Data commit

Task/Queue-Definition/Common-Lisp/queue-definition.lisp

@@ -0,0 +1,26 @@
+(defstruct (queue (:constructor %make-queue))
+  (items '() :type list)
+  (tail '() :type list))
+
+(defun make-queue ()
+  "Returns an empty queue."
+  (%make-queue))
+
+(defun queue-empty-p (queue)
+  "Returns true if the queue is empty."
+  (endp (queue-items queue)))
+
+(defun enqueue (item queue)
+  "Enqueue item in queue. Returns the queue."
+  (prog1 queue
+    (if (queue-empty-p queue)
+      (setf (queue-items queue) (list item)
+            (queue-tail queue) (queue-items queue))
+      (setf (cdr (queue-tail queue)) (list item)
+            (queue-tail queue) (cdr (queue-tail queue))))))
+
+(defun dequeue (queue)
+  "Dequeues an item from queue. Signals an error if queue is empty."
+  (if (queue-empty-p queue)
+    (error "Cannot dequeue from empty queue.")
+    (pop (queue-items queue))))