presto-spi/src/main/java/io/prestosql/spi/block/VariableWidthBlock.java/VariableWidthBlock/getSliceLength
class VariableWidthBlock:    @Override
    public int getSliceLength(int position)
    {
        checkReadablePosition(position);
        return getPositionOffset(position + 1) - getPositionOffset(position);
    }