Skip lower bound compute IFF one is descendant of the other (l.b is 0)

Co-authored-by: Veselin Nikolov <veselin.nikolov@neotechnology.com>

Author: IoannisPanagiotas

algo/src/main/java/org/neo4j/gds/hdbscan/DualTreeMSTAlgorithm.java

@@ -175,7 +175,7 @@ boolean score(KdNode kdNodeQ, KdNode kdNodeR) {
         if (singleQ == singleR) return false;
 
         if (kdNodeBound.get(qId) == Double.MAX_VALUE) return true;
-
+        if ( kdTree.descentOfOther(kdNodeQ,kdNodeR)) return true;
         var lowerBoundQR = kdNodeQ.aabb().lowerBoundFor(kdNodeR.aabb());
         return lowerBoundQR < Math.sqrt(kdNodeBound.get(qId));
 

algo/src/main/java/org/neo4j/gds/hdbscan/KdTree.java

@@ -70,6 +70,15 @@ LongStream nodesContained(KdNode node) {
         return LongStream.range(start, end).map(ids::get);
     }
 
+   boolean descentOfOther(KdNode nodeA,KdNode nodeB){
+        long startA = nodeA.start();
+        long endA = nodeA.end();
+
+        long startB = nodeB.start();
+        long endB = nodeB.end();
+
+        return startA <= startB && endB <= endA || startB <= startA && endA <= endB;
+    }
 
     long  treeNodeCount(){
         return treeNodeCount;

algo/src/test/java/org/neo4j/gds/hdbscan/KdTreeTest.java

@@ -89,4 +89,25 @@ void shouldNotFindItself() {
             );
     }
 
+    @Test
+    void shouldCorrectlyFigureOutDescendants() {
+        var root = KdNode.createLeaf(0, 10, 100, null);
+        var l1 = KdNode.createLeaf(1, 10, 50, null);
+        var l2 = KdNode.createLeaf(1, 20, 50, null);
+        var l3 = KdNode.createLeaf(1, 100, 150, null);
+
+        var kdtree = new KdTree(null, null, root, 3);
+
+        assertThat(kdtree.descentOfOther(root, l1)).isTrue();
+        assertThat(kdtree.descentOfOther(l1, root)).isTrue();
+
+        assertThat(kdtree.descentOfOther(root, l2)).isTrue();
+        assertThat(kdtree.descentOfOther(l2, root)).isTrue();
+
+        assertThat(kdtree.descentOfOther(root, l3)).isFalse();
+        assertThat(kdtree.descentOfOther(l3, root)).isFalse();
+
+
+    }
+
 }