fix: reject tabu size 0; validate VALUE/MOVE_TABU configs; remove dead else-if

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: triceo

core/src/main/java/ai/timefold/solver/core/impl/localsearch/decider/acceptor/AcceptorFactory.java

@@ -54,7 +54,7 @@ public Acceptor<Solution_> buildAcceptor(HeuristicConfigPolicy<Solution_> config
                 .collect(Collectors.toList());
 
         if (acceptorList.size() == 1) {
-            return acceptorList.get(0);
+            return acceptorList.getFirst();
         } else if (acceptorList.size() > 1) {
             return new CompositeAcceptor<>(acceptorList);
         } else {
@@ -134,14 +134,12 @@ private Optional<EntityTabuAcceptor<Solution_>> buildEntityTabuAcceptor(Heuristi
     private Optional<ValueTabuAcceptor<Solution_>> buildValueTabuAcceptor(HeuristicConfigPolicy<Solution_> configPolicy) {
         if (acceptorTypeListsContainsAcceptorType(AcceptorType.VALUE_TABU)
                 || acceptorConfig.getValueTabuSize() != null || acceptorConfig.getFadingValueTabuSize() != null) {
-            var acceptor = new ValueTabuAcceptor<Solution_>(configPolicy.getLogIndentation());
-            if (acceptorConfig.getValueTabuSize() != null) {
-                acceptor.setTabuSizeStrategy(new FixedTabuSizeStrategy<>(acceptorConfig.getValueTabuSize()));
+            if (acceptorConfig.getValueTabuSize() == null && acceptorConfig.getFadingValueTabuSize() == null) {
+                throw new IllegalArgumentException(
+                        "The acceptorType (%s) requires either valueTabuSize or fadingValueTabuSize to be configured."
+                                .formatted(AcceptorType.VALUE_TABU));
             }
-            if (acceptorConfig.getFadingValueTabuSize() != null) {
-                acceptor.setFadingTabuSizeStrategy(new FixedTabuSizeStrategy<>(acceptorConfig.getFadingValueTabuSize()));
-            }
-
+            var acceptor = new ValueTabuAcceptor<Solution_>(configPolicy.getLogIndentation());
             if (acceptorConfig.getValueTabuSize() != null) {
                 acceptor.setTabuSizeStrategy(new FixedTabuSizeStrategy<>(acceptorConfig.getValueTabuSize()));
             }
@@ -159,6 +157,11 @@ private Optional<ValueTabuAcceptor<Solution_>> buildValueTabuAcceptor(HeuristicC
     private Optional<MoveTabuAcceptor<Solution_>> buildMoveTabuAcceptor(HeuristicConfigPolicy<Solution_> configPolicy) {
         if (acceptorTypeListsContainsAcceptorType(AcceptorType.MOVE_TABU)
                 || acceptorConfig.getMoveTabuSize() != null || acceptorConfig.getFadingMoveTabuSize() != null) {
+            if (acceptorConfig.getMoveTabuSize() == null && acceptorConfig.getFadingMoveTabuSize() == null) {
+                throw new IllegalArgumentException(
+                        "The acceptorType (%s) requires either moveTabuSize or fadingMoveTabuSize to be configured."
+                                .formatted(AcceptorType.MOVE_TABU));
+            }
             var acceptor = new MoveTabuAcceptor<Solution_>(configPolicy.getLogIndentation());
             if (acceptorConfig.getMoveTabuSize() != null) {
                 acceptor.setTabuSizeStrategy(new FixedTabuSizeStrategy<>(acceptorConfig.getMoveTabuSize()));

core/src/main/java/ai/timefold/solver/core/impl/localsearch/decider/acceptor/tabu/AbstractTabuAcceptor.java

@@ -141,9 +141,6 @@ public boolean isAccepted(LocalSearchMoveScope<Solution_> moveScope) {
             logger.trace("{}        Proposed move ({}) is tabu and is therefore not accepted.",
                     logIndentation, moveScope.getMove());
             return false;
-        } else if (workingFadingTabuSize == 0) {
-            // No comment as the move is not tabu; the tabu list needs to be adjusted.
-            return true;
         }
         // From this point, we are guaranteed to be in a fading tabu.
         var acceptChance = calculateFadingTabuAcceptChance(tabuStepCount - workingTabuSize);

core/src/main/java/ai/timefold/solver/core/impl/localsearch/decider/acceptor/tabu/size/FixedTabuSizeStrategy.java

@@ -8,9 +8,9 @@ public final class FixedTabuSizeStrategy<Solution_> extends AbstractTabuSizeStra
 
     public FixedTabuSizeStrategy(int tabuSize) {
         this.tabuSize = tabuSize;
-        if (tabuSize < 0) {
-            throw new IllegalArgumentException("The tabuSize (" + tabuSize
-                    + ") cannot be negative.");
+        if (tabuSize < 1) {
+            throw new IllegalArgumentException("The tabuSize (%d) must be at least 1."
+                    .formatted(tabuSize));
         }
     }
 

core/src/test/java/ai/timefold/solver/core/impl/localsearch/decider/acceptor/AcceptorFactoryTest.java

@@ -110,4 +110,22 @@ <Solution_> void diversifiedLateAcceptanceAcceptor() {
         AcceptorFactory<Solution_> badAcceptorFactory = AcceptorFactory.create(localSearchAcceptorConfig);
         assertThatIllegalStateException().isThrownBy(() -> badAcceptorFactory.buildAcceptor(heuristicConfigPolicy));
     }
+
+    @Test
+    <Solution_> void valueTabuWithoutSizes_throwsException() {
+        var config = new LocalSearchAcceptorConfig()
+                .withAcceptorTypeList(List.of(AcceptorType.VALUE_TABU));
+        var factory = AcceptorFactory.create(config);
+        assertThatIllegalArgumentException()
+                .isThrownBy(() -> factory.buildAcceptor(mock(HeuristicConfigPolicy.class)));
+    }
+
+    @Test
+    <Solution_> void moveTabuWithoutSizes_throwsException() {
+        var config = new LocalSearchAcceptorConfig()
+                .withAcceptorTypeList(List.of(AcceptorType.MOVE_TABU));
+        var factory = AcceptorFactory.create(config);
+        assertThatIllegalArgumentException()
+                .isThrownBy(() -> factory.buildAcceptor(mock(HeuristicConfigPolicy.class)));
+    }
 }

core/src/test/java/ai/timefold/solver/core/impl/localsearch/decider/acceptor/tabu/size/FixedTabuSizeStrategyTest.java

@@ -1,6 +1,7 @@
 package ai.timefold.solver.core.impl.localsearch.decider.acceptor.tabu.size;
 
 import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatIllegalArgumentException;
 import static org.mockito.Mockito.mock;
 
 import ai.timefold.solver.core.impl.localsearch.scope.LocalSearchStepScope;
@@ -16,4 +17,10 @@ void tabuSize() {
         assertThat(new FixedTabuSizeStrategy(17).determineTabuSize(stepScope)).isEqualTo(17);
     }
 
+    @Test
+    void invalidTabuSize() {
+        assertThatIllegalArgumentException().isThrownBy(() -> new FixedTabuSizeStrategy<>(0));
+        assertThatIllegalArgumentException().isThrownBy(() -> new FixedTabuSizeStrategy<>(-1));
+    }
+
 }