presto-orc/src/main/java/io/prestosql/orc/writer/MapColumnWriter.java/MapColumnWriter/writeColumnarMap
class MapColumnWriter:    private void writeColumnarMap(ColumnarMap columnarMap)
    {
        // write nulls and lengths
        for (int position = 0; position < columnarMap.getPositionCount(); position++) {
            boolean present = !columnarMap.isNull(position);
            presentStream.writeBoolean(present);
            if (present) {
                nonNullValueCount++;
                lengthStream.writeLong(columnarMap.getEntryCount(position));
            }
        }

        // write keys and value
        Block keysBlock = columnarMap.getKeysBlock();
        if (keysBlock.getPositionCount() > 0) {
            keyWriter.writeBlock(keysBlock);
            valueWriter.writeBlock(columnarMap.getValuesBlock());
        }
    }