diff --git a/advent-of-code-2018.asd b/advent-of-code-2018.asd
index ff1bac9..c2c9fc2 100644
--- a/advent-of-code-2018.asd
+++ b/advent-of-code-2018.asd
@@ -26,4 +26,5 @@
                (:file "day16")
                (:file "day17")
                (:file "day18")
-               (:file "day19")))
+               (:file "day19")
+               (:file "day23")))
diff --git a/day23.lisp b/day23.lisp
new file mode 100644
index 0000000..b6c352b
--- /dev/null
+++ b/day23.lisp
@@ -0,0 +1,61 @@
+;;;; day23.lisp
+
+(in-package :advent-of-code-2018)
+
+(defun manhattan (pos1 pos2)
+  (reduce #'+ (mapcar #'abs (map 'list #'- pos1 pos2))))
+
+(defun day23-position (robot)
+  (butlast robot))
+
+(defun day23-radius (robot)
+  (fourth robot))
+
+(defun day23 ()
+  (let* ((data (sort (mapcar #'extract-integers (read-puzzlefile "input23.txt"))
+                     #'> :key #'day23-radius))
+         (biggest (first data)))
+    (format t "The bot with the biggest range has ~a bots in range.~%"
+            (count-if (lambda (pos) (<= (manhattan pos (day23-position biggest)) (day23-radius biggest)))
+                      data))))
+
+(defun day23-dist-to-origin (bot)
+  (- (manhattan '(0 0 0) (day23-position bot))
+     (day23-radius bot)))
+
+(defun max-overlap (nodes possibilities adjacent)
+  (if (null possibilities)
+      (return-from max-overlap nodes))
+  (let* ((new-nodes (aref adjacent (first possibilities)))
+         ; next line is not correct. should just be (rest possibilities)
+         ; but then it runs forever
+         (todo (set-difference (rest possibilities) new-nodes))
+         (best (max-overlap (cons (first possibilities) nodes)
+                            (intersection possibilities new-nodes)
+                            adjacent))
+         (other-best (max-overlap nodes todo adjacent)))
+    (if (> (length best) (length other-best))
+        best
+        other-best)))
+
+(defun day23-build-adjacency-matrix (bots)
+  (loop :with adjacent := (make-array (length bots) :initial-element nil)
+        :for todo := bots :then (rest todo)
+        :while todo
+        :for index := 0 :then (1+ index)
+        :for bot := (first todo)
+        :do (loop :for other :in (rest todo)
+                  :for other-index := (1+ index) :then (1+ other-index)
+                  :when (day23-has-overlap-p bot other)
+                  :do (push index (aref adjacent other-index))
+                  :and :do (push other-index (aref adjacent index)))
+        :finally (return adjacent)))
+
+(defun day23-extra ()
+  (let* ((bots (mapcar #'extract-integers (read-puzzlefile "input23.txt")))
+         (adjacent (day23-build-adjacency-matrix  bots))
+         (overlap-ids (max-overlap nil (loop for i below (length bots) collect i)
+                       adjacent))
+         (overlap-bots (mapcar (lambda (id) (nth id bots)) overlap-ids)))
+    (format t "The best points are ~a units from the origin.~%"
+            (reduce #'max (mapcar #'day23-dist-to-origin overlap-bots)))))