Count number of visible lines in document with folding?

Question

I'd like to write a plugin that automatically sets the foldlevel of a newly opened document so that as much of the document fits on the screen as possible. (In other words, I'd like to make sure short documents open fully expanded while long documents open mostly collapsed.)

Is there any built-in function in vim that reports the number of visible lines in a document, given the current fold state or a particular fold level? (I.e., closed folds count as one line, but lines you'd have to scroll down to see are counted, too.)

Answer 1

Nice idea! Here's a basic function that achieves your goal using a different technique:

function! FitOnPage()
  normal! zR
  while &foldlevel > 0
    if line('w0') == 1 && line('w$') == line('$')
      break
    endif
    normal! zm
  endwhile
endfunction

It works by first expanding all the folds. It then closes folds (globally) until the entire document is on screen or all the folds are closed.

It checks if the whole file is on screen by checking if the first and last lines displayed are the first and last lines of the file.

You could improve this a bit by closing folds individually, but that would be a lot more complicated.