private MockId getDeletedKeyFromIndexSegment(Offset indexSegmentStartOffset) {
    MockId deletedId = null;
    for (Map.Entry<MockId, TreeSet<IndexValue>> indexSegmentEntry : state.referenceIndex.get(indexSegmentStartOffset)
        .entrySet()) {
      if (indexSegmentEntry.getValue().last().isDelete()) {
        deletedId = indexSegmentEntry.getKey();
        break;
      }
    }
    return deletedId;
  }
Gets a deleted key from the index segment with start offset {@code indexSegmentStartOffset}.
@param indexSegmentStartOffset the start {@link Offset} of the index segment from which a deleted key is required
@return a deleted key from the index segment with start offset {@code indexSegmentStartOffset}.
ambry-store/src/test/java/com/github/ambry/store/IndexTest.java