perf: improve tuple store performance (#1329)

We were being too clever and the clever approach doesn't seem justified.
This improves performance in the benchmarks by 3 to 6 percent.

core/src/main/java/ai/timefold/solver/core/impl/score/stream/bavet/common/AbstractJoinNode.java

@@ -91,8 +91,8 @@ private void updateOutTupleLeft(OutTuple_ outTuple, LeftTuple_ leftTuple) {
     private void doUpdateOutTuple(OutTuple_ outTuple) {
         var state = outTuple.state;
         if (!state.isActive()) { // Impossible because they shouldn't linger in the indexes.
-            throw new IllegalStateException("Impossible state: The tuple (" + outTuple.state + ") in node (" +
-                    this + ") is in an unexpected state (" + outTuple.state + ").");
+            throw new IllegalStateException("Impossible state: The tuple (%s) in node (%s) is in an unexpected state (%s)."
+                    .formatted(outTuple, this, outTuple.state));
         } else if (state != TupleState.OK) { // Already in the queue in the correct state.
             return;
         }
@@ -155,10 +155,9 @@ protected final void retractOutTuple(OutTuple_ outTuple) {
         ElementAwareListEntry<OutTuple_> outEntryRight = outTuple.removeStore(outputStoreIndexRightOutEntry);
         outEntryRight.remove();
         var state = outTuple.state;
-        if (!state.isActive()) {
-            // Impossible because they shouldn't linger in the indexes.
-            throw new IllegalStateException("Impossible state: The tuple (" + outTuple.state + ") in node (" + this
-                    + ") is in an unexpected state (" + outTuple.state + ").");
+        if (!state.isActive()) { // Impossible because they shouldn't linger in the indexes.
+            throw new IllegalStateException("Impossible state: The tuple (%s) in node (%s) is in an unexpected state (%s)."
+                    .formatted(outTuple, this, outTuple.state));
         }
         propagationQueue.retract(outTuple, state == TupleState.CREATING ? TupleState.ABORTING : TupleState.DYING);
     }

core/src/main/java/ai/timefold/solver/core/impl/score/stream/bavet/common/tuple/AbstractTuple.java

@@ -19,43 +19,26 @@
  */
 public abstract sealed class AbstractTuple permits UniTuple, BiTuple, TriTuple, QuadTuple {
 
-    /*
-     * We create a lot of tuples, many of them having store size of 1.
-     * If an array of size 1 was created for each such tuple, memory would be wasted and indirection created.
-     * This trade-off of increased memory efficiency for marginally slower access time is proven beneficial.
-     */
-    private final boolean storeIsArray;
-
-    private Object store;
+    private static final Object[] EMPTY_STORE = new Object[0];
+
+    private final Object[] store;
     public TupleState state = TupleState.DEAD; // It's the node's job to mark a new tuple as CREATING.
 
     protected AbstractTuple(int storeSize) {
-        this.store = (storeSize < 2) ? null : new Object[storeSize];
-        this.storeIsArray = store != null;
+        this.store = storeSize == 0 ? EMPTY_STORE : new Object[storeSize];
     }
 
     public final <Value_> Value_ getStore(int index) {
-        return (Value_) (storeIsArray ? ((Object[]) store)[index] : store);
+        return (Value_) store[index];
     }
 
     public final void setStore(int index, Object value) {
-        if (storeIsArray) {
-            ((Object[]) store)[index] = value;
-        } else {
-            store = value;
-        }
+        store[index] = value;
     }
 
-    public <Value_> Value_ removeStore(int index) {
-        Value_ value;
-        if (storeIsArray) {
-            Object[] array = (Object[]) store;
-            value = (Value_) array[index];
-            array[index] = null;
-        } else {
-            value = (Value_) store;
-            store = null;
-        }
+    public final <Value_> Value_ removeStore(int index) {
+        Value_ value = getStore(index);
+        setStore(index, null);
         return value;
     }