Clean up dead code and unused imports

- SSTableCursorWriter.currentOffsetInPartition had zero callers: its logic moved into
  BigCursorIndexWriter when the index-writer seam was extracted, but the dead private
  method was left behind.
- Five imports in SSTableCursorWriter (Ints, FSWriteError, RowIndexEntry, BloomFilter,
  ByteArrayUtil) are unused: BloomFilter/RowIndexEntry usage moved into
  BigCursorIndexWriter along with the rest of the index machinery.
- DifferentialCompactionTester recompiled the "expired" field's normalization regex on
  every dump line instead of once; now a static final Pattern.
- CursorCompactor.Purger took a nowInSec constructor parameter and stored it in a field
  that was never read; shouldPurge() only ever uses controller.gcBefore. Removed the
  dead field and parameter.

No behavior change.

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

@@ -315,7 +315,7 @@ public class CursorCompactor extends CompactionInfo.Holder
         this.sstableCursorsEqualsNext = new boolean[sstables.size()];
         this.enforceStrictLiveness = controller.cfs.metadata.get().enforceStrictLiveness();
 
-        purger = new Purger(type, controller, nowInSec);
+        purger = new Purger(type, controller);
 
         detachedLastWrittenKey = metadata.partitioner.createReusableKey(128);
     }
@@ -1423,8 +1423,6 @@ public class CursorCompactor extends CompactionInfo.Holder
      */
     static class Purger implements DeletionPurger
     {
-        private final long nowInSec;
-
         private final long oldestUnrepairedTombstone;
         private final boolean onlyPurgeRepairedTombstones;
         private final boolean shouldIgnoreGcGraceForAnyKey;
@@ -1438,12 +1436,11 @@ public class CursorCompactor extends CompactionInfo.Holder
 
         private long compactedUnfiltered;
 
-        Purger(OperationType type, AbstractCompactionController controller, long nowInSec)
+        Purger(OperationType type, AbstractCompactionController controller)
         {
             oldestUnrepairedTombstone = controller.compactingRepaired() ? Long.MAX_VALUE : Integer.MIN_VALUE;
             onlyPurgeRepairedTombstones = controller.cfs.getCompactionStrategyManager().onlyPurgeRepairedTombstones();
             shouldIgnoreGcGraceForAnyKey = controller.cfs.shouldIgnoreGcGraceForAnyKey();
-            this.nowInSec = nowInSec;
             this.controller = controller;
             this.type = type;
         }

src/java/org/apache/cassandra/io/sstable/SSTableCursorWriter.java

@@ -22,7 +22,6 @@ import java.io.IOException;
 import java.nio.ByteBuffer;
 
 import com.google.common.annotations.VisibleForTesting;
-import com.google.common.primitives.Ints;
 
 import org.agrona.collections.IntArrayList;
 
@@ -44,19 +43,15 @@ import org.apache.cassandra.db.rows.SerializationHelper;
 import org.apache.cassandra.db.rows.Unfiltered;
 import org.apache.cassandra.db.rows.UnfilteredSerializer;
 import org.apache.cassandra.dht.IPartitioner;
-import org.apache.cassandra.io.FSWriteError;
 import org.apache.cassandra.io.sstable.format.SSTableReader;
 import org.apache.cassandra.io.sstable.format.SortedTableWriter;
 import org.apache.cassandra.io.sstable.format.big.BigFormatPartitionWriter;
 import org.apache.cassandra.io.sstable.format.big.BigTableWriter;
-import org.apache.cassandra.io.sstable.format.big.RowIndexEntry;
 import org.apache.cassandra.io.sstable.metadata.MetadataCollector;
 import org.apache.cassandra.io.util.DataOutputBuffer;
 import org.apache.cassandra.io.util.DataOutputPlus;
 import org.apache.cassandra.io.util.SequentialWriter;
 import org.apache.cassandra.schema.ColumnMetadata;
-import org.apache.cassandra.utils.BloomFilter;
-import org.apache.cassandra.utils.ByteArrayUtil;
 import org.apache.cassandra.utils.ByteBufferUtil;
 import org.apache.cassandra.utils.concurrent.Ref;
 
@@ -513,12 +508,6 @@ public class SSTableCursorWriter implements AutoCloseable
         metadataCollector.updateClusteringValues(unfilteredDescriptor);
     }
 
-
-    private long currentOffsetInPartition(long position)
-    {
-        return position - partitionStart;
-    }
-
     /**
      * Garbage-free equivalent of {@link org.apache.cassandra.db.Columns.Serializer}'s
      * serializeSubset for a PARTIAL subset (not all-present, not all-missing — callers

test/unit/org/apache/cassandra/db/compaction/differential/DifferentialCompactionTester.java

@@ -92,6 +92,12 @@ public abstract class DifferentialCompactionTester extends CQLTester
     /** Fixed "now" used for JSON dumps so rendering cannot depend on wall clock. */
     private static final long DUMP_NOW_SEC = 0;
 
+    // sstabledump's "expired" fields come from WALL CLOCK, not the fixed nowInSec above
+    // (JsonTransformer), so the two paths' captures can render them differently; the flag is
+    // derived from expires_at, which is still compared.
+    private static final java.util.regex.Pattern EXPIRED_FLAG =
+        java.util.regex.Pattern.compile("\"expired\"\\s*:\\s*(true|false)");
+
     /**
      * Scale mode for very large scenarios (millions of rows): the logical dump is streamed
      * into a SHA-256 digest instead of being retained as a String, so capture memory stays
@@ -422,7 +428,7 @@ public abstract class DifferentialCompactionTester extends CQLTester
                                             sstable.metadata(), DUMP_NOW_SEC, baos);
             }
             json = baos.toString(StandardCharsets.UTF_8)
-                       .replaceAll("\"expired\"\\s*:\\s*(true|false)", "\"expired\":\"normalized\"");
+                       .transform(s -> EXPIRED_FLAG.matcher(s).replaceAll("\"expired\":\"normalized\""));
         }
 
         // 3. stats spot-check summary
@@ -633,9 +639,9 @@ public abstract class DifferentialCompactionTester extends CQLTester
 
         private void update(byte[] bytes, int length)
         {
-            byte[] normalized = new String(bytes, 0, length, StandardCharsets.UTF_8)
-                                .replaceAll("\"expired\"\\s*:\\s*(true|false)", "\"expired\":\"normalized\"")
-                                .getBytes(StandardCharsets.UTF_8);
+            byte[] normalized = EXPIRED_FLAG.matcher(new String(bytes, 0, length, StandardCharsets.UTF_8))
+                                            .replaceAll("\"expired\":\"normalized\"")
+                                            .getBytes(StandardCharsets.UTF_8);
             digest.update(normalized);
             bytesSeen += normalized.length;
         }