Emacs: read-only copy of current buffer

Very often I split the emacs frame into two windows with the same buffer in both. For example on the left side I have a class definition and on the right side I implement it (so that part of the buffer is below the definition). While this works like a charm, sometimes I stumble upon an obstacle: if I want to change the class definition for testing, I cannot see the old definition for comparision. It has changed after all.

So I wrote this function which creates a copy of the current buffer, names it appropriately, makes it read-only, splits the current window and places the read-only copy on the right. Now I can see the old implementation while changing the current one, I can copy parts of the old stuff back into the current (because UNDO might remove some of the new things I want to keep) and so on. I bound this to C-c C-p, which you might want to change.

Here's the code. Put into .emacs to try it out:

;; ** Make a read-only copy of the current buffer
;; I just create a new read-only buffer and copy the contents of the
;; current one into it, which can be used as backup
(defvar copy-counter 0)
(defun get-copy-buffer-name()
“return unique copy buffer name”
(let (
(name (concat "*COPY “ (buffer-name (current-buffer)) ” (RO)"))
)
(if (not (get-buffer name))
(progn
(setq copy-counter (1+ copy-counter))
(concat name "<" (number-to-string copy-counter) ">")
)
(concat name)
)))
(defun copy-buffer-read-only()
“Create a read-only copy of the current buffer”
(interactive)
(let (
(old-buffer (current-buffer))
(new-buffer-name (get-copy-buffer-name))
)
(progn
(delete-other-windows)
(split-window-horizontally)
(other-window 1)
(if (not (eq (get-buffer new-buffer-name) nil))
(kill-buffer (get-buffer new-buffer-name))
)
(set-buffer (get-buffer-create new-buffer-name))
(insert-buffer-substring old-buffer)
(read-only-mode)
(switch-to-buffer new-buffer-name)
(other-window 1)
)))
(defalias ‘cp ‘copy-buffer-read-only)
(global-set-key (kbd “C-c C-p”)         ‘copy-buffer-read-only)

There's even a screenshot (the read-only copy can be seen on the right):