presto-spi/src/main/java/io/prestosql/spi/block/VariableWidthBlockBuilder.java/VariableWidthBlockBuilder/getSliceLength
class VariableWidthBlockBuilder:    @Override
    public int getSliceLength(int position)
    {
        checkValidPosition(position, positions);
        return getOffset((position + 1)) - getOffset(position);
    }