@RequiresNonNull({"joinStart", "nFieldsInJoinFactor"})
  private ImmutableBitSet factorBitmap(
          @UnknownInitialization HiveLoptMultiJoin this,
          ImmutableBitSet fieldRefBitmap) {
    ImmutableBitSet.Builder factorRefBitmap = ImmutableBitSet.builder();
    for (int field : fieldRefBitmap) {
      int factor = findRef(field);
      factorRefBitmap.set(factor);
    }
    return factorRefBitmap.build();
  }
Sets the bitmap indicating which factors a filter references based on
which fields it references.

@param fieldRefBitmap bitmap representing fields referenced
@return bitmap representing factors referenced that will
be set by this method
ql/src/java/org/apache/hadoop/hive/ql/optimizer/calcite/rules/HiveLoptMultiJoin.java