/* quicklistNode is a 32 byte struct describing a listpack for a quicklist.
 * We use bit fields keep the quicklistNode at 32 bytes.
 * count: 16 bits, max 65536 (max lp bytes is 65k, so max count actually < 32k).
 * encoding: 2 bits, RAW=1, LZF=2.
 * container: 2 bits, PLAIN=1, PACKED=2.
 * recompress: 1 bit, bool, true if node is temporary decompressed for usage.
 * attempted_compress: 1 bit, boolean, used for verifying during testing.
 * extra: 10 bits, free for future use; pads out the remainder of 32 bits */
typedef struct quicklistNode {
    // 双向链表前驱节点
    struct quicklistNode *prev;
    // 双向链表的后节点
    struct quicklistNode *next;
    // 不设置压缩数据参数 recompres 指向一个 ziplist 结构
    // 不设置压缩数据参数 recompres 指向一个 quicklistLZF 结构
    unsigned char *entry;
    // 压缩列表 ziplist 的总长度
    size_t sz;             /* entry size in bytes */
    // 每个 ziplist 中 entry 的个数
    unsigned int count : 16;     /* count of items in listpack */
    // 表示是否采用了 LZF 压缩 quickList 节点 1 表示压缩过，2 表示没有压缩站 2bit 长度
    unsigned int encoding : 2;   /* RAW==1 or LZF==2 */
    // 表示是否开启 ziplist 进行压缩
    unsigned int container : 2;  /* PLAIN==1 or PACKED==2 */
    // 表示该节点是否被压缩过
    unsigned int recompress : 1; /* was this node previous compressed? */
    // 测试使用
    unsigned int attempted_compress : 1; /* node can't compress; too small */
    // 额外拓展位，占 10bit 长度
    unsigned int extra : 10; /* more bits to steal for future usage */
} quicklistNode;
/* 
  当指定使用 lzf 压缩算法压缩 ziplist entry 节点时
  quicklistNode 结构的 zl 成员执行 quicklistLZF 结构
  quicklistLZF is a 8+N byte struct holding 'sz' followed by 'compressed'.
 * 'sz' is byte length of 'compressed' field.
 * 'compressed' is LZF data with total (compressed) length 'sz'
 * NOTE: uncompressed length is stored in quicklistNode->sz.
 * When quicklistNode->entry is compressed, node->entry points to a quicklistLZF */
typedef struct quicklistLZF {
    // 表示被LZF 压缩后的 ziplist 的大小
    size_t sz; /* LZF size in bytes*/
    // 压缩有的数据，柔性数组
    char compressed[];
} quicklistLZF;

/* quicklist is a 40 byte struct (on 64-bit systems) describing a quicklist.
 * 'count' is the number of total entries.
 * 'len' is the number of quicklist nodes.
 * 'compress' is: 0 if compression disabled, otherwise it's the number
 *                of quicklistNodes to leave uncompressed at ends of quicklist.
 * 'fill' is the user-requested (or default) fill factor.
 * 'bookmarks are an optional feature that is used by realloc this struct,
 *      so that they don't consume memory when not used. */
typedef struct quicklist {
    // 链表表头
    quicklistNode *head;
    // 链表表尾巴
    quicklistNode *tail;
    // 所有 quicklistNode 节点中所有的 entry 个数
    unsigned long count;        /* total count of all entries in all listpacks */
    // quickListNode 节点个数，也就是 quickList 的长度
    unsigned long len;          /* number of quicklistNodes */
    //单个节点的填充因子，也就是 ziplist 的大小
    signed int fill : QL_FILL_BITS;       /* fill factor for individual nodes */
    // 保存压缩成都只，配置文件设置，64位操作系统占 16bit , 6 表示压缩
    unsigned int compress : QL_COMP_BITS; /* depth of end nodes not to compress;0=off */
    unsigned int bookmark_count: QL_BM_BITS;
    quicklistBookmark bookmarks[];
} quicklist;

list-compress-depth 0