--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/elisp/stack-mode/fifo.el	Wed Sep 16 16:53:07 2015 -0500
@@ -0,0 +1,55 @@
+;;; fifo.el --- FIFO queue.
+
+;; Copyright (c) 2015 Chris Done.
+
+;; This file is free software; you can redistribute it and/or modify
+;; it under the terms of the GNU General Public License as published by
+;; the Free Software Foundation; either version 3, or (at your option)
+;; any later version.
+
+;; This file is distributed in the hope that it will be useful,
+;; but WITHOUT ANY WARRANTY; without even the implied warranty of
+;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;; GNU General Public License for more details.
+
+;; You should have received a copy of the GNU General Public License
+;; along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+;;; Code:
+
+(require 'cl-lib)
+
+(defun fifo-make ()
+  "Make a fifo queue."
+  (cons 0 nil))
+
+(defun fifo-push (q a)
+  "Push a new item onto the queue."
+  (cl-assert (consp q) nil "Must be a queue.")
+  (setcar q (1+ (car q)))
+  (let ((next q)
+        (continue t))
+    (while (not (null (cdr next)))
+      (setq next (cdr next)))
+    (setcdr next (cons a nil)))
+  q)
+
+(defun fifo-pop (q)
+  "Pop the next item on the queue."
+  (cl-assert (consp q) nil "Must be a queue.")
+  (cl-assert (consp (cdr q)) nil "No items to pop from queue.")
+  (setcar q (1- (car q)))
+  (let ((a (car (cdr q))))
+    (setcdr q (cdr (cdr q)))
+    a))
+
+(defun fifo-size (q)
+  "Get the size of the queue."
+  (cl-assert (consp q) nil "Must be a queue.")
+  (car q))
+
+(defun fifo-null-p (q)
+  "Is the queue empty?"
+  (= (fifo-size q) 0))
+
+(provide 'fifo)