diff --git a/advent-of-code-2018.asd b/advent-of-code-2018.asd
index 9ea15c4..263cf6f 100644
--- a/advent-of-code-2018.asd
+++ b/advent-of-code-2018.asd
@@ -13,4 +13,5 @@
                (:file "day2")
                (:file "day3")
                (:file "day4")
-               (:file "day5")))
+               (:file "day5")
+               (:file "day6")))
diff --git a/day6.lisp b/day6.lisp
new file mode 100644
index 0000000..0c1dac2
--- /dev/null
+++ b/day6.lisp
@@ -0,0 +1,97 @@
+;;;; day6.lisp
+
+(in-package :advent-of-code-2018)
+
+(defun manhattan (x1 y1 x2 y2)
+  (declare (optimize (speed 3) (debug 0) (safety 1))
+           (fixnum x1 y1 x2 y2))
+  (the fixnum (+ (abs (- x1 x2))
+                 (abs (- y1 y2)))))
+
+(defun day6-insert (newx newy name grid)
+  (let ((width (array-dimension grid 0))
+        (height (array-dimension grid 1)))
+    (loop
+      :for x :below width
+      :do (loop
+            :for y :below height
+            :for dist := (manhattan x y newx newy)
+            :for cell := (aref grid x y)
+            :when (or (null cell) (< dist (first cell)))
+            :do (setf (aref grid x y) (list dist name (or (third cell) 0)))
+            :else :when (= dist (first cell))
+            :do (setf (second (aref grid x y)) nil)
+            :end
+            :do (incf (third (aref grid x y)) dist)))))
+
+(defun day6-shift-coords (dx dy)
+  (lambda (coord)
+    (cons (- (car coord) dx) (- (cdr coord) dy))))
+
+(defun day6-count (grid)
+  (let ((counters (make-hash-table))
+        (infinites (make-hash-table))
+        (width (array-dimension grid 0))
+        (height (array-dimension grid 1))
+        (safe-dist 10000)
+        (good-region 0))
+    (loop :for x :below width
+          :do (loop :for y :from 0 :below height
+                    :for (id total) := (cdr (aref grid x y))
+                    :when id
+                    :do (incf (gethash id counters 0))
+                    :and :when (or (= x 0) (= x (1- width))
+                              (= y 0) (= y (1- height)))
+                    :do (setf (gethash id infinites) t) :end
+                    :when (< total safe-dist) :do (incf good-region)))
+    ;remove infinite areas
+    (maphash (lambda (k v) (when v) (remhash k counters)) infinites)
+    (values (second (multiple-value-list (max-key counters )))
+            good-region)))
+
+(defun day6-prep-input ()
+  (loop-line-by-line (puzzlepath "input6.txt")
+    :for (x . y) := (ppcre:register-groups-bind
+                        ((#'parse-integer x y)) ("(\\d+), (\\d+)" line)
+                      (cons x y))
+    :collecting (cons x y) :into coords
+    :minimizing x :into min-x
+    :maximizing x :into max-x
+    :minimizing y :into min-y
+    :maximizing y :into max-y
+    :finally (return (list coords min-x max-x min-y max-y))))
+
+;;; this is correct for part 1, but not for part 2.
+;;; Luckily it gives the right result for my input
+(defun day6 ()
+  (destructuring-bind
+      (raw-coords min-x max-x min-y max-y) (day6-prep-input)
+    (let ((grid (make-array (list (- max-x min-x -2) (- max-y min-y -2)) :initial-element nil))
+          (coords (mapcar (day6-shift-coords (- min-x 1) (- min-y 1)) raw-coords)))
+      ;; insert all points
+      (loop
+        :for (x . y) :in coords
+        :for id :from 0
+        :do (day6-insert x y (intern (format nil "~a" id)) grid))
+      (multiple-value-bind (largest-area save-spots) (day6-count grid)
+        (format t "The largest finite region is ~a.~%There are ~a safe spots.~%"
+                largest-area save-spots)))))
+
+;;; performant stand-alone solution for part 2
+;;; runtime ~0.13s
+;;; with (debug 0) runtime ~0.1s
+;;; OPTIMIZATION: compute better padding values for each side
+(defun day6-extra ()
+  (declare (optimize (speed 3) (debug 0) (safety 1)))
+  (format t "There are ~a safe spots.~%"
+          (let ((max-dist 10000))
+            (destructuring-bind (coords min-x max-x min-y max-y) (day6-prep-input)
+              (declare (fixnum min-x max-x min-y max-y) (list coords))
+              (loop
+                :with padding := (ceiling max-dist (length coords))
+                :for x :from (- min-x padding) :upto (+ max-x padding)
+                :sum (loop
+                       :for y :from (- min-y padding) :to (+ max-y padding)
+                       :count (> max-dist (loop :for (cx . cy) :in coords
+                                                :sum (manhattan x y cx cy) fixnum)))
+                fixnum)))))