Clean up BVH tree API

CityDoctorParent/CityDoctorModel/src/main/java/de/hft/stuttgart/citydoctor2/datastructure/bht/BoundingVolumeHierarchyBuilder.java

@@ -146,6 +146,8 @@ final class BoundingVolumeHierarchyBuilder<E> {
 	private boolean shouldStop(List<BvhBuildItem<E>> items, int depth, AABB totalAabb) {
 		return items.size() <= config.getMaxLeafSize()
 				|| depth >= config.getMaxDepth()
+				// Very flat bounds are kept as one terminal group to avoid repeated
+				// ineffective spatial splits on effectively lower-dimensional data.
 				|| totalAabb.isDegenerate(config.getDegenerateTolerance());
 	}
 
@@ -156,6 +158,8 @@ final class BoundingVolumeHierarchyBuilder<E> {
 		}
 
 		Node<E> leafGroup = new Node<>(null, totalAabb);
+		// A terminal group keeps its aggregate AABB for pruning, while the children
+		// remain the actual element leaves returned by queries.
 		for (BvhBuildItem<E> item : items) {
 			leafGroup.getChildren().add(new Node<>(item.element, item.aabb));
 		}

CityDoctorParent/CityDoctorModel/src/main/java/de/hft/stuttgart/citydoctor2/datastructure/bht/BoundingVolumeHierarchyTree.java

@@ -6,12 +6,9 @@ import java.util.Objects;
 import java.util.function.Function;
 import java.util.function.Predicate;
 
-import de.hft.stuttgart.citydoctor2.datastructure.ConcretePolygon;
-
 public class BoundingVolumeHierarchyTree<E> {
 
-	private Node<E> root;
-	private final Function<E, AABB> aabbFunction;
+	private final Node<E> root;
 
 	static final int DEFAULT_MIN_DEPTH = 4;
 	static final int DEFAULT_MAX_DEPTH = 32;
@@ -21,7 +18,6 @@ public class BoundingVolumeHierarchyTree<E> {
 
 	private BoundingVolumeHierarchyTree(Builder<E> builder) {
 		Objects.requireNonNull(builder, "builder");
-		this.aabbFunction = Objects.requireNonNull(builder.getAabbFunction(), "aabbFunction");
 		this.root = new BoundingVolumeHierarchyBuilder<>(builder).buildRoot();
 	}
 
@@ -110,14 +106,11 @@ public class BoundingVolumeHierarchyTree<E> {
 		return root;
 	}
 
-	public void setRoot(Node<E> root) {
-		this.root = root;
-	}
-
 	/**
 	 * Returns all elements whose node AABBs overlap or touch the query AABB.
 	 */
 	public List<E> getAllIntersectingElements(AABB query) {
+		Objects.requireNonNull(query, "query");
 		List<E> result = new ArrayList<>();
 		collectMatchingElements(
 				root,
@@ -133,6 +126,7 @@ public class BoundingVolumeHierarchyTree<E> {
 	 * missed when their parent node AABB extends beyond the container.
 	 */
 	public List<E> getAllElementsContainedIn(AABB container) {
+		Objects.requireNonNull(container, "container");
 		List<E> result = new ArrayList<>();
 		collectMatchingElements(
 				root,
@@ -161,14 +155,13 @@ public class BoundingVolumeHierarchyTree<E> {
 		}
 	}
 
-	public static AABB getAggregateAABB(List<ConcretePolygon> polygons) {
-		if (polygons.size() == 1) {
-			return AABB.of(polygons.get(0));
+	public static <E> AABB getAggregateAABB(List<E> elements, Function<E, AABB> aabbFunc) {
+		Objects.requireNonNull(elements, "elements");
+		Objects.requireNonNull(aabbFunc, "aabbFunc");
+		if (elements.isEmpty()) {
+			throw new IllegalArgumentException("elements must not be empty");
 		}
-		return getAggregateAABB(polygons, AABB::of);
-	}
 
-	public static <E> AABB getAggregateAABB(List<E> elements, Function<E, AABB> aabbFunc) {
 		double minX = Double.POSITIVE_INFINITY;
 		double minY = Double.POSITIVE_INFINITY;
 		double minZ = Double.POSITIVE_INFINITY;
@@ -177,7 +170,7 @@ public class BoundingVolumeHierarchyTree<E> {
 		double maxZ = Double.NEGATIVE_INFINITY;
 
 		for (E e : elements) {
-			AABB aabb = aabbFunc.apply(e);
+			AABB aabb = Objects.requireNonNull(aabbFunc.apply(e), "aabbFunc returned null");
 			minX = Math.min(minX, aabb.getMinX());
 			minY = Math.min(minY, aabb.getMinY());
 			minZ = Math.min(minZ, aabb.getMinZ());