/* 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.
 * 'bookmakrs are an optional feature that is used by realloc this struct,
 *      so that they don't consume memory when not used. */
typedef struct quicklist {
    quicklistNode *head;
    quicklistNode *tail;
    unsigned long count;        /* total count of all entries in all ziplists */
    unsigned long len;          /* number of quicklistNodes */
    int fill : QL_FILL_BITS;              /* ziplist大小限定，由list-max-ziplist-size给定 */
    unsigned int compress : QL_COMP_BITS; /* 节点压缩深度设置，由list-compress-depth给定 */
    unsigned int bookmark_count: QL_BM_BITS;
    quicklistBookmark bookmarks[];
} quicklist;

* quicklistNode is a 32 byte struct describing a ziplist for a quicklist.
 * We use bit fields keep the quicklistNode at 32 bytes.
 * count: 16 bits, max 65536 (max zl bytes is 65k, so max count actually < 32k).
 * encoding: 2 bits, RAW=1, LZF=2.
 * container: 2 bits, NONE=1, ZIPLIST=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 {
    struct quicklistNode *prev;
    struct quicklistNode *next;
    unsigned char *zl;
    unsigned int sz;             /* ziplist size in bytes */
    unsigned int count : 16;     /* count of items in ziplist */
    unsigned int encoding : 2;   /* RAW==1 or LZF==2 */
    unsigned int container : 2;  /* NONE==1 or ZIPLIST==2 */
    unsigned int recompress : 1; /* was this node previous compressed? */
    unsigned int attempted_compress : 1; /* node can't compress; too small */
    unsigned int extra : 10; /* more bits to steal for future usage */
} quicklistNode;

/* quicklistLZF is a 4+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->zl is compressed, node->zl points to a quicklistLZF */
typedef struct quicklistLZF {
    unsigned int sz; /* LZF size in bytes*/
    char compressed[];
} quicklistLZF;

typedef struct quicklistIter {
    const quicklist *quicklist;
    quicklistNode *current;
    unsigned char *zi;
    long offset; /* offset in current ziplist */
    int direction;
} quicklistIter;

typedef struct quicklistEntry {
    const quicklist *quicklist;
    quicklistNode *node;
    unsigned char *zi;
    unsigned char *value;
    long long longval;
    unsigned int sz;
    int offset;
} quicklistEntry;

# Lists may also be compressed.
# Compress depth is the number of quicklist ziplist nodes from *each* side of
# the list to *exclude* from compression.  The head and tail of the list
# are always uncompressed for fast push/pop operations.  Settings are:
# 0: disable all list compression
# 1: depth 1 means "don't start compressing until after 1 node into the list,
#    going from either the head or tail"
#    So: [head]->node->node->...->node->[tail]
#    [head], [tail] will always be uncompressed; inner nodes will compress.
# 2: [head]->[next]->node->node->...->node->[prev]->[tail]
#    2 here means: don't compress head or head->next or tail->prev or tail,
#    but compress all nodes between them.
# 3: [head]->[next]->[next]->node->node->...->node->[prev]->[prev]->[tail]
# etc.
list-compress-depth 0

/* Wrapper to allow argument-based switching between HEAD/TAIL pop */
void quicklistPush(quicklist *quicklist, void *value, const size_t sz,
                   int where) {
    if (where == QUICKLIST_HEAD) {	//插入到quicklist的前面
        quicklistPushHead(quicklist, value, sz);
    } else if (where == QUICKLIST_TAIL) {	//插入到quicklist的尾部
        quicklistPushTail(quicklist, value, sz);
    }
}

/* Add new entry to head node of quicklist.
 *
 * Returns 0 if used existing head.
 * Returns 1 if new head created. */
int quicklistPushHead(quicklist *quicklist, void *value, size_t sz) {
    quicklistNode *orig_head = quicklist->head;
    assert(sz < UINT32_MAX); /* TODO: add support for quicklist nodes that are sds encoded (not zipped) */
    if (likely(
            _quicklistNodeAllowInsert(quicklist->head, quicklist->fill, sz))) {	//如果还允许插入
        quicklist->head->zl =
            ziplistPush(quicklist->head->zl, value, sz, ZIPLIST_HEAD);	// 就插入头部的ziplist
        quicklistNodeUpdateSz(quicklist->head);
    } else {
        quicklistNode *node = quicklistCreateNode();	// 创建一个quicklist的node
        node->zl = ziplistPush(ziplistNew(), value, sz, ZIPLIST_HEAD);	//node中的ziplist插入新值

        quicklistNodeUpdateSz(node);	//node更新长度
        _quicklistInsertNodeBefore(quicklist, quicklist->head, node);
    }
    quicklist->count++;	//quciklist的entry总和增加
    quicklist->head->count++;	//ziplist的长度增加
    return (orig_head != quicklist->head);
}

likely:阅读和理解代码的角度来看，是一样的.目的是将“分支转移”的信息提供给编译器，这样编译器可以对代码进行优化， 以减少指令跳转带来的性能下降。
https://blog.csdn.net/edonlii/article/details/8877239

#define sizeMeetsSafetyLimit(sz) ((sz) <= SIZE_SAFETY_LIMIT)
/* Maximum size in bytes of any multi-element ziplist.
 * Larger values will live in their own isolated ziplists. */
#define SIZE_SAFETY_LIMIT 8192

REDIS_STATIC int _quicklistNodeAllowInsert(const quicklistNode *node,
                                           const int fill, const size_t sz) {	//fill负数是ziplist的entry大小的限定，正数是entry个数的限定
    if (unlikely(!node))
        return 0;

    int ziplist_overhead;
    /* size of previous offset */
    if (sz < 254)
        ziplist_overhead = 1;
    else
        ziplist_overhead = 5;

    /* size of forward offset */
    if (sz < 64)
        ziplist_overhead += 1;
    else if (likely(sz < 16384))
        ziplist_overhead += 2;
    else
        ziplist_overhead += 5;

    /* new_sz overestimates if 'sz' encodes to an integer type */
    unsigned int new_sz = node->sz + sz + ziplist_overhead;
    if (likely(_quicklistNodeSizeMeetsOptimizationRequirement(new_sz, fill)))	//如果fill是负数并且符合要求
        return 1;
    /* when we return 1 above we know that the limit is a size limit (which is
     * safe, see comments next to optimization_level and SIZE_SAFETY_LIMIT) */
    else if (!sizeMeetsSafetyLimit(new_sz))	//新的entry默认还是有上限的
        return 0;
    else if ((int)node->count < fill) 	//fill是证书，代表ziplist中的最大entry个数
        return 1;
    else
        return 0;
}

static const size_t optimization_level[] = {4096, 8192, 16384, 32768, 65536};

REDIS_STATIC int
_quicklistNodeSizeMeetsOptimizationRequirement(const size_t sz,
                                               const int fill) {
    if (fill >= 0)	//如果是对entry个数的限定。先返回0，不判断
        return 0;

    size_t offset = (-fill) - 1;	//对entry大小的判断
    if (offset < (sizeof(optimization_level) / sizeof(*optimization_level))) {	//判断entry的大小
        if (sz <= optimization_level[offset]) {
            return 1;
        } else {
            return 0;
        }
    } else {
        return 0;
    }
}

#define quicklistNodeUpdateSz(node)                                            \
    do {                                                                       \
        (node)->sz = ziplistBlobLen((node)->zl);                               \
    } while (0)

/* Wrappers for node inserting around existing node. */
REDIS_STATIC void _quicklistInsertNodeBefore(quicklist *quicklist,
                                             quicklistNode *old_node,
                                             quicklistNode *new_node) {
    __quicklistInsertNode(quicklist, old_node, new_node, 0);
}

/* Insert 'new_node' after 'old_node' if 'after' is 1.
 * Insert 'new_node' before 'old_node' if 'after' is 0.
 * Note: 'new_node' is *always* uncompressed, so if we assign it to
 *       head or tail, we do not need to uncompress it. */
REDIS_STATIC void __quicklistInsertNode(quicklist *quicklist,
                                        quicklistNode *old_node,
                                        quicklistNode *new_node, int after) {
    if (after) {
        new_node->prev = old_node;
        if (old_node) {
            new_node->next = old_node->next;
            if (old_node->next)
                old_node->next->prev = new_node;
            old_node->next = new_node;
        }
        if (quicklist->tail == old_node)
            quicklist->tail = new_node;
    } else {
        new_node->next = old_node;
        if (old_node) {
            new_node->prev = old_node->prev;
            if (old_node->prev)
                old_node->prev->next = new_node;
            old_node->prev = new_node;
        }
        if (quicklist->head == old_node)
            quicklist->head = new_node;
    }
    /* If this insert creates the only element so far, initialize head/tail. */
    if (quicklist->len == 0) {
        quicklist->head = quicklist->tail = new_node;
    }

    if (old_node)
        quicklistCompress(quicklist, old_node);

    quicklist->len++;
}

/* Add new entry to tail node of quicklist.
 *
 * Returns 0 if used existing tail.
 * Returns 1 if new tail created. */
int quicklistPushTail(quicklist *quicklist, void *value, size_t sz) {
    quicklistNode *orig_tail = quicklist->tail;
    if (likely(
            _quicklistNodeAllowInsert(quicklist->tail, quicklist->fill, sz))) {
        quicklist->tail->zl =
            ziplistPush(quicklist->tail->zl, value, sz, ZIPLIST_TAIL);
        quicklistNodeUpdateSz(quicklist->tail);
    } else {
        quicklistNode *node = quicklistCreateNode();
        node->zl = ziplistPush(ziplistNew(), value, sz, ZIPLIST_TAIL);

        quicklistNodeUpdateSz(node);
        _quicklistInsertNodeAfter(quicklist, quicklist->tail, node);
    }
    quicklist->count++;
    quicklist->tail->count++;
    return (orig_tail != quicklist->tail);
}