public void moveForward(int steps) {
      safetyPiles[safetyPileIndex] += steps;
    }
Set safetyIndex for corresponding safetyPile.

@param safetyPilePosition the index of the first element that cannot be evicted. in other
    words, elements whose index are <b>less than</b> the safetyPilePosition can be evicted.
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/queryengine/transformation/dag/memory/SafetyLine.java