CNDB-11950: cache sstable density result to speed up test (#1454)

### What is the issue
riptano/cndb#11950

### What does this PR fix and why was it fixed
We saw many test failures in `UnifiedCompactionStrategyTest` after
#1407. After investigating it a bit, it seems that the root cause to the
unit test failure is likely the cost associated with the mockito calls
to get different values.

However, without changing anything in Mockito, I was able to optimize
the `UCS::getLevels` method enough to make the test suite go from timing
out to taking 3 minutes 11 seconds when running `ant test
-Dtest.name=UnifiedCompactionStrategyTest` on the command line.

Let's see if the test passes in butler.

### Checklist before you submit for review
- [ ] Make sure there is a PR in the CNDB project updating the Converged
Cassandra version
- [ ] Use `NoSpamLogger` for log lines that may appear frequently in the
logs
- [ ] Verify test results on Butler
- [ ] Test coverage for new/modified code is > 80%
- [ ] Proper code formatting
- [ ] Proper title for each commit staring with the project-issue
number, like CNDB-1234
- [ ] Each commit has a meaningful description
- [ ] Each commit is not very long and contains related changes
- [ ] Renames, moves and reformatting are in distinct commits

src/java/org/apache/cassandra/db/compaction/UnifiedCompactionStrategy.java

@@ -28,7 +28,6 @@
 import java.util.Set;
 import java.util.TreeMap;
 import java.util.UUID;
-import java.util.concurrent.atomic.AtomicLong;
 import java.util.function.BiPredicate;
 import java.util.regex.Matcher;
 import java.util.regex.Pattern;
@@ -873,14 +872,20 @@ public Map<Arena, List<Level>> getLevels(Collection<? extends CompactionSSTable>
         for (Arena arena : arenas)
         {
             List<Level> levels = new ArrayList<>(MAX_LEVELS);
-            arena.sstables.sort(currentShardManager::compareByDensity);
+
+            // Precompute the density, then sort.
+            List<SSTableWithDensity> ssTableWithDensityList = new ArrayList<>(arena.sstables.size());
+            for (CompactionSSTable sstable : arena.sstables)
+                ssTableWithDensityList.add(new SSTableWithDensity(sstable, currentShardManager.density(sstable)));
+            Collections.sort(ssTableWithDensityList);
 
             double maxSize = controller.getMaxLevelDensity(0, controller.getBaseSstableSize(controller.getFanout(0)) / currentShardManager.localSpaceCoverage());
             int index = 0;
             Level level = new Level(controller, index, 0, maxSize);
-            for (CompactionSSTable candidate : arena.sstables)
+            for (SSTableWithDensity candidateWithDensity : ssTableWithDensityList)
             {
-                final double size = currentShardManager.density(candidate);
+                final CompactionSSTable candidate = candidateWithDensity.sstable;
+                final double size = candidateWithDensity.density;
                 if (size < level.max)
                 {
                     level.add(candidate);
@@ -1445,4 +1450,25 @@ public String toString()
                                  FBUtilities.prettyPrintMemory(spaceAvailable), rateLimitLog, remainingAdaptiveCompactions);
         }
     }
+
+    /**
+     * Utility wrapper to efficiently store the density of an SSTable with the SSTable itself.
+     */
+    private static class SSTableWithDensity implements Comparable<SSTableWithDensity>
+    {
+        final CompactionSSTable sstable;
+        final double density;
+
+        SSTableWithDensity(CompactionSSTable sstable, double density)
+        {
+            this.sstable = sstable;
+            this.density = density;
+        }
+
+        @Override
+        public int compareTo(SSTableWithDensity o)
+        {
+            return Double.compare(density, o.density);
+        }
+    }
 }