结构差异

结构定义在server.h文件
方法实现在t_zset.c文件(ziplist是压缩)

  1. zskiplistNode

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    typedef struct zskiplistNode {
        sds ele; // 原本是 robj *obj,表示保存的成员对象
        double score; // 分值
        struct zskiplistNode *backward; // 后退指针
        struct zskiplistLevel {
            struct zskiplistNode *forward; // 前进指针
            unsigned long span;            // 跨度
        } level[];
    } zskiplistNode;

  2. zskiplist

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    typedef struct zskiplist {
        struct zskiplistNode *header, *tail; // 表头和表尾
        unsigned long length; // 长度,即节点数()不含表头
        int level; // 跳跃表内层数最大的节点的层数
    } zskiplist;

一些常量

  • ZSKIPLIST_P = 0.25,随机level的概率
  • ZSKIPLIST_MAXLEVEL = 64,最大level

level的幂次定律

幂次定律: 越大的数出现的概率越小
level为 n的概率大概是 (3/4)^n

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int zslRandomLevel(void) {
    int level = 1;
    // 每次1/4的概率往上加
    while ((random()&0xFFFF) < (ZSKIPLIST_P * 0xFFFF))
        level += 1;
    return (level<ZSKIPLIST_MAXLEVEL) ? level : ZSKIPLIST_MAXLEVEL;
}

API

  1. zslCreate

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    zskiplist *zslCreate(void) {
        int j;
        zskiplist *zsl;

        zsl = zmalloc(sizeof(*zsl));
        zsl->level = 1;
        zsl->length = 0;
        //	所以头节点是不计算长度的,而且初始化了最大的level但不计算level(算1)
        zsl->header = zslCreateNode(ZSKIPLIST_MAXLEVEL,0,NULL);
        for (j = 0; j < ZSKIPLIST_MAXLEVEL; j++) { 
            zsl->header->level[j].forward = NULL;
            zsl->header->level[j].span = 0;
        }
        zsl->header->backward = NULL;
        zsl->tail = NULL;
        return zsl;
    }

  2. zslInsert

    前提保证了元素不存在
    看起来有点复杂的

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zskiplistNode *zslInsert(zskiplist *zsl, double score, sds ele) {
	// 更新数组,存储新node插入,各层需要修改的node,也就是新node在各层插入在哪个node之后
    zskiplistNode *update[ZSKIPLIST_MAXLEVEL], *x;
    // 排序数组,存着各层从header到update里面的node的跨度和
    unsigned int rank[ZSKIPLIST_MAXLEVEL];
    int i, level;
	// 非数字断言
    serverAssert(!isnan(score));
    x = zsl->header;
    for (i = zsl->level-1; i >= 0; i--) { // 进行最大level次的遍历
        /* 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();
    // 如果随机出来的level更大,要补充update
    // update的值直接设置成header,跨度是length,因为是一步到达
    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++) {
    // 每一层
    // 把新node放在update[i]和它的forward之间
        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 */
        // 因为之前的rank累积关系,新node和update之间的node跨度就是(rank[0] - rank[i])
        //  x->level[i].span而这个值很可能是1
        // 在新加层里面,因为指向的是Null,所以span值没有关系
        x->level[i].span = update[i]->level[i].span - (rank[0] - rank[i]);
        update[i]->level[i].span = (rank[0] - rank[i]) + 1; // 因为新加了一个node,所以+1
    }

    /* increment span for untouched levels */
	
    // 新层因为新节点的出现,是forward到新节点的,sapn+1
    for (i = level; i < zsl->level; i++) {
        update[i]->level[i].span++;
    }
	// 第一层是顺序完全的
    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;
}

  1. zslDelete

  2. zslUpdateScore

    是先delete,在insert,不复用