Thinking about undo/redo is a great place to start thinking about your text editor's underlying data structure. I went down this rabbit hole a while back. I had to really dig[1]: it's been 5 years... --- Data structures aren't the only interesting rabbit-hole, though. UI/UX doesn't get nearly as much attention as it deserves. There are really only two that I am aware of: Notepad and Vim. Vim's modal editing results i…
I still use a lot of nvi on openbsd. and it has a strange quirk to it's undo. so "u" undoes your last action. but if you hit it again it undoes the undo, a redo if you will. I did not really think about it much, just accepted that was how nvi worked, a single level undo. But that is not true at all. the trick is you have to "u" undo then "." repeat previous action and you get the full undo stack. and, like the parent…
How undo and redo commands work depends on the 'u' flag in 'cpoptions'.
There is the Vim way ('u' excluded) and the Vi-compatible way ('u' included).
In the Vim way, "uu" undoes two changes. In the Vi-compatible way, "uu" does
nothing (undoes an undo).
'u' excluded, the Vim way:
You can go back in time with the undo command. You can then go forward again
with the redo command. If you make a new change after the undo command,
the redo will not be possible anymore.
'u' included, the Vi-compatible way:
The undo command undoes the previous change, and also the previous undo
command. The redo command repeats the previous undo command. It does NOT
repeat a change command, use "." for that.
(Actually, I have so rarely used vi that I'm relying on the implications of the Vim manual that it's original vi behavior.)