Redis基础数据结构——skipList

skipList（跳表）一种支持二分查找的链表，结构简单并且性能不亚于红黑树

与传统链表不同的是：

节点包含多个指针，指针的跨度不同，指针层级越高，跨度越大

元素按照升序排列存储：节点安装score值排序，score值一样则按元素排序

 

在翻阅源码的时候，我发现没有像之前的源码一样有个专门为skiplist生成的skiplist.c和skiplist.h，全局搜索之后才发现原来skiplist只存在于server.h（定义）和t_zet.c中。

skiplist的数据结构

/* ZSETs use a specialized version of Skiplists */
typedef struct zskiplistNode {
    sds ele;//元素内容
    double score;//排序依据
    struct zskiplistNode *backward;//前驱指针（固定执行前一个，因此不需要多个）
    struct zskiplistLevel {//后驱指针数组
        struct zskiplistNode *forward;//指针
        unsigned long span;//指针的层级
    } level[];
} zskiplistNode;

typedef struct zskiplist {
    struct zskiplistNode *header, *tail;
    unsigned long length;
    int level;
} zskiplist;

注释写：zset使用的一种特殊版本的skiplist，也就是说：skiplist是专门为优化zset用的，并不是独立的数据结构。

 

skiplist的插入

逻辑：

1.先在每层查找查找一下当前节点要插入的位置（小于当前score的，后置score相同ele更小的节点，记作update[i]，并设置当前span）

2.随机生成新节点层高，如果新层高大于当前最大，则覆盖skiplist的level

3.创建新节点，由于前面记录了每层的前驱节点，因此只需要根据前驱节点插入到对应的位置即可

4.未插入的节点的层，直接更新span(跨度)+1

5.如果新节点不是最后一个，则将后继节点的前驱指针指上；如果新节点是最后一个，更新尾指针*tail

/* Insert a new node in the skiplist. Assumes the element does not already
 * exist (up to the caller to enforce that). The skiplist takes ownership
 * of the passed SDS string 'ele'. */
zskiplistNode *zslInsert(zskiplist *zsl, double score, sds ele) {
    zskiplistNode *update[ZSKIPLIST_MAXLEVEL], *x;
    unsigned long rank[ZSKIPLIST_MAXLEVEL];
    int i, level;

    serverAssert(!isnan(score));
    x = zsl->header;
    for (i = zsl->level-1; i >= 0; i--) {
        /* store rank that is crossed to reach the insert position */
        rank[i] = i == (zsl->level-1) ? 0 : rank[i+1];
        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;
            x = x->level[i].forward;
        }
        update[i] = x;
    }
    /* 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();
    if (level > zsl->level) {
        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);
    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++;
    }

    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;
}