>> and when you want the other one, it's quite painful.

It's really not that painful. A logical line of text may span multiple visual lines because of word wrapping. A visual line will never span more than one logical line. So you always have a 1-to-N relationship between logical and visual lines, and everyone knows you have to go to M-to-N before things get objectively painful.

Here's the thing about text editors: they are extremely well known problems. It's really hard to say anything about writing a text editor is "difficult" when we have hundreds of examples of free, open source software that we can sit with and analyze for requirements. There really aren't many new lessons to learn, certainly not in the memory representation for the file.

It's really nice to talk about parallelizing your text editor operations, but here in the year twenty thousand and seventeen, Vi is still single-threaded and not at all hardware accelerated and it still runs essentially infinitely faster than most other text editors. The only time Vi's single-threadedness becomes a problem is when plugin authors can't be arsed to not load up Python.

What data structure does Vi use? An array of lines.