(defvar region-history (make-ring region-history-size))

(cl-defstruct region-history-entry buffer beg end)

(defun region-history-set-size (symbol value)
  (set-default symbol value)
  (region-history-extend-if-necessary))

(defun region-history-extend-if-necessary ()
  (let ((current-size (ring-size region-history)))
    (when (> region-history-size
             (ring-size region-history))
      (ring-extend region-history
                   (- region-history-size current-size)))))

(defcustom region-history-size 50
  "Maximum number of entries in transient mark history."
  :type 'number
  :set 'region-history-set-size)

(defun region-history-insert (beg end)
  (unless (markerp beg)
    (error "First argument must be a marker, %s provided" (type-of (point-marker))))
  (unless (markerp end)
    (error "Second argument must be a marker, %s provided" (type-of (point-marker))))
  (let ((buffer (marker-buffer beg)))
    (unless (eq (marker-buffer end) buffer)
      (error "Markers do not belong to the same buffer"))
    (ring-insert region-history
                 (make-region-history-entry
                  :buffer buffer
                  :beg beg
                  :end end))))

(defun region-history-is-entry-valid (entry)
  (and (buffer-live-p (region-history-entry-buffer entry))
       (not (eq (marker-position (region-history-entry-beg entry))
                (marker-position (region-history-entry-end entry))))))

(defun region-history-prev ()
  (interactive)
  (message "%S" (ring-length region-history))
  (unless (ring-empty-p region-history)
   (with-region-history-mode-disabled
     (let ((entry (ring-remove region-history 0)))
       (if (region-history-is-entry-valid entry)
           (progn
             (ring-insert-at-beginning region-history entry)
             (switch-to-buffer (region-history-entry-buffer entry))
             (set-mark (region-history-entry-beg entry))
             (goto-char (region-history-entry-end entry)))
         (region-history-prev))))))

(defun region-history-next ()
  (interactive)
  (unless (ring-empty-p region-history)
   (with-region-history-mode-disabled
     (let ((entry (ring-remove region-history)))
       (if (region-history-is-entry-valid entry)
           (progn
             (ring-insert region-history entry)
             (switch-to-buffer (region-history-entry-buffer entry))
             (set-mark (region-history-entry-beg entry))
             (goto-char (region-history-entry-end entry)))
         (region-history-next))))))

(defmacro with-region-history-mode-disabled (&rest body)
  "Evaluate body with region-history-mode disabled."
  (declare (indent 0) (debug t))
  `(unwind-protect
        (progn (region-history-mode -1)
               ,@body)
     (region-history-mode 1)))

(defun region-history-reset ()
  (interactive)
  (setq region-history (make-ring region-history-size)))

(defun region-history-deactivate-mark-hook ()
  (region-history-insert (mark-marker) (point-marker)))

(defvar region-history-mode-)

(define-minor-mode region-history-mode
    "Minor mode for saving and navigating history of selected regions."
  :global t
  :keymap
  (easy-mmode-define-keymap
   (list (cons (kbd "M-p") 'region-history-prev)
         (cons (kbd "M-n") 'region-history-next)))
  (if region-history-mode
      (add-hook 'deactivate-mark-hook 'region-history-deactivate-mark-hook)
    (remove-hook 'deactivate-mark-hook 'region-history-deactivate-mark-hook)))