feat: add replaceValue... methods to MutableSolutionView

core/src/main/java/ai/timefold/solver/core/impl/move/MoveDirector.java

@@ -1,5 +1,7 @@
 package ai.timefold.solver.core.impl.move;
 
+import java.util.ArrayList;
+import java.util.Collections;
 import java.util.List;
 import java.util.Objects;
 import java.util.function.BiFunction;
@@ -197,6 +199,32 @@
         return element;
     }
 
+    @Override
+    public <Entity_, Value_> Value_ replaceValueBetweenLists(
+            PlanningListVariableMetaModel<Solution_, Entity_, Value_> variableMetaModel, Entity_ sourceEntity, int sourceIndex,
+            Entity_ replacementEntity, int replacementIndex) {
+        if (sourceEntity == replacementEntity) {
+            throw new IllegalArgumentException(
+                    "Source entity (%s) and replacement entity (%s) must be different when replacing values between lists."
+                            .formatted(sourceEntity, replacementEntity));
+        }
+
+        var variableDescriptor = extractVariableDescriptor(variableMetaModel);
+        var toReplace = (Value_) variableDescriptor.getElement(sourceEntity, sourceIndex);
+        externalScoreDirector.beforeListVariableElementUnassigned(variableDescriptor, toReplace);
+        externalScoreDirector.beforeListVariableChanged(variableDescriptor, sourceEntity, sourceIndex, sourceIndex + 1);
+        externalScoreDirector.beforeListVariableChanged(variableDescriptor, replacementEntity, replacementIndex,
+                replacementIndex + 1);
+        var toMove = variableDescriptor.removeElement(replacementEntity, replacementIndex);
+        variableDescriptor.setElement(sourceEntity, sourceIndex, toMove);
+        externalScoreDirector.afterListVariableChanged(variableDescriptor, replacementEntity, replacementIndex,
+                replacementIndex);
+        externalScoreDirector.afterListVariableChanged(variableDescriptor, sourceEntity, sourceIndex, sourceIndex + 1);
+        externalScoreDirector.afterListVariableElementUnassigned(variableDescriptor, toReplace);
+        externalScoreDirector.triggerVariableListeners();
+        return toReplace;
+    }
+
     @SuppressWarnings("unchecked")
     @Override
     public final <Entity_, Value_> Value_ moveValueInList(
@@ -212,9 +240,6 @@
                             .formatted(sourceIndex, destinationIndex));
         }
 
-        var fromIndex = Math.min(sourceIndex, destinationIndex);
-        var toIndex = Math.max(sourceIndex, destinationIndex) + 1;
-
         var variableDescriptor = extractVariableDescriptor(variableMetaModel);
         var list = variableDescriptor.getValue(sourceEntity);
         var listSize = list.size();
@@ -227,12 +252,87 @@
                             .formatted(destinationIndex, listSize));
         }
 
+        var fromIndex = Math.min(sourceIndex, destinationIndex);
+        var toIndex = Math.max(sourceIndex, destinationIndex) + 1;
         externalScoreDirector.beforeListVariableChanged(variableDescriptor, sourceEntity, fromIndex, toIndex);
-        var element = (Value_) list.remove(sourceIndex);
-        list.add(destinationIndex, element);
+        moveInList(list, sourceIndex, destinationIndex);
         externalScoreDirector.afterListVariableChanged(variableDescriptor, sourceEntity, fromIndex, toIndex);
         externalScoreDirector.triggerVariableListeners();
-        return element;
+        return (Value_) list.get(destinationIndex);
+    }
+
+    @Override
+    public <Entity_, Value_> Value_ replaceValueInList(
+            PlanningListVariableMetaModel<Solution_, Entity_, Value_> variableMetaModel, Entity_ sourceEntity, int sourceIndex,
+            int replacementIndex) {
+        if (sourceIndex == replacementIndex) {
+            throw new IllegalArgumentException(
+                    "When replacing values in the same list, sourceIndex (%d) and replacementIndex (%d) must be different."
+                            .formatted(sourceIndex, replacementIndex));
+        } else if (sourceIndex < 0 || replacementIndex < 0) {
+            throw new IllegalArgumentException(
+                    "The sourceIndex (%d) and replacementIndex (%d) must both be >= 0."
+                            .formatted(sourceIndex, replacementIndex));
+        }
+
+        var variableDescriptor = extractVariableDescriptor(variableMetaModel);
+        var fromIndex = Math.min(sourceIndex, replacementIndex);
+        var toIndex = Math.max(sourceIndex, replacementIndex) + 1;
+        var list = variableDescriptor.getValue(sourceEntity);
+        var toReplace = (Value_) list.get(replacementIndex);
+        externalScoreDirector.beforeListVariableElementUnassigned(variableDescriptor, toReplace);
+        externalScoreDirector.beforeListVariableChanged(variableDescriptor, sourceEntity, fromIndex, toIndex);
+        list.set(replacementIndex, list.get(sourceIndex));
+        // Remove from sourceIndex after setting the replacement to preserve index validity,
+        // as the replacement may occur after the source position.
+        list.remove(sourceIndex);
+        externalScoreDirector.afterListVariableChanged(variableDescriptor, sourceEntity, fromIndex, toIndex - 1);
+        externalScoreDirector.afterListVariableElementUnassigned(variableDescriptor, toReplace);
+        externalScoreDirector.triggerVariableListeners();
+        return toReplace;
+    }
+
+    /**
+     * Moves the element at index {@code from} to index {@code to} in a list,
+     * choosing the faster of two strategies based on the move's distance and position within the list.
+     *
+     * <p>
+     * <b>Strategy selection</b> (lo = min(from, to), d = |from − to|):
+     * <ul>
+     * <li>Use {@code Collections.rotate} when {@code d * 8 < n − lo}
+     * (distance is small relative to the remaining tail).</li>
+     * <li>Use {@code remove + add} otherwise.</li>
+     * </ul>
+     *
+     * <p>
+     * <b>Why position matters</b>: {@code remove+add} shifts {@code (n−1−from) + (n−1−to)} elements in total.
+     * When one endpoint is near the tail, one of those copies is nearly free,
+     * making {@code remove+add} cheap even for large lists.
+     * {@code rotate} always pays for the full sublist span,
+     * so it only wins when that span is short relative to what {@code removeAdd} would have to copy.
+     *
+     * <p>
+     * The threshold constant 8 was determined empirically by benchmarking on HotSpot
+     * with a microbenchmark that performed moves of varying distances and positions within lists of varying sizes.
+     *
+     * @param list the list to mutate; assumes {@link ArrayList}
+     * @param from index of the element to move
+     * @param to index the element should occupy after the move
+     */
+    private static <T> void moveInList(List<T> list, int from, int to) {
+        var distance = Math.abs(from - to);
+        if (distance == 1) {
+            Collections.swap(list, from, to);
+            return;
+        }
+        var lowerIndex = Math.min(from, to);
+        var distanceTimesEight = (long) distance * 8L; // Prevents unlikely yet possible overflow.
+        var tailLength = (long) list.size() - lowerIndex;
+        if (distanceTimesEight < tailLength) {
+            Collections.rotate(list.subList(lowerIndex, lowerIndex + distance + 1), from < to ? -1 : 1);
+        } else {
+            list.add(to, list.remove(from));
+        }
     }
 
     @Override
@@ -276,15 +376,11 @@
         }
 
         var variableDescriptor = extractVariableDescriptor(variableMetaModel);
-        var leftElement = variableDescriptor.getElement(entity, leftIndex);
-        var rightElement = variableDescriptor.getElement(entity, rightIndex);
-
         var fromIndex = Math.min(leftIndex, rightIndex);
         var toIndex = Math.max(leftIndex, rightIndex) + 1;
         externalScoreDirector.beforeListVariableChanged(variableDescriptor, entity, fromIndex, toIndex);
         var list = variableDescriptor.getValue(entity);
-        list.set(leftIndex, rightElement);
-        list.set(rightIndex, leftElement);
+        Collections.swap(list, leftIndex, rightIndex);
         externalScoreDirector.afterListVariableChanged(variableDescriptor, entity, fromIndex, toIndex);
         externalScoreDirector.triggerVariableListeners();
     }

core/src/main/java/ai/timefold/solver/core/preview/api/move/MutableSolutionView.java

@@ -177,6 +177,31 @@ <Entity_, Value_> void changeVariable(PlanningVariableMetaModel<Solution_, Entit
     <Entity_, Value_> Value_ moveValueBetweenLists(PlanningListVariableMetaModel<Solution_, Entity_, Value_> variableMetaModel,
             Entity_ sourceEntity, int sourceIndex, Entity_ destinationEntity, int destinationIndex);
 
+    /**
+     * Replaces a value from one entity's {@link PlanningListVariable planning list variable} with another.
+     * The value is removed from the replacementIndex, shifting all later values to the left.
+     * The value is then added at the sourceIndex, replacing the pre-existing value and unassigning it.
+     * This means that the replacement list will be one item shorter after the move.
+     *
+     * @param variableMetaModel Describes the variable to be changed.
+     * @param sourceEntity The entity in which the value will be replaced.
+     * @param sourceIndex The index in the source entity's list variable which contains the value to be replaced;
+     *        Acceptable values range from zero to one less than list size.
+     * @param replacementEntity The entity from which the value will be taken.
+     * @param replacementIndex The index in the destination entity's list variable which contains the value to be moved;
+     *        Acceptable values range from zero to one less than list size.
+     *        All values at or after the index are shifted to the left.
+     * @return the value that was replaced
+     * @throws IndexOutOfBoundsException if either index is out of bounds
+     * @throws IllegalArgumentException if sourceEntity == replacementEntity;
+     *         use {@link #replaceValueInList(PlanningListVariableMetaModel, Object, int, int)} instead.
+     * @see #moveValueBetweenLists(PlanningListVariableMetaModel, Object, int, Object, int) Similar operation that moves the
+     *      value to the destination without removing the pre-existing value.
+     */
+    <Entity_, Value_> Value_ replaceValueBetweenLists(
+            PlanningListVariableMetaModel<Solution_, Entity_, Value_> variableMetaModel, Entity_ sourceEntity, int sourceIndex,
+            Entity_ replacementEntity, int replacementIndex);
+
     /**
      * As defined by {@link #moveValueBetweenLists(PlanningListVariableMetaModel, Object, int, Object, int)},
      * but using {@link PositionInList} to specify the source and destination positions.
@@ -207,12 +232,39 @@ default <Entity_, Value_> Value_ moveValueBetweenLists(
      * @return the value that was moved
      * @throws IndexOutOfBoundsException if either index is out of bounds
      * @throws IllegalArgumentException if sourceIndex == destinationIndex
+     * @see #replaceValueInList(PlanningListVariableMetaModel, Object, int, int) Similar operation that replaces the value at
+     *      the destination index instead.
      * @see #shiftValue(PlanningListVariableMetaModel, Object, int, int) Equivalent operation using offset calculation instead
      *      of index arithmetics.
      */
     <Entity_, Value_> Value_ moveValueInList(PlanningListVariableMetaModel<Solution_, Entity_, Value_> variableMetaModel,
             Entity_ sourceEntity, int sourceIndex, int destinationIndex);
 
+    /**
+     * Moves a value within one entity's {@link PlanningListVariable planning list variable}.
+     * Behaves as if the value is first put in the replacementIndex,
+     * and then removed from the sourceIndex, shifting all later values to the left
+     * The value previously at the replacementIndex is unassigned.
+     *
+     * @param variableMetaModel Describes the variable to be changed.
+     * @param sourceEntity The entity whose variable value is to be moved.
+     * @param sourceIndex The index in the source entity's list variable which contains the value to be moved;
+     *        Acceptable values range from zero to one less than list size.
+     *        All values after the index are shifted to the left.
+     * @param replacementIndex The index in the source entity's list variable to which the value will be moved;
+     *        Acceptable values range from zero to one less than list size.
+     *        The value previously at this index is unassigned.
+     * @return the value that was replaced
+     * @throws IndexOutOfBoundsException if either index is out of bounds
+     * @throws IllegalArgumentException if sourceIndex == replacementIndex
+     * @see #moveValueInList(PlanningListVariableMetaModel, Object, int, int) Similar operation that moves the value to the
+     *      destination index instead.
+     * @see #shiftValue(PlanningListVariableMetaModel, Object, int, int) Equivalent operation using offset calculation instead
+     *      of index arithmetic.
+     */
+    <Entity_, Value_> Value_ replaceValueInList(PlanningListVariableMetaModel<Solution_, Entity_, Value_> variableMetaModel,
+            Entity_ sourceEntity, int sourceIndex, int replacementIndex);
+
     /**
      * Moves a value within one entity's {@link PlanningListVariable planning list variable},
      * by the given offset.