REDIS_STATIC void __quicklistCompress(const quicklist *quicklist,
                                      quicklistNode *node) {
    /* If length is less than our compress depth (from both sides),
     * we can't compress anything. */
    if (!quicklistAllowsCompression(quicklist) ||
        quicklist->len < (unsigned int)(quicklist->compress * 2))
        return;

# if 0

    /* Optimized cases for small depth counts */
    if (quicklist->compress == 1) {
        quicklistNode *h = quicklist->head, *t = quicklist->tail;
        quicklistDecompressNode(h);
        quicklistDecompressNode(t);
        if (h != node && t != node)
            quicklistCompressNode(node);
        return;
    } else if (quicklist->compress == 2) {
        quicklistNode *h = quicklist->head, *hn = h->next, *hnn = hn->next;
        quicklistNode *t = quicklist->tail, *tp = t->prev, *tpp = tp->prev;
        quicklistDecompressNode(h);
        quicklistDecompressNode(hn);
        quicklistDecompressNode(t);
        quicklistDecompressNode(tp);
        if (h != node && hn != node && t != node && tp != node) {
            quicklistCompressNode(node);
        }
        if (hnn != t) {
            quicklistCompressNode(hnn);
        }
        if (tpp != h) {
            quicklistCompressNode(tpp);
        }
        return;
    }

# endif

    /* Iterate until we reach compress depth for both sides of the list.a
     * Note: because we do length checks at the *top* of this function,
     *       we can skip explicit null checks below. Everything exists. */
    quicklistNode *forward = quicklist->head;
    quicklistNode *reverse = quicklist->tail;
    int depth = 0;
    int in_depth = 0;
    while (depth++ < quicklist->compress) {
        quicklistDecompressNode(forward);
        quicklistDecompressNode(reverse);

        if (forward == node || reverse == node)
            in_depth = 1;

        if (forward == reverse)
            return;

        forward = forward->next;
        reverse = reverse->prev;
    }

    if (!in_depth)
        quicklistCompressNode(node);

    if (depth > 2) {
        /* At this point, forward and reverse are one node beyond depth */
        quicklistCompressNode(forward);
        quicklistCompressNode(reverse);
    }
}

quicklist的compress字段表示不压缩的结束节点的深度。举个例子：当quicklist中有4个quicklistnodes，quicklist的compress字段为2时，调用uu quicklistcompress（），while语句后，深度为3，forward指向第3个quicklistnode，reverse指向第2个quicklistnode，然后quicklistcompressnode（forward）；quicklistcompressnode（反向）；因此，quicklist只有第一个quicklistnode和第四个quicklistnode未压缩。是这样吗？quicklist必须有前2个quicklistnode和后2个quicklistnode未压缩？