The tpRunCycle() function

The member 'reverse_run ' is indicated that the tp runs reverse, but look at followings, the tcqItem() function just ignore "n < 0", so how is it can run reverse ?

/* Get pointers to current and relevant future segments. It's ok here if
* future segments don't exist (NULL pointers) as we check for this later).
*/

int queue_dir_step = tp->reverse_run ? -1 : 1;
tc = tcqItem(&tp->queue, 0);
nexttc = tcqItem(&tp->queue, queue_dir_step * 1);

---

/* gets the n-th TC element in the queue, without removing it */TC_STRUCT * tcqItem(TC_QUEUE_STRUCT const * const tcq, int n)
{
if (tcqCheck(tcq) || (n < 0) || (n >= tcq->_len)) return NULL;

return &(tcq->queue[(tcq->start + n) % tcq->size]);
}