Implement a transformation which takes value from another column of output file

[anatoly-scherbakov]

How to call this?

• output
• column
• from-column
• copy-column
• duplicate

This task also means that we need to process columns in the order of their dependencies one from another; this makes this task rather complicated.

[anatoly-scherbakov]

I will go with from.

[anatoly-scherbakov]

When transformation chain for column a refers to column b, it requires that the value for b for current line must be already computed.

Right now, the cells of output row are calculated in the order in which they come in output file, left to right. However, since column 1 might in fact reference column 2, such a simplistic calculation is not always possible.

What we have to do is:

• Determine the order of processing for the available columns;
• Calculate columns in that specified order;
• When calculating every column, we need to have access to results of calculating previous columns (as vector? as linked map?)
• When outputting the row, we need to order it properly for output.

[anatoly-scherbakov]

I believe we will use a mutable BTreeMap<String, Option<String>> for a while to store values of the current output row. This is not efficient, but I'd like to get a proof of concept working first.

We will separately store a Vec<String> sequence of columns in the order of processing them.