How can I move by screen lines as quickly as forward-line?

Question

next-line is very slow, and this is creating problems. I'll give some background then explain why.

By default, linum.el treats a wrapped line as a single line, and it takes into account hidden lines when numbering. I need it to number visual lines only - i.e. it should ignore hidden lines, and count wrapped lines as multiple lines.

For example, this is how it works by default:

1 This is a line.
2 This is a long line which has been auto-wrapped
 >by emacs.
3 This is another line.

This is the behaviour I want:

1 This is a line.
2 This is a long line which has been auto-wrapped
3>by emacs.
4 This is another line.

I have this behaviour working, but it's very slow. By default, linum.el uses forward-line to number each line. This is the code that actually moves down by one line and increments the line counter:

(let ((inhibit-point-motion-hooks t))
  (forward-line))
(setq line (1+ line))

I modified it to use next-line:

(let ((inhibit-point-motion-hooks t))
  ;; (forward-line)
  (next-line))
(setq line (1+ line))

This works, but next-line is very slow in comparison to forward-line. Line numbers must be updated frequently so this slows down Emacs considerably.

I don't actually need to number every line. I am happy to use forward-line and number only the beginning of lines, as so:

1 This is a line.
2 This is a long line which has been auto-wrapped
 >by emacs.
4 This is another line.

But if I want to do this, I need to be able to check how many screen lines I moved when I called forward-line.

Does anyone have any ideas on how to solve this?

Answer 1

After delving into the source for next-line, I found a solution.

line-move-visual is the command I want. next-line is an interactive command with side effects that does a lot of unnecessary things, line-move-visual has the core behaviour I'm looking for.

This is the relevant passage after modification:

(let ((inhibit-point-motion-hooks t))
  ;; (forward-line)
  (line-move-visual 1 t))
(setq line (1+ line))

This is the full spec for the command:

(line-move-visual ARG &optional NOERROR)

Suppressing errors is necessary to prevent it dinging when it reaches the end of a buffer.

After some testing it's still a lot slower than forward-line, but it's fast enough to be usable if I delay updates until I'm idling.