* lisp/emacs-lisp/tabulated-list.el: New optional print method

(tabulated-list-print): New optional argument, UPDATE.  If
non-nil, the list is printed by only adding and deleting the
changed entries, instead of erasing the whole buffer.  This method
is much faster when few or no entries have changed.

* doc/lispref/modes.texi (Tabulated List Mode): Document it.

* etc/NEWS: Document it.

1 files changed, 45 insertions, 9 deletions

diff --git a/lisp/emacs-lisp/tabulated-list.el b/lisp/emacs-lisp/tabulated-list.el
index 9d55ab8f533..58b8fd62d2f 100644
--- a/lisp/emacs-lisp/tabulated-list.el
+++ b/lisp/emacs-lisp/tabulated-list.el
@@ -298,7 +298,7 @@ column.  Negate the predicate that would be returned if
           (lambda (a b) (not (funcall sorter a b)))
         sorter))))
 
-(defun tabulated-list-print (&optional remember-pos)
+(defun tabulated-list-print (&optional remember-pos update)
   "Populate the current Tabulated List mode buffer.
 This sorts the `tabulated-list-entries' list if sorting is
 specified by `tabulated-list-sort-key'.  It then erases the
@@ -306,7 +306,14 @@ buffer and inserts the entries with `tabulated-list-printer'.
 
 Optional argument REMEMBER-POS, if non-nil, means to move point
 to the entry with the same ID element as the current line and
-recenter window line accordingly."
+recenter window line accordingly.
+
+Non-nil UPDATE argument means to use an alternative printing
+method which is faster if most entries haven't changed since the
+last print.  The only difference in outcome is that tags will not
+be removed from entries that haven't changed (see
+`tabulated-list-put-tag').  Don't use this immediately after
+changing `tabulated-list-sort-key'."
   (let ((inhibit-read-only t)
 	(entries (if (functionp tabulated-list-entries)
 		     (funcall tabulated-list-entries)
@@ -319,18 +326,47 @@ recenter window line accordingly."
                  (count-screen-lines (window-start) (point))))
 	 (setq entry-id (tabulated-list-get-id))
 	 (setq saved-col (current-column)))
-    (erase-buffer)
-    (unless tabulated-list-use-header-line
-      (tabulated-list-print-fake-header))
     ;; Sort the entries, if necessary.
     (setq entries (sort entries sorter))
     (unless (functionp tabulated-list-entries)
       (setq tabulated-list-entries entries))
+    ;; Without a sorter, we have no way to just update.
+    (when (and update (not sorter))
+      (setq update nil))
+    (if update (goto-char (point-min))
+      ;; Redo the buffer, unless we're just updating.
+      (erase-buffer)
+      (unless tabulated-list-use-header-line
+        (tabulated-list-print-fake-header)))
+    ;; Finally, print the resulting list.
     (dolist (elt entries)
-      (and entry-id
-	   (equal entry-id (car elt))
-	   (setq saved-pt (point)))
-      (apply tabulated-list-printer elt))
+      (let ((id (car elt)))
+        (and entry-id
+             (equal entry-id id)
+             (setq entry-id nil
+                   saved-pt (point)))
+        ;; If the buffer this empty, simply print each elt.
+        (if (eobp)
+            (apply tabulated-list-printer elt)
+          (while (let ((local-id (tabulated-list-get-id)))
+                   ;; If we find id, then nothing to update.
+                   (cond ((equal id local-id)
+                          (forward-line 1)
+                          nil)
+                         ;; If this entry sorts after id (or it's the
+                         ;; end), then just insert id and move on.
+                         ((funcall sorter elt
+                                   ;; FIXME: Might be faster if
+                                   ;; don't construct this list.
+                                   (list local-id (tabulated-list-get-entry)))
+                          (apply tabulated-list-printer elt)
+                          nil)
+                         ;; We find an entry that sorts before id,
+                         ;; it needs to be deleted.
+                         (t t)))
+            (let ((old (point)))
+              (forward-line 1)
+              (delete-region old (point)))))))
     (set-buffer-modified-p nil)
     ;; If REMEMBER-POS was specified, move to the "old" location.
     (if saved-pt