redis中的数据结构—跳跃表

typedef struct zskiplistNode {//跳跃表节点
    sds ele; //值，sds结构
    double score; //分数，根据分数来排序
    struct zskiplistNode *backward; //前驱节点
    struct zskiplistLevel { //层，数组，新建时随机建立层数，不过越高概率越低
        struct zskiplistNode *forward; //对应层的下一个节点
        unsigned long span;//与同层的下一个节点之间的跨度（距离）
    } level[];
} zskiplistNode;

typedef struct zskiplist {//跳跃表链表结构，
    struct zskiplistNode *header, *tail; //头尾指针
    unsigned long length;//跳跃表节点个数
    int level; //最高层的层数
} zskiplist;

zskiplistNode *zslInsert(zskiplist *zsl, double score, sds ele) {
    zskiplistNode *update[ZSKIPLIST_MAXLEVEL], *x; //存储新节点每一层的前驱节点
    unsigned int rank[ZSKIPLIST_MAXLEVEL]; //存储新节点每一层前驱结点距离头的跨度
    int i, level;

    serverAssert(!isnan(score));
    x = zsl->header; //从头节点开始寻找
    for (i = zsl->level-1; i >= 0; i--) {//从最顶层开始找
        /* store rank that is crossed to reach the insert position */
        rank[i] = i == (zsl->level-1) ? 0 : rank[i+1]; //累加上一层得到的rank
       //如果当前层有后继节点，且该节点分数小于等于新节点分数||键值字典比较小于新节点
         while (x->level[i].forward &&
                (x->level[i].forward->score < score ||
                    (x->level[i].forward->score == score &&
                    sdscmp(x->level[i].forward->ele,ele) < 0)))
        {   
            rank[i] += x->level[i].span; //加上跨度
            x = x->level[i].forward;//则x指向为下一个节点
        }
        update[i] = x; //获得新节点每一层的前驱节点
    }
    /* we assume the element is not already inside, since we allow duplicated
    /* scores, reinserting the same element should never happen since the
    /* caller of zslInsert() should test in the hash table if the element is
    /* already inside or not. 
     
    level = zslRandomLevel();//为新节点随机获取层数
    if (level > zsl->level) { //如果大于当前最大层
        for (i = zsl->level; i < level; i++) {
            rank[i] = 0; //
            update[i] = zsl->header;
            update[i]->level[i].span = zsl->length;//?????
        }
        zsl->level = level;
    }
    x = zslCreateNode(level,score,ele);
    //交换指针指向，还有根据rank据算跨度
    for (i = 0; i < level; i++) {
        x->level[i].forward = update[i]->level[i].forward; 
        update[i]->level[i].forward = x;

        /* update span covered by update[i] as x is inserted here */
        x->level[i].span = update[i]->level[i].span - (rank[0] - rank[i]);
        update[i]->level[i].span = (rank[0] - rank[i]) + 1;
    }

    /* increment span for untouched levels */
    for (i = level; i < zsl->level; i++) {
        update[i]->level[i].span++;//新节点直接前驱中比level更高的层跨度+1；
    }

    x->backward = (update[0] == zsl->header) ? NULL : update[0];
    if (x->level[0].forward)
        x->level[0].forward->backward = x;
    else
        zsl->tail = x;
    zsl->length++;
    return x;