Bug in level-off detection fixed. The use of nogoods as proposed in the textbook seems not to guarantee termination.

Author: RuedigerLunde

aima-core/src/main/java/aima/core/logic/planning/GraphPlanAlgorithm.java

@@ -34,6 +34,13 @@
  */
 public class GraphPlanAlgorithm {
 
+    /** Graph which was created in the last <code>graphPlan</code> call. */
+    private Graph graph;
+
+    public Graph getGraph() {
+        return graph;
+    }
+
     /**
      * function GRAPHPLAN(problem) returns solution or failure
      *
@@ -42,7 +49,7 @@ public class GraphPlanAlgorithm {
      */
     public List<List<ActionSchema>> graphPlan(Problem problem) {
         // graph ← INITIAL-PLANNING-GRAPH(problem)
-        Graph graph = new Graph(problem);
+        graph = new Graph(problem);
         // goals ← CONJUNCTS(problem.GOAL)
         List<Literal> goals = problem.getGoalState().getFluents();
         // nogoods ← an empty hash table
@@ -59,7 +66,9 @@ public List<List<ActionSchema>> graphPlan(Problem problem) {
                 // if solution ≠ failure then return solution
                 if (solution != null && solution.size() != 0)
                     return solution;
-            }
+            } else
+                // seems to be missing in the book - but needed to guarantee termination! (RLu)
+                nogoods.put(tl, goals);
             // if graph and nogoods have both leveled off then return failure
             if (levelledOff(graph) && leveledOff(nogoods))
                 return null;

aima-core/src/main/java/aima/core/logic/planning/Level.java

@@ -9,12 +9,12 @@
  * @author Ruediger Lunde
  */
 public class Level<CURR, PREV> {
-    List<CURR> levelObjects = new ArrayList<>();
+    private List<CURR> levelObjects = new ArrayList<>();
     HashMap<CURR, List<PREV>> prevLinks = new HashMap<>();
     HashMap<CURR, List<PREV>> nextLinks = new HashMap<>();
-    HashMap<CURR, List<CURR>> mutexLinks = new HashMap<>();; //can be planned alternatively
+    private HashMap<CURR, List<CURR>> mutexLinks = new HashMap<>();; //can be planned alternatively
 
-    Level<PREV, CURR> prevLevel;
+    private Level<PREV, CURR> prevLevel;
 
     public Level(List<CURR> levelObjects, Problem problem) {
         // store level objects and prevLinks

aima-core/src/test/java/aima/test/core/unit/logic/planning/GraphPlanAlgorithmTest.java

@@ -1,12 +1,10 @@
 package aima.test.core.unit.logic.planning;
 
-import aima.core.logic.planning.ActionSchema;
-import aima.core.logic.planning.GraphPlanAlgorithm;
-import aima.core.logic.planning.PlanningProblemFactory;
-import aima.core.logic.planning.Problem;
+import aima.core.logic.planning.*;
 import org.junit.Assert;
 import org.junit.Test;
 
+import javax.swing.*;
 import java.util.Collections;
 import java.util.List;
 
@@ -15,7 +13,7 @@
  */
 public class GraphPlanAlgorithmTest {
     @Test
-    public void test() {
+    public void spareTireTest() {
         GraphPlanAlgorithm algorithm = new GraphPlanAlgorithm();
         Problem spareTireProblem = PlanningProblemFactory.spareTireProblem();
         List<List<ActionSchema>> solution = algorithm.graphPlan(spareTireProblem);
@@ -33,4 +31,16 @@ public void test() {
         Assert.assertTrue(solution.get(0).contains(removeSpareTrunk));
         Assert.assertTrue(solution.get(1).contains(putOnSpareAxle));
     }
+
+    @Test
+    public void levelOffTest() {
+        GraphPlanAlgorithm algorithm = new GraphPlanAlgorithm();
+        Problem stProblem = PlanningProblemFactory.spareTireProblem();
+        State initialState = new State("Tire(Flat)^Tire(Spare)^At(Flat,Axle)"); //^At(Spare,Trunk)");
+        Problem modifiedProblem = new Problem(initialState, stProblem.getGoalState(), stProblem.getActionSchemas());
+        List<List<ActionSchema>> solution = algorithm.graphPlan(modifiedProblem);
+        Assert.assertEquals(4, algorithm.getGraph().numLevels());
+        Assert.assertNull(solution);
+
+    }
 }