chore: improve the new list variable logic (#1248)

For some reason, this commit was not merged together with ee760ca and I
only discovered it now.

core/src/main/java/ai/timefold/solver/core/impl/domain/variable/ExternalizedListVariableStateSupply.java

@@ -27,23 +27,23 @@ public ExternalizedListVariableStateSupply(ListVariableDescriptor<Solution_> sou
 
     @Override
     public void externalize(IndexShadowVariableDescriptor<Solution_> shadowVariableDescriptor) {
-        listVariableState.linkDescriptor(shadowVariableDescriptor);
+        listVariableState.linkShadowVariable(shadowVariableDescriptor);
     }
 
     @Override
     public void externalize(InverseRelationShadowVariableDescriptor<Solution_> shadowVariableDescriptor) {
-        listVariableState.linkDescriptor(shadowVariableDescriptor);
+        listVariableState.linkShadowVariable(shadowVariableDescriptor);
     }
 
     @Override
     public void externalize(PreviousElementShadowVariableDescriptor<Solution_> shadowVariableDescriptor) {
-        listVariableState.linkDescriptor(shadowVariableDescriptor);
+        listVariableState.linkShadowVariable(shadowVariableDescriptor);
         previousExternalized = true;
     }
 
     @Override
     public void externalize(NextElementShadowVariableDescriptor<Solution_> shadowVariableDescriptor) {
-        listVariableState.linkDescriptor(shadowVariableDescriptor);
+        listVariableState.linkShadowVariable(shadowVariableDescriptor);
         nextExternalized = true;
     }
 

core/src/main/java/ai/timefold/solver/core/impl/domain/variable/ExternalizedNextPrevElementVariableProcessor.java

@@ -0,0 +1,58 @@
+package ai.timefold.solver.core.impl.domain.variable;
+
+import java.util.List;
+import java.util.Objects;
+
+import ai.timefold.solver.core.impl.domain.variable.descriptor.ShadowVariableDescriptor;
+import ai.timefold.solver.core.impl.domain.variable.nextprev.NextElementShadowVariableDescriptor;
+import ai.timefold.solver.core.impl.domain.variable.nextprev.PreviousElementShadowVariableDescriptor;
+import ai.timefold.solver.core.impl.score.director.InnerScoreDirector;
+
+final class ExternalizedNextPrevElementVariableProcessor<Solution_> {
+
+    public static <Solution_> ExternalizedNextPrevElementVariableProcessor<Solution_>
+            ofPrevious(PreviousElementShadowVariableDescriptor<Solution_> shadowVariableDescriptor) {
+        return new ExternalizedNextPrevElementVariableProcessor<>(shadowVariableDescriptor, -1);
+    }
+
+    public static <Solution_> ExternalizedNextPrevElementVariableProcessor<Solution_>
+            ofNext(NextElementShadowVariableDescriptor<Solution_> shadowVariableDescriptor) {
+        return new ExternalizedNextPrevElementVariableProcessor<>(shadowVariableDescriptor, 1);
+    }
+
+    private final ShadowVariableDescriptor<Solution_> shadowVariableDescriptor;
+    private final int modifier;
+
+    private ExternalizedNextPrevElementVariableProcessor(ShadowVariableDescriptor<Solution_> shadowVariableDescriptor,
+            int modifier) {
+        this.shadowVariableDescriptor = Objects.requireNonNull(shadowVariableDescriptor);
+        this.modifier = modifier;
+    }
+
+    public void setElement(InnerScoreDirector<Solution_, ?> scoreDirector, List<Object> listVariable, Object element,
+            int index) {
+        var target = index + modifier;
+        if (target < 0 || target >= listVariable.size()) {
+            setValue(scoreDirector, element, null);
+        } else {
+            setValue(scoreDirector, element, listVariable.get(target));
+        }
+    }
+
+    private void setValue(InnerScoreDirector<Solution_, ?> scoreDirector, Object element, Object value) {
+        if (getElement(element) != value) {
+            scoreDirector.beforeVariableChanged(shadowVariableDescriptor, element);
+            shadowVariableDescriptor.setValue(element, value);
+            scoreDirector.afterVariableChanged(shadowVariableDescriptor, element);
+        }
+    }
+
+    public Object getElement(Object element) {
+        return shadowVariableDescriptor.getValue(element);
+    }
+
+    public void unsetElement(InnerScoreDirector<Solution_, ?> scoreDirector, Object element) {
+        setValue(scoreDirector, element, null);
+    }
+
+}

core/src/main/java/ai/timefold/solver/core/impl/domain/variable/ListVariableState.java

@@ -20,10 +20,9 @@ final class ListVariableState<Solution_> {
 
     private ExternalizedIndexVariableProcessor<Solution_> externalizedIndexProcessor = null;
     private ExternalizedListInverseVariableProcessor<Solution_> externalizedInverseProcessor = null;
-    private AbstractExternalizedNextPrevElementVariableProcessor<Solution_> externalizedPreviousElementProcessor = null;
-    private AbstractExternalizedNextPrevElementVariableProcessor<Solution_> externalizedNextElementProcessor = null;
+    private ExternalizedNextPrevElementVariableProcessor<Solution_> externalizedPreviousElementProcessor = null;
+    private ExternalizedNextPrevElementVariableProcessor<Solution_> externalizedNextElementProcessor = null;
 
-    private boolean canUseExternalizedLocation = false;
     private boolean requiresLocationMap = true;
     private InnerScoreDirector<Solution_, ?> scoreDirector;
     private int unassignedCount = 0;
@@ -33,30 +32,29 @@ public ListVariableState(ListVariableDescriptor<Solution_> sourceVariableDescrip
         this.sourceVariableDescriptor = sourceVariableDescriptor;
     }
 
-    public void linkDescriptor(IndexShadowVariableDescriptor<Solution_> shadowVariableDescriptor) {
+    public void linkShadowVariable(IndexShadowVariableDescriptor<Solution_> shadowVariableDescriptor) {
         this.externalizedIndexProcessor = new ExternalizedIndexVariableProcessor<>(shadowVariableDescriptor);
     }
 
-    public void linkDescriptor(InverseRelationShadowVariableDescriptor<Solution_> shadowVariableDescriptor) {
+    public void linkShadowVariable(InverseRelationShadowVariableDescriptor<Solution_> shadowVariableDescriptor) {
         this.externalizedInverseProcessor =
                 new ExternalizedListInverseVariableProcessor<>(shadowVariableDescriptor, sourceVariableDescriptor);
     }
 
-    public void linkDescriptor(PreviousElementShadowVariableDescriptor<Solution_> shadowVariableDescriptor) {
+    public void linkShadowVariable(PreviousElementShadowVariableDescriptor<Solution_> shadowVariableDescriptor) {
         this.externalizedPreviousElementProcessor =
-                new ExternalizedPreviousElementVariableProcessor<>(shadowVariableDescriptor);
+                ExternalizedNextPrevElementVariableProcessor.ofPrevious(shadowVariableDescriptor);
     }
 
-    public void linkDescriptor(NextElementShadowVariableDescriptor<Solution_> shadowVariableDescriptor) {
-        this.externalizedNextElementProcessor = new ExternalizedNextElementVariableProcessor<>(shadowVariableDescriptor);
+    public void linkShadowVariable(NextElementShadowVariableDescriptor<Solution_> shadowVariableDescriptor) {
+        this.externalizedNextElementProcessor = ExternalizedNextPrevElementVariableProcessor.ofNext(shadowVariableDescriptor);
     }
 
     public void initialize(InnerScoreDirector<Solution_, ?> scoreDirector, int initialUnassignedCount) {
         this.scoreDirector = scoreDirector;
         this.unassignedCount = initialUnassignedCount;
 
-        this.canUseExternalizedLocation = externalizedIndexProcessor != null && externalizedInverseProcessor != null;
-        this.requiresLocationMap = !canUseExternalizedLocation
+        this.requiresLocationMap = externalizedIndexProcessor == null || externalizedInverseProcessor == null
                 || externalizedPreviousElementProcessor == null || externalizedNextElementProcessor == null;
         if (requiresLocationMap) {
             if (elementLocationMap == null) {
@@ -220,14 +218,14 @@ private enum ChangeType {
     }
 
     public ElementLocation getLocationInList(Object planningValue) {
-        if (!canUseExternalizedLocation) {
+        if (requiresLocationMap) {
             return Objects.requireNonNullElse(elementLocationMap.get(planningValue), ElementLocation.unassigned());
-        } else {
-            var inverse = getInverseSingleton(planningValue);
+        } else { // At this point, both inverse and index are externalized.
+            var inverse = externalizedInverseProcessor.getInverseSingleton(planningValue);
             if (inverse == null) {
                 return ElementLocation.unassigned();
             }
-            return ElementLocation.of(inverse, getIndex(planningValue));
+            return ElementLocation.of(inverse, externalizedIndexProcessor.getIndex(planningValue));
         }
     }