emacs - elisp:如何获得当前点上方一行的点？

Question

例如，我试图返回当前 (point) 上一行位置的缓冲区字符位置。我的完整功能如下。

我相信 (point) 是字符位置，所以我想减去当前点和当前点上方位置之间的字符数。但是，这取决于当前点上方的线的长度(并不总是等于 frame-char-height)。

I am trying to mimic the Eclipse comment feature, where when an area is selected, the bottom-most line (the line with the pointer) is not included in the commented region:

(defun comment-eclipse (&optional arg)
  (interactive)
  (let ((start (line-beginning-position))
        (end (line-end-position)))
    (when (or (not transient-mark-mode) (region-active-p))
      (setq start (save-excursion
                    (goto-char (region-beginning))
                    (beginning-of-line)
                    (point))
            end (save-excursion
                  (goto-char (region-end))
                  (end-of-line)
                  (point)))) ;; HERE: I want to return something like (- (point) (line-length))
    (comment-or-uncomment-region start end)))

如有任何关于如何实现此目标的建议，我们将不胜感激。

更新

感谢 lunaryorn 在下面的回答，我改进了我的功能如下:

(defun comment-eclipse (&optional arg)
  (interactive)
  (let ((start (line-beginning-position))
        (end (line-end-position)))
    (when (or (not transient-mark-mode) (region-active-p))
      (setq start (save-excursion
                    (goto-char (region-beginning))
                    (beginning-of-line)
                    (point))
            end (save-excursion
                  (goto-char (region-end));;move point to region end
                  (end-of-line);;move point to end of line
                  (forward-line -1)
                  (end-of-line)
                  (point))))
    (comment-or-uncomment-region start end)))

Answer 1

使用 current-column 的组合获取一行中的当前列，forward-line 导航到另一行，以及 move-to- column 恢复新行上的列:

(let ((column (current-column)))
  (forward-line -1)
  (move-to-column column))