perf: more efficient LoopedTracker

Author: triceo

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

@@ -1,41 +1,33 @@
 package ai.timefold.solver.core.impl.domain.variable.declarative;
 
-import java.util.BitSet;
+import ai.timefold.solver.core.impl.util.DynamicIntArray;
 
 import org.jspecify.annotations.NullMarked;
 
 @NullMarked
 public final class LoopedTracker {
 
-    // Simple LoopedStatus[] array would have occupied too much memory with large node counts.
-    // Furthermore, allocating and/or clearing these large arrays is expensive as well.
-    private final BitSet present;
-    private final BitSet looped;
+    // For some reason, the array was getting re-created on every values() call.
+    // So, we cache a single instance.
+    private static final LoopedStatus[] VALUES = LoopedStatus.values();
+
+    private final DynamicIntArray looped;
 
     public LoopedTracker(int nodeCount) {
-        this.present = new BitSet(nodeCount);
-        this.looped = new BitSet(nodeCount);
+        this.looped = new DynamicIntArray(nodeCount);
     }
 
     public void mark(int node, LoopedStatus status) {
-        if (status == LoopedStatus.UNKNOWN) {
-            present.clear(node);
-            looped.clear(node);
-        } else {
-            present.set(node);
-            looped.set(node, status == LoopedStatus.LOOPED);
-        }
+        looped.set(node, status.ordinal());
     }
 
     public LoopedStatus status(int node) {
-        if (present.isEmpty() || !present.get(node)) {
-            return LoopedStatus.UNKNOWN;
-        }
-        return looped.get(node) ? LoopedStatus.LOOPED : LoopedStatus.NOT_LOOPED;
+        // When in the unallocated part of the dynamic array, the value returned is zero.
+        // Therefore it is imperative that LoopedStatus.UNKNOWN be the first element in the enum.
+        return VALUES[looped.get(node)];
     }
 
     public void clear() {
-        present.clear();
         looped.clear();
     }