presto-spi/src/main/java/io/prestosql/spi/block/SingleMapBlockWriter.java/SingleMapBlockWriter/writeLong
class SingleMapBlockWriter:    @Override
    public BlockBuilder writeLong(long value)
    {
        if (writeToValueNext) {
            valueBlockBuilder.writeLong(value);
        }
        else {
            keyBlockBuilder.writeLong(value);
        }
        return this;
    }