Demonstrate and fix bug

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

Author: IoannisPanagiotas

algo/src/main/java/org/neo4j/gds/influenceMaximization/CELF.java

@@ -95,15 +95,15 @@ protected boolean lessThan(long a, long b) {
     public LongDoubleScatterMap compute() {
         //Find the first node with greedy algorithm
         progressTracker.beginSubTask();
-        greedyPart();
+        var firstSeedNode = greedyPart();
         //Find the next k-1 nodes using the list-sorting procedure
-        lazyForwardPart();
+        lazyForwardPart(firstSeedNode);
         progressTracker.endSubTask();
 
         return seedSetNodes;
     }
 
-    private void greedyPart() {
+    private long greedyPart() {
         HugeDoubleArray singleSpreadArray = HugeDoubleArray.newArray(graph.nodeCount());
         progressTracker.beginSubTask(graph.nodeCount());
         var tasks = PartitionUtils.rangePartition(
@@ -136,15 +136,16 @@ private void greedyPart() {
         gain = spreads.cost(highestNode);
         spreads.pop();
         seedSetNodes.put(highestNode, gain);
+        return highestNode;
     }
 
-    private void lazyForwardPart() {
+    private void lazyForwardPart(long firstSeedNode) {
 
         var independentCascade = ICLazyForwardMC.create(
             graph,
             propagationProbability,
             monteCarloSimulations,
-            seedSetNodes.keys[0],
+            firstSeedNode,
             (int) seedSetCount,
             concurrency,
             executorService,

alpha/alpha-algo/src/test/java/org/neo4j/gds/impl/influenceMaximization/CelfTest.java

@@ -0,0 +1,53 @@
+/*
+ * Copyright (c) "Neo4j"
+ * Neo4j Sweden AB [http://neo4j.com]
+ *
+ * This file is part of Neo4j.
+ *
+ * Neo4j is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+package org.neo4j.gds.impl.influenceMaximization;
+
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.ValueSource;
+import org.neo4j.gds.api.schema.Direction;
+import org.neo4j.gds.beta.generator.RandomGraphGenerator;
+import org.neo4j.gds.beta.generator.RelationshipDistribution;
+import org.neo4j.gds.core.concurrency.Pools;
+import org.neo4j.gds.core.utils.progress.tasks.ProgressTracker;
+import org.neo4j.gds.influenceMaximization.CELF;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+ class CelfTest {
+
+     @ParameterizedTest
+     @ValueSource(ints = {2, 7})
+     void shouldNotReturnNegativeGains(int seedSize) {
+         var graph = RandomGraphGenerator
+             .builder()
+             .averageDegree(5)
+             .relationshipDistribution(RelationshipDistribution.POWER_LAW)
+             .direction(Direction.DIRECTED)
+             .nodeCount(60)
+             .seed(42)
+             .build()
+             .generate();
+
+         var celf = new CELF(graph, seedSize, 0.1, 3, Pools.DEFAULT, 1, 10, 5, ProgressTracker.NULL_TRACKER).compute();
+         for (var a : celf) {
+             assertThat(a.value).isNotNegative();
+         }
+     }
+}