
					util.HandleError(fmt.Errorf("error pruning image from stream: %v", err))
					continue
				}

				streamNode.ImageStream = updatedStream

				ref, err := imageapi.DockerImageReferenceForStream(stream)
				if err != nil {
					util.HandleError(fmt.Errorf("error constructing DockerImageReference for %q: %v", repoName, err))
					continue
				}

				glog.V(4).Infof("Invoking pruneManifest for registry %q, repo %q, image %q", ref.Registry, repoName, imageNode.Image.Name)
				if err := pruneManifest(ref.Registry, repoName, imageNode.Image.Name); err != nil {
					util.HandleError(fmt.Errorf("error pruning manifest for registry %q, repo %q, image %q: %v", ref.Registry, repoName, imageNode.Image.Name, err))
				}
			}
		}

		// remove pruned image node from graph, for layer pruning later
		g.RemoveNode(imageNode)
	}
}

// resolveImageStreamDockerRepository looks up the ImageStream[Tag/Image] and converts it to a
// the docker repository reference with no tag information
func (g *BuildGenerator) resolveImageStreamDockerRepository(ctx kapi.Context, from kapi.ObjectReference, defaultNamespace string) (string, error) {
	namespace := defaultNamespace
	if len(from.Namespace) > 0 {
		namespace = from.Namespace
	}

	glog.V(4).Infof("Resolving ImageStreamReference %s of Kind %s in namespace %s", from.Name, from.Kind, namespace)
	switch from.Kind {
	case "ImageStreamImage":
		imageStreamImage, err := g.Client.GetImageStreamImage(kapi.WithNamespace(ctx, namespace), from.Name)
		if err != nil {
			glog.V(2).Infof("Error ImageStreamReference %s in namespace %s: %v", from.Name, namespace, err)
			if errors.IsNotFound(err) {
				return "", err
			}
			return "", err
		}
		image := imageStreamImage.Image
		glog.V(4).Infof("Resolved ImageStreamReference %s to image %s with reference %s in namespace %s", from.Name, image.Name, image.DockerImageReference, namespace)
		return image.DockerImageReference, nil
	case "ImageStreamTag":
		name := strings.Split(from.Name, ":")[0]
		is, err := g.Client.GetImageStream(kapi.WithNamespace(ctx, namespace), name)
		if err != nil {
			glog.V(2).Infof("Error getting ImageStream %s/%s: %v", namespace, name, err)
			if errors.IsNotFound(err) {
				return "", err
			}
			return "", err
		}
		image, err := imageapi.DockerImageReferenceForStream(is)
		if err != nil {
			glog.V(2).Infof("Error resolving Docker image reference for %s/%s: %v", namespace, name, err)
			return "", err
		}
		glog.V(4).Infof("Resolved ImageStreamTag %s/%s to repository %s", namespace, from.Name, image)
		return image.String(), nil
	case "DockerImage":
		return from.Name, nil
	default:
		return "", fmt.Errorf("Unknown From Kind %s", from.Kind)
	}
}

// pruneLayers creates a mapping of registryURLs to
// server.DeleteLayersRequest objects, invoking layerPruneFunc for each
// registryURL and request.
func pruneLayers(g graph.Graph, layerNodes []*graph.ImageLayerNode, pruneLayer LayerPruneFunc, pruneBlob BlobPruneFunc) {
	for _, layerNode := range layerNodes {
		glog.V(4).Infof("Examining layer %q", layerNode.Layer)

		if !layerIsPrunable(g, layerNode) {
			glog.V(4).Infof("Layer %q has image references - not pruning", layerNode.Layer)
			continue
		}

		registries := util.NewStringSet()

		// get streams that reference layer
		streamNodes := streamLayerReferences(g, layerNode)

		for _, streamNode := range streamNodes {
			stream := streamNode.ImageStream
			streamName := fmt.Sprintf("%s/%s", stream.Namespace, stream.Name)
			glog.V(4).Infof("Layer has an ImageStream predecessor: %s", streamName)

			ref, err := imageapi.DockerImageReferenceForStream(stream)
			if err != nil {
				util.HandleError(fmt.Errorf("error constructing DockerImageReference for %q: %v", streamName, err))
				continue
			}

			if !registries.Has(ref.Registry) {
				registries.Insert(ref.Registry)
				glog.V(4).Infof("Invoking pruneBlob with registry=%q, blob=%q", ref.Registry, layerNode.Layer)
				if err := pruneBlob(ref.Registry, layerNode.Layer); err != nil {
					util.HandleError(fmt.Errorf("error invoking pruneBlob: %v", err))
				}
			}

			repoName := fmt.Sprintf("%s/%s", ref.Namespace, ref.Name)
			glog.V(4).Infof("Invoking pruneLayer with registry=%q, repo=%q, layer=%q", ref.Registry, repoName, layerNode.Layer)
			if err := pruneLayer(ref.Registry, repoName, layerNode.Layer); err != nil {
				util.HandleError(fmt.Errorf("error invoking pruneLayer: %v", err))
			}
		}
	}
}