@Override
  public CompactionWindow newIncomingWindow(long now) {
    return new Window(baseWindowMillis, now / baseWindowMillis, getMaxTierAgeCutoff(now));
  }
A timestamp t is within the window iff t / size == divPosition.
hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/compactions/ExponentialCompactionWindowFactory.java