Compare if the file content is changed on the buffer

Question

I want to do something like if the "buffer-changed" but "content-not-changed", reverted the buffer.

When I work on the checking if the content changed, I tested the following code, but it can't work, it always return nil even if I just save the buffer.

(defun file-content-keep-same(BUFFER)
  (interactive)
  (save-restriction 
  (widen)
  (let ((original-buffer BUFFER)
    (original-size (buffer-size BUFFER))
    (filename-on-disk (buffer-file-name (current-buffer))))
    (message "buffer size %d" original-size)
    (with-temp-buffer
      (insert-file-contents filename-on-disk)
      (message "%s %d" filename-on-disk (point-max))
      (zerop
       (compare-buffer-substrings original-buffer 1 original-size
                 (current-buffer) 1 (point-max))))
    )
  ))

where is it wrong for above code?

Answer 1

where is it wrong for above code?

(buffer-size) ≠ (point-max), your use of compare-buffer-substrings is incorrect. A better idea is omitting the START1/END1/START2/END2 argument by supplying nil, then (point-min) and (point-max) will be used automatically in corresponding buffers.

---

Here is my attempt (for code simplicity, it doesn't take account of narrowing)

(defun same-buffer-file-p (buffer file)
  "Return t if BUFFER and FILE have the same contents."
  (with-temp-buffer
    (insert-file-contents file)
    (zerop
     (let ((case-fold-search nil))
       (compare-buffer-substrings buffer nil nil
                                  (current-buffer) nil nil)))))

;;; Tests

(write-region "hello" nil "hello.txt")

(with-temp-buffer
  (insert "hello")
  (same-buffer-file-p (current-buffer) "hello.txt"))
;; => t

(with-temp-buffer
  (insert "HELLO")
  (same-buffer-file-p (current-buffer) "hello.txt"))
;; => nil

---

By the way, Emacs already provides similar feature in C-x s (save-some-buffers) then d (or simply M-x diff-buffer-with-file).