chore: save split random on step start so we can restore it

This allows move iterators to generate additional moves without
affecting the random (and thus reach the same result when those
moves are not generated provided the same number of steps).

Author: Christopher-Chianelli

core/src/main/java/ai/timefold/solver/core/impl/solver/AbstractSolver.java

@@ -2,6 +2,7 @@
 
 import java.util.Iterator;
 import java.util.List;
+import java.util.random.RandomGenerator;
 
 import ai.timefold.solver.core.api.domain.solution.PlanningSolution;
 import ai.timefold.solver.core.api.solver.Solver;
@@ -12,6 +13,7 @@
 import ai.timefold.solver.core.impl.phase.scope.AbstractPhaseScope;
 import ai.timefold.solver.core.impl.phase.scope.AbstractStepScope;
 import ai.timefold.solver.core.impl.solver.event.SolverEventSupport;
+import ai.timefold.solver.core.impl.solver.random.DelegatingSplittableRandomGenerator;
 import ai.timefold.solver.core.impl.solver.recaller.BestSolutionRecaller;
 import ai.timefold.solver.core.impl.solver.scope.SolverScope;
 import ai.timefold.solver.core.impl.solver.termination.UniversalTermination;
@@ -44,6 +46,8 @@ public abstract class AbstractSolver<Solution_> implements Solver<Solution_> {
     protected final UniversalTermination<Solution_> globalTermination;
     protected final List<Phase<Solution_>> phaseList;
 
+    private RandomGenerator.SplittableGenerator savedRandom;
+
     // ************************************************************************
     // Constructors and simple getters/setters
     // ************************************************************************
@@ -123,10 +127,12 @@ public void stepStarted(AbstractStepScope<Solution_> stepScope) {
         bestSolutionRecaller.stepStarted(stepScope);
         phaseLifecycleSupport.fireStepStarted(stepScope);
         globalTermination.stepStarted(stepScope);
+        savedRandom = ((DelegatingSplittableRandomGenerator) stepScope.getWorkingRandom()).split();
         // Do not propagate to phases; the active phase does that for itself and they should not propagate further.
     }
 
     public void stepEnded(AbstractStepScope<Solution_> stepScope) {
+        ((DelegatingSplittableRandomGenerator) stepScope.getWorkingRandom()).setDelegate(savedRandom);
         bestSolutionRecaller.stepEnded(stepScope);
         phaseLifecycleSupport.fireStepEnded(stepScope);
         globalTermination.stepEnded(stepScope);

core/src/main/java/ai/timefold/solver/core/impl/solver/random/DefaultRandomFactory.java

@@ -32,7 +32,8 @@ public DefaultRandomFactory(RandomType randomType, Long randomSeed) {
     public RandomGenerator createRandom() {
         switch (randomType) {
             case JDK:
-                return randomSeed == null ? new DelegatingSplittableRandomGenerator(new Random().nextLong()) : new DelegatingSplittableRandomGenerator(randomSeed);
+                return randomSeed == null ? new DelegatingSplittableRandomGenerator(new Random().nextLong())
+                        : new DelegatingSplittableRandomGenerator(randomSeed);
             case MERSENNE_TWISTER:
                 return new RandomAdaptor(randomSeed == null ? new MersenneTwister() : new MersenneTwister(randomSeed));
             case WELL512A:

core/src/test/java/ai/timefold/solver/core/impl/solver/DefaultSolverTest.java

@@ -1397,8 +1397,8 @@ void solveStaleBuiltinShadows() {
 
         var solution = PlannerTestUtils.solve(solverConfig, problem);
 
-        assertThat(solution.getEntityList().getFirst().getValue().getCode()).isEqualTo("v1");
-        assertThat(solution.getEntityList().get(1).getValue().getCode()).isEqualTo("v2");
+        assertThat(solution.getEntityList().getFirst().getValue().getCode()).isEqualTo("v2");
+        assertThat(solution.getEntityList().get(1).getValue().getCode()).isEqualTo("v1");
 
         assertThat(solution.getScore()).isEqualTo(SimpleScore.of(-2));
     }