How to preserve the visual height of the cursor after an operation?

Question

Q: How do I find the visual line number of point (so that I can restore it after an operation)?

By “visual line number”, I'm referring to the number N of lines between the top of the window (or the screen) and the cursor, not the top of the buffer.

At first I thought I could calculate N with the following snippet:

(- (line-number-at (point))
   (line-number-at (window-start)))

However, to complicate things, I have sections of the buffer which are invisible. So the snippet above usually returns values much larger than N.

---

Context:
I need to perform an operation that essentially erases the entire buffer and writes it again, but I want it to be mostly invisible to the user.

Since the buffer contents are created anew, save-excursion doesn't help in this situation. Still, I manage to preserve point position by saving it as a number, instead of a marker.

(let ((point (point)))
  (recreate-buffer)
  (goto-char point))

Q (alternative wording): Is there a similar method I can use to preserve the visual height of the cursor?
By that, I mean that if the cursor is initially on the N^th visible line of the screen, the window should be scrolled after the operation so that this remains true.

Answer 1

You can use (count-screen-lines &optional beg end count-final-newline window) (manual page) to find the number of lines shown between two points.

You can do the following to re-create the point's position on the screen:

(let ((lines-down (count-screen-lines nil (point) t))
      (lines-from-top-of-window (count-screen-lines (window-start) (point) t))
      (diff-beg-of-line (- (point) (line-beginning-position))))
  ;; Code that redraws screen goes here:
  ;; <code></code>
  ;; Now, to find your old position:
  (if (equal lines-down 0)
      ;; When the point is 1, lines-down is 0. In all other cases,
      ;; `count-screen-lines' is 1-indexed by the line. Set
      ;; `lines-down' to 1 to compensate for this edge case.
      (setq lines-down 1))
  (move-to-window-line (- lines-down 1))
  (recenter lines-from-top-of-window)
  (goto-char (+ (point) diff-beg-of-line)))