(defun flags (a)
  (let* ((vec (coerce a 'vector))
         (size (length vec))
         (peaks (make-array size :initial-element nil))
         (next (make-array size :initial-element -1)))
    (loop for i from 1 below size
          do (setf (aref peaks i)
                   (and (< (aref vec (1- i)) (aref vec i))
                        (> (aref vec i) (if (< (1+ i) size) (aref vec (1+ i)) 0)))))
    (setf (aref next (1- size)) -1)
    (loop for i from (- size 2) downto 0
          do (setf (aref next i) (if (aref peaks i) i (aref next (1+ i)))))
    (let ((i 1) (result 0))
      (loop while (<= (* i (1- i)) size)
            do (let ((pos 0) (num 0))
                 (loop while (and (< pos size) (< num i))
                       do (progn
                            (setf pos (aref next pos))
                            (when (= pos -1) (return))
                            (incf num)
                            (incf pos i)))
                 (incf i)
                 (setf result (max result num))))
      result)))