diff --git a/advent-of-code-2018.asd b/advent-of-code-2018.asd
index 49d6d5f..d334571 100644
--- a/advent-of-code-2018.asd
+++ b/advent-of-code-2018.asd
@@ -16,4 +16,5 @@
                (:file "day5")
                (:file "day6")
                (:file "day7")
-               (:file "day8")))
+               (:file "day8")
+               (:file "day9")))
diff --git a/day9.lisp b/day9.lisp
new file mode 100644
index 0000000..075edfb
--- /dev/null
+++ b/day9.lisp
@@ -0,0 +1,50 @@
+;;;; day9.lisp
+
+(in-package :advent-of-code-2018)
+
+(defun day9-parse-input ()
+  (let ((input (split-seq (first (read-puzzlefile "input9.txt")) #\Space)))
+    (list (parse-integer (first input))
+          (parse-integer (nth 6 input)))))
+
+(defun day9-make-board ()
+  (let ((zero (list 0)))
+    (setf (cdr zero) zero)))
+
+(defun day9-insert-after (board marble)
+  (let ((new (list marble)))
+    (setf (cdr new) (cdr board)
+          (cdr board) new)))
+
+(defun day9-remove-next (board)
+  (let ((removed (cdr board)))
+    (setf (cdr board) (cddr board))
+    removed))
+
+(defun day9-play (players last-marble)
+  (declare (optimize (debug 3) (safety 3)))
+  (loop
+    :with scores := (make-hash-table)
+    :with board := (day9-make-board)
+    :with trailing := board
+    :with length := 1
+    :for marble :from 1 :upto last-marble
+    :when (= marble 8) :do (setf trailing board)
+    :unless (> 5 (mod marble 23)) :do (setf trailing (cddr trailing))
+    :unless (= 0 (mod marble 23))
+    :do (setf board (day9-insert-after (cdr board) marble))
+    :and :do (incf length)
+    :else :do (let ((removed (day9-remove-next trailing)))
+                (setf board (cdr removed) trailing (cdr removed))
+                (incf (gethash (mod marble players) scores 0) (+ marble (car removed)))
+                (decf length)
+                ;(format t "~a points for player ~a.~%" (+ marble (car removed)) (1+ (mod marble players)))
+                )
+    :finally (return (multiple-value-list (max-key scores)))))
+
+(defun day9 ()
+  (destructuring-bind (players marbles) (day9-parse-input)
+    (destructuring-bind (player points) (day9-play players marbles)
+      (format t "The first game wins elf #~a with ~a points.~%" player points))
+    (destructuring-bind (player points) (day9-play players (* 100 marbles))
+      (format t "The second game wins elf #~a with ~a points.~%" player points))))