Redis有序集合的实现&跳跃表源码学习

typedef struct zset {
    dict *dict;
    zskiplist *zsl;
} zset;

/* ZSETs use a specialized version of Skiplists */
# 跳跃表节点（ZSET版）
typedef struct zskiplistNode {
    # 使用sds来存储成员名字
    sds ele;
    # 浮点型分数
    double score;
    # 每个zskiplist节点都带有向前的指针
    struct zskiplistNode *backward;
    # zskiplist分层，每层中包含指向其他zskiplist节点的指针
    struct zskiplistLevel {
        # zskiplist节点指针
        struct zskiplistNode *forward;
        # 本层指向的下个节点离本节点的跨度
        unsigned long span;
    } level[];
} zskiplistNode;

# 跳跃表
typedef struct zskiplist {
    # 分别指向头尾的指针
    struct zskiplistNode *header, *tail;
    # 长度 即跳跃表中包含的节点数目（头节点不算）
    unsigned long length;
    # 层数 即跳跃表中各节点层数的最大值（头节点不算）
    int level;
} zskiplist;

# 输入一个zset的skiplist、新成员的得分和名字
zskiplistNode *zslInsert(zskiplist *zsl, double score, sds ele) {    
    zskiplistNode *update[ZSKIPLIST_MAXLEVEL], *x;
    unsigned int rank[ZSKIPLIST_MAXLEVEL];
    int i, level;

    serverAssert(!isnan(score));
    x = zsl->header;

    # 从头遍历跳跃表来查找当前元素应该插入在哪个节点之后
    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 &&
                # 排名是由分数和sds名字共同决定的，同分数下按节点名排序
                (x->level[i].forward->score < score ||
                    (x->level[i].forward->score == score &&
                    sdscmp(x->level[i].forward->ele,ele) < 0)))
        {
            # 注意这一句，说明span是用来便于计算节点排名的
            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();
    # 判断是否要重写头节点的level值
    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]);
        # span实际就是zset两个成员之间的rank差值
        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;
}