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redis数据结构-跳跃表

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跳跃表

redis的有序集合,使用的是hash字典+跳跃表实现的

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

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

跳跃表基本结构

file

跳跃表结构分为N层,数据量从上到下,redis的跳跃表总共有64层,最多可以容纳2^64个元素,每个kv块对应的是zskiplistNode 结构.

typedef struct zskiplistNode {
    sds ele;
    double score;
    struct zskiplistNode *backward;
    struct zskiplistLevel {
        struct zskiplistNode *forward;
        unsigned long span;
    } level[];
} zskiplistNode;

kvheade 的value值为空,score为0,每次查找时,从head的最顶层开始查找,通过score范围查找,再进行降级:

    x = zsl->header;//从header开始查找
    for (i = zsl->level-1; i >= 0; i--) {//从最顶层开始查找
        while (x->level[i].forward &&//判断是否存在小于当前分数的元素
                (x->level[i].forward->score < curscore ||//比较分数是否小于
                    (x->level[i].forward->score == curscore &&//当分数相同时
                     sdscmp(x->level[i].forward->ele,ele) < 0)))//比较key字符
        {
            x = x->level[i].forward;
        }
        update[i] = x;
    }

当分数一样时,需要根据sdscmp函数判断key,根据key的长度,字符来判断大小:

/* Compare two sds strings s1 and s2 with memcmp().
 *
 * Return value:
 *
 *     positive if s1 > s2.
 *     negative if s1 < s2.
 *     0 if s1 and s2 are exactly the same binary string.
 *
 * If two strings share exactly the same prefix, but one of the two has
 * additional characters, the longer string is considered to be greater than
 * the smaller one. */
int sdscmp(const sds s1, const sds s2) {
    size_t l1, l2, minlen;
    int cmp;

    l1 = sdslen(s1);
    l2 = sdslen(s2);
    minlen = (l1 < l2) ? l1 : l2;
    cmp = memcmp(s1,s2,minlen);
    if (cmp == 0) return l1>l2? 1: (l1<l2? -1: 0);
    return cmp;
}

插入逻辑

对于每个新插入的节点,都默认为第1层,同时有一定概率升级层数.

int zslRandomLevel(void) {
    int level = 1;
    while ((random()&0xFFFF) < (ZSKIPLIST_P * 0xFFFF))
        level += 1;
    return (level<ZSKIPLIST_MAXLEVEL) ? level : ZSKIPLIST_MAXLEVEL;
}

可以看到,在redis中,概率为ZSKIPLIST_P,默认为25%几率升级

在插入前,需要先获取到每一层小于该分数的第一个字值,定位出来,因为需要更新前置元素,同时还要获取排名进行更新

    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;
    }

如果随机的层数大于当前层数,则需要创建一个新的层

    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;
    }

然后就是创建一个zskiplistNode结构体,进行插入

 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;
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