presto-rcfile/src/main/java/io/prestosql/rcfile/text/StructEncoding.java/StructEncoding/encodeValueInto
class StructEncoding:    @Override
    public void encodeValueInto(int depth, Block block, int position, SliceOutput output)
            throws RcFileCorruptionException
    {
        byte separator = getSeparator(depth);

        Block row = block.getObject(position, Block.class);
        for (int fieldIndex = 0; fieldIndex < structFields.size(); fieldIndex++) {
            if (fieldIndex > 0) {
                output.writeByte(separator);
            }

            if (row.isNull(fieldIndex)) {
                output.writeBytes(nullSequence);
            }
            else {
                structFields.get(fieldIndex).encodeValueInto(depth + 1, row, fieldIndex, output);
            }
        }
    }