(defun setnext (world i j)
  (let* ((current (world-current world))
         (next (world-next world))
         (neighbors (count-neighbors current i j)))
    (if (zerop (bit current i j))
        (cond ((not (= neighbors 3))
               (setf (bit next i j) 0)) 
              (t (setf (bit next i j) 1)
                 (incf (world-numdots world)))) 
      (cond ((or (= neighbors 2) 
                 (= neighbors 3))
             (setf (bit next i j) 1))   
            (t (setf (bit next i j) 0)
               (decf (world-numdots world)))))
    (unless (zerop (bit next i j))
      (when (< i (world-xmin world)) (setf (world-xmin world) i))
      (when (> i (world-xmax world)) (setf (world-xmax world) i))
      (when (< j (world-ymin world)) (setf (world-ymin world) j))
      (when (> j (world-ymax world)) (setf (world-ymax world) j)))))


(defun life (source)
  (let* ((size (length (car source)))
         (life (make-world
                :size size
                :current (make-array (list size size) :element-type 'bit
                                     :initial-contents source)
                :next (make-array (list size size) :element-type 'bit
                                  :initial-element 0)
                :numdots 0)))
    (dotimes (i size)
      (dotimes (j size)
        (unless (zerop (bit (world-current life) i j))
          (incf (world-numdots life))
          (when (< i (world-xmin life)) (setf (world-xmin life) i))
          (when (> i (world-xmax life)) (setf (world-xmax life) i))
          (when (< j (world-ymin life)) (setf (world-ymin life) j))
          (when (> j (world-ymax life)) (setf (world-ymax life) j)))))
    (propagate life 0)))