How can I set foldlevelstart in vim to just fold nothing initially, still allowing zr to work immediately?

Question

In vim, I typically set foldmethod to indent for most file types. In general, I like files to open with all folds open. However, it is useful to start immediately using zm to start closing folds globally across the file, one level at a time, allowing me to see the overall structure of the file without details getting in the way.

The only general solution I have found is to set foldlevelstart to a very high number, e.g. 99, to make sure that all files (bearing in mind different files will have different levels of maximum indent) start completely unfolded. However, I'd then have to use zm repeatedly to reduce foldlevel down to the maximum fold level of the file, normally much less than 99, which is cumbersome and impractical. I could set foldlevel manually using the vim command line, but I'd still need to know the maximum indent in the file.

Is there a practical way to open a file with foldlevel set to exactly one more than the maximum current indent/fold level in the file itself?

For example, given the following file:

a
  b
    c
      d1
      d2
    e
      f1
      f2

The first keypress of zm would show this:

a
  b
    c
+--  2 lines: d1------
    e
+--  2 lines: f1------

(Note, however, that it should work for the general case where the initial maximum indent of the file could be any value).

Answer 1

This finds the highest foldlevel in the current file:

:let &foldlevel = max(map(range(1, line('$')), 'foldlevel(v:val)'))

To set this automatically, you can use an :autocmd:

:autocmd BufWinEnter * let &foldlevel = max(map(range(1, line('$')), 'foldlevel(v:val)'))

Answer 2

The option set nofoldenable lets you start off the file with all folds open

Answer 3

Instead of using zr, use zR, which will open all folds. Similarly, zM will close all folds.