Handle empty graphs in path finding compute facade

Author: vnickolov

algo/src/main/java/org/neo4j/gds/dag/topologicalsort/TopologicalSortResult.java

@@ -19,8 +19,8 @@
  */
 package org.neo4j.gds.dag.topologicalsort;
 
-import org.neo4j.gds.collections.haa.HugeAtomicDoubleArray;
 import org.neo4j.gds.collections.ha.HugeLongArray;
+import org.neo4j.gds.collections.haa.HugeAtomicDoubleArray;
 
 import java.util.Optional;
 import java.util.concurrent.atomic.AtomicLong;
@@ -29,6 +29,9 @@
  * An implementation of the result with message queues instead of synchronization.
  */
 public class TopologicalSortResult {
+
+    public static final TopologicalSortResult EMPTY = new TopologicalSortResult(0, Optional.empty());
+
     private final HugeLongArray sortedNodes;
     private final AtomicLong addIndex = new AtomicLong(0);
     private final Optional<HugeAtomicDoubleArray> maxSourceDistances;

algo/src/main/java/org/neo4j/gds/paths/bellmanford/BellmanFordResult.java

@@ -25,4 +25,11 @@ public record BellmanFordResult(
     PathFindingResult shortestPaths,
     PathFindingResult negativeCycles,
     boolean containsNegativeCycle
-) {}
+) {
+    public static final BellmanFordResult EMPTY = new BellmanFordResult(
+        PathFindingResult.EMPTY,
+        PathFindingResult.EMPTY,
+        false
+    );
+
+}

algo/src/main/java/org/neo4j/gds/paths/dijkstra/PathFindingResult.java

@@ -30,6 +30,9 @@
 import java.util.stream.Stream;
 
 public class PathFindingResult {
+
+    public static final PathFindingResult EMPTY = new PathFindingResult(Stream.empty());
+
     private final Stream<PathResult> paths;
 
     private final Runnable closeStreamAction;

algo/src/main/java/org/neo4j/gds/pricesteiner/PrizeSteinerTreeResult.java

@@ -32,6 +32,12 @@ public record PrizeSteinerTreeResult(
     public static final long  PRUNED=-2;
     public static final long  ROOT=-1;
 
-
+    public static final PrizeSteinerTreeResult EMPTY = new PrizeSteinerTreeResult(
+        HugeLongArray.newArray(0),
+        HugeDoubleArray.newArray(0),
+        0,
+        0.0,
+        0.0
+    );
 
 }

algo/src/main/java/org/neo4j/gds/spanningtree/SpanningTree.java

@@ -31,6 +31,15 @@
  */
 public class SpanningTree {
 
+    public static final SpanningTree EMPTY = new SpanningTree(
+        -1L,
+        0,
+        0,
+        HugeLongArray.newArray(0),
+        n -> 0.0,
+        0.0
+    );
+
     final long head;
     final long nodeCount;
     final long effectiveNodeCount;
@@ -54,7 +63,6 @@ public SpanningTree(
         this.totalWeight = totalWeight;
     }
 
- 
     public long effectiveNodeCount() {
         return effectiveNodeCount;
     }

algo/src/main/java/org/neo4j/gds/steiner/SteinerTreeResult.java

@@ -28,4 +28,13 @@ public record SteinerTreeResult(
         double totalCost,
         long effectiveNodeCount,
         long effectiveTargetNodesCount
-    ) {}
+    ) {
+
+    public static final SteinerTreeResult EMPTY = new SteinerTreeResult(
+        HugeLongArray.newArray(0),
+        HugeDoubleArray.newArray(0),
+        0.0,
+        0,
+        0
+    );
+}

algorithms-compute-facade/src/main/java/org/neo4j/gds/pathfinding/PathFindingComputeFacade.java

@@ -28,6 +28,7 @@
 import org.neo4j.gds.async.AsyncAlgorithmCaller;
 import org.neo4j.gds.collections.ha.HugeLongArray;
 import org.neo4j.gds.collections.haa.HugeAtomicLongArray;
+import org.neo4j.gds.core.utils.paged.ParalleLongPageCreator;
 import org.neo4j.gds.core.utils.progress.JobId;
 import org.neo4j.gds.dag.longestPath.DagLongestPath;
 import org.neo4j.gds.dag.longestPath.DagLongestPathParameters;
@@ -115,6 +116,10 @@ public CompletableFuture<Stream<AllShortestPathsStreamResult>> allShortestPaths(
         AllShortestPathsParameters parameters,
         JobId jobId
     ) {
+        if (graph.isEmpty()) {
+            return CompletableFuture.completedFuture(Stream.empty());
+        }
+
         // Create ProgressTracker
         // `allShortestPaths` doesn't use progress tracker (yet 🤔)
         var progressTracker = progressTrackerFactory.nullTracker();
@@ -142,6 +147,11 @@ public CompletableFuture<BellmanFordResult> bellmanFord(
         JobId jobId,
         boolean logProgress
     ) {
+        // If the input graph is empty return a completed future with empty result
+        if (graph.isEmpty()) {
+            return CompletableFuture.completedFuture(BellmanFordResult.EMPTY);
+        }
+
         // Create ProgressTracker
         var progressTracker = progressTrackerFactory.create(
             BellmanFordProgressTask.create(),
@@ -174,6 +184,11 @@ public CompletableFuture<HugeLongArray> breadthFirstSearch(
         JobId jobId,
         boolean logProgress
     ) {
+        // If the input graph is empty return a completed future with empty result
+        if (graph.isEmpty()) {
+            return CompletableFuture.completedFuture(HugeLongArray.newArray(0L));
+        }
+
         // Create ProgressTracker
         var progressTracker = progressTrackerFactory.create(
             BFSProgressTask.create(),
@@ -211,6 +226,11 @@ public CompletableFuture<PathFindingResult> deltaStepping(
         JobId jobId,
         boolean logProgress
     ) {
+        // If the input graph is empty return a completed future with empty result
+        if (graph.isEmpty()) {
+            return CompletableFuture.completedFuture(PathFindingResult.EMPTY);
+        }
+
         // Create ProgressTracker
         var progressTracker = progressTrackerFactory.create(
             DeltaSteppingProgressTask.create(),
@@ -235,8 +255,12 @@ public CompletableFuture<HugeLongArray> depthFirstSearch(
         TraversalParameters parameters,
         JobId jobId,
         boolean logProgress
-
     ) {
+        // If the input graph is empty return a completed future with empty result
+        if (graph.isEmpty()) {
+            return CompletableFuture.completedFuture(HugeLongArray.newArray(0L));
+        }
+
         // Create ProgressTracker
         var progressTracker = progressTrackerFactory.create(
             DFSProgressTask.create(),
@@ -272,6 +296,11 @@ public CompletableFuture<SpanningTree> kSpanningTree(
         JobId jobId,
         boolean logProgress
     ) {
+        // If the input graph is empty return a completed future with empty result
+        if (graph.isEmpty()) {
+            return CompletableFuture.completedFuture(SpanningTree.EMPTY);
+        }
+
         // Create ProgressTracker
         var progressTracker = progressTrackerFactory.create(
             KSpanningTreeTask.create(graph.relationshipCount()),
@@ -303,6 +332,11 @@ public CompletableFuture<PathFindingResult> longestPath(
         JobId jobId,
         boolean logProgress
     ) {
+        // If the input graph is empty return a completed future with empty result
+        if (graph.isEmpty()) {
+            return CompletableFuture.completedFuture(PathFindingResult.EMPTY);
+        }
+
         // Create ProgressTracker
         var progressTracker = progressTrackerFactory.create(
             LongestPathTask.create(graph.nodeCount()),
@@ -332,6 +366,11 @@ public CompletableFuture<Stream<long[]>> randomWalk(
         JobId jobId,
         boolean logProgress
     ) {
+        // If the input graph is empty return a completed future with empty result
+        if (graph.isEmpty()) {
+            return CompletableFuture.completedFuture(Stream.empty());
+        }
+
         // Create ProgressTracker
         var progressTracker = progressTrackerFactory.create(
             RandomWalkProgressTask.create(graph),
@@ -361,6 +400,13 @@ public CompletableFuture<HugeAtomicLongArray> randomWalkCountingNodeVisits(
         JobId jobId,
         boolean logProgress
     ) {
+        // If the input graph is empty return a completed future with empty result
+        if (graph.isEmpty()) {
+            return CompletableFuture.completedFuture(HugeAtomicLongArray.of(
+                0,
+                ParalleLongPageCreator.passThrough(parameters.concurrency())
+            ));
+        }
 
         // Create ProgressTracker
         var progressTracker = progressTrackerFactory.create(
@@ -392,6 +438,11 @@ public CompletableFuture<PrizeSteinerTreeResult> pcst(
         JobId jobId,
         boolean logProgress
     ) {
+        // If the input graph is empty return a completed future with empty result
+        if (graph.isEmpty()) {
+            return CompletableFuture.completedFuture(PrizeSteinerTreeResult.EMPTY);
+        }
+
         // Create ProgressTracker
         var progressTracker = progressTrackerFactory.create(
             PCSTProgressTrackerTaskCreator.progressTask(graph.nodeCount(), graph.relationshipCount()),
@@ -421,6 +472,11 @@ public CompletableFuture<PathFindingResult> singlePairShortestPathAStar(
         JobId jobId,
         boolean logProgress
     ) {
+        // If the input graph is empty return a completed future with empty result
+        if (graph.isEmpty()) {
+            return CompletableFuture.completedFuture(PathFindingResult.EMPTY);
+        }
+
         // Create ProgressTracker
         var progressTracker = progressTrackerFactory.create(
             RelationshipCountProgressTaskFactory.create(AlgorithmLabel.AStar, graph.relationshipCount()),
@@ -450,6 +506,11 @@ public CompletableFuture<PathFindingResult> singlePairShortestPathDijkstra(
         JobId jobId,
         boolean logProgress
     ) {
+        // If the input graph is empty return a completed future with empty result
+        if (graph.isEmpty()) {
+            return CompletableFuture.completedFuture(PathFindingResult.EMPTY);
+        }
+
         // Create ProgressTracker
         var progressTracker = progressTrackerFactory.create(
             RelationshipCountProgressTaskFactory.create(AlgorithmLabel.Dijkstra, graph.relationshipCount()),
@@ -482,6 +543,11 @@ public CompletableFuture<PathFindingResult> singlePairShortestPathYens(
         JobId jobId,
         boolean logProgress
     ) {
+        // If the input graph is empty return a completed future with empty result
+        if (graph.isEmpty()) {
+            return CompletableFuture.completedFuture(PathFindingResult.EMPTY);
+        }
+
         // Create ProgressTracker
         var progressTracker = progressTrackerFactory.create(
             YensProgressTask.create(
@@ -514,6 +580,11 @@ public CompletableFuture<PathFindingResult> singleSourceShortestPathDijkstra(
         JobId jobId,
         boolean logProgress
     ) {
+        // If the input graph is empty return a completed future with empty result
+        if (graph.isEmpty()) {
+            return CompletableFuture.completedFuture(PathFindingResult.EMPTY);
+        }
+
         // Create ProgressTracker
         var progressTracker = progressTrackerFactory.create(
             RelationshipCountProgressTaskFactory.create(AlgorithmLabel.SingleSourceDijkstra, graph.relationshipCount()),
@@ -545,6 +616,11 @@ public CompletableFuture<SpanningTree> spanningTree(
         JobId jobId,
         boolean logProgress
     ) {
+        // If the input graph is empty return a completed future with empty result
+        if (graph.isEmpty()) {
+            return CompletableFuture.completedFuture(SpanningTree.EMPTY);
+        }
+
         // Create ProgressTracker
         var progressTracker = progressTrackerFactory.create(
             RelationshipCountProgressTaskFactory.create(AlgorithmLabel.SpanningTree, graph.relationshipCount()),
@@ -575,6 +651,10 @@ public CompletableFuture<SteinerTreeResult> steinerTree(
         JobId jobId,
         boolean logProgress
     ) {
+        // If the input graph is empty return a completed future with empty result
+        if (graph.isEmpty()) {
+            return CompletableFuture.completedFuture(SteinerTreeResult.EMPTY);
+        }
 
         // Create ProgressTracker
         var progressTracker = progressTrackerFactory.create(
@@ -616,6 +696,11 @@ public CompletableFuture<TopologicalSortResult> topologicalSort(
         JobId jobId,
         boolean logProgress
     ) {
+        // If the input graph is empty return a completed future with empty result
+        if (graph.isEmpty()) {
+            return CompletableFuture.completedFuture(TopologicalSortResult.EMPTY);
+        }
+
         // Create ProgressTracker
         var progressTracker = progressTrackerFactory.create(
             TopSortTask.create(graph),