(defun number-of-disc-intersections (a)
  (let* ((vec (coerce a 'vector))
         (c (length vec))
         (start (make-array c :initial-element 0))
         (end (make-array c :initial-element 0))
         (sum 0)
         (active 0))
    (loop for k from 0 below c
          for v = (aref vec k)
          do (let ((key1 (if (< k v) 0 (- k v)))
                    (key2 (if (>= (+ k v) c) (1- c) (+ k v))))
               (incf (aref start key1))
               (incf (aref end key2))))
    (loop for k from 0 below c
          do (progn
               (incf sum (+ (* active (aref start k))
                            (/ (* (aref start k) (1- (aref start k))) 2)))
               (incf active (- (aref start k) (aref end k)))
               (when (> sum 10000000)
                 (return-from number-of-disc-intersections -1))))
    sum))