Redis 1-2、跳跃表数据结构

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李延 LV6 2021-07-19

悬赏：20积分

# 跳跃表算法说明

[https://blog.csdn.net/qpzkobe/article/details/80056807](https://blog.csdn.net/qpzkobe/article/details/80056807)

# redis中的数据结构
在server.h中定义

每个节点的定义
```c
typedef struct zskiplistNode {
    // 节点保存的值
    sds ele;
    //分值
    double score;
    // 后退指针，指向上一个节点
    struct zskiplistNode *backward;
    //层，保存跳跃表每层的数据
    struct zskiplistLevel {
        //前进指针
        struct zskiplistNode *forward;
        //跨度。到下几个节点间隔的数量
        unsigned long span;
    } level[];
} zskiplistNode;

```

跳跃表定义：

```java
typedef struct zskiplist {
    //跳跃表头、尾节点
    struct zskiplistNode *header, *tail;
    //跳跃表长度
    unsigned long length;
    //跳跃表层层数
    int level;
} zskiplist;

```

# redis中实现

## zslInsert

在跳跃表中插入一个节点

```java

//在跳跃表中插入一个节点
zskiplistNode *zslInsert(zskiplist *zsl, double score, sds ele) {
    //update记录我们要插入的节点在每层节点的最后一个值
    zskiplistNode *update[ZSKIPLIST_MAXLEVEL], *x;
    //rank 记录，要插入的节点在每层的位置
    unsigned int rank[ZSKIPLIST_MAXLEVEL];
    int i, level;

serverAssert(!isnan(score));
    //获取头节点
    x = zsl->header;
    //从最高层查询
    for (i = zsl->level-1; i >= 0; i--) {
        /* store rank that is crossed to reach the insert position */
        //初始化当前层的rank。当是最开始时，为0。其他情况与上一层相同
        rank[i] = i == (zsl->level-1) ? 0 : rank[i+1];
        /**
         * 当前层有前进节点，并且
         * 当前节点score比要找的小，或者
         * 当评分一样，当前节点对象比要找的值小。
         *
         * 说明我们要插入的节点在当前节点的后面
         *
         */
        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)))
        {
            //更新当前rank值，跳跃到下一个节点
            rank[i] += x->level[i].span;
            //x 跟新为前进节点
            x = x->level[i].forward;
        }
        //退出上面的循环。说明我们要插入的节点在x的前面。将其记录在update中。
        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. */
    //随机层数，
    level = zslRandomLevel();
    //如果比之前的大，初始化后面rank与update的值
    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;
    }
    //为我们插入的数据创建节点
    x = zslCreateNode(level,score,ele);

//插入数据，在每层中都插入我们的当前节点
    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 */
    //计算我们新加入层数的update值
    for (i = level; i < zsl->level; i++) {
        update[i]->level[i].span++;
    }

//跟新当前节点的backward值
    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;
}

```

## zslGetElementByRank

根据排序名次查询节点

```java

zskiplistNode* zslGetElementByRank(zskiplist *zsl, unsigned long rank) {
    zskiplistNode *x;
    unsigned long traversed = 0;
    int i;
    //获取头
    x = zsl->header;
    //从最高层查找，根据span计算下一个节点的名次。
    for (i = zsl->level-1; i >= 0; i--) {
        while (x->level[i].forward && (traversed + x->level[i].span) <= rank)
        {
            traversed += x->level[i].span;
            x = x->level[i].forward;
        }
        if (traversed == rank) {
            return x;
        }
    }
    return NULL;
}

```

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