redis源码阅读(二)

typedef struct zskiplistNode {     
    sds ele;                              //数据域, sds类型
    double score;                         //分值 
    struct zskiplistNode *backward;       //后向指针，双向链表
    struct zskiplistLevel {               //每一个结点的层级
        struct zskiplistNode *forward;    //当前层的前向结点
        unsigned int span;                //当前层距离下一个结点的跨度
    } level[];                            //level本身是一个柔性数组，最大值为32，由 ZSKIPLIST_MAXLEVEL 定义
} zskiplistNode;

typedef struct zskiplist {
    struct zskiplistNode *header;     //头部指针， 指向跳跃表的第一个节点
    struct zskiplistNode *tail;       //尾部指针，指向跳跃表最后一个节点
    unsigned long length;             //长度，即一共有多少个元素，除开header的长度
    int level;                        //最大层级，即跳表目前的最大层级
} zskiplist;

/* Create a new skiplist. */
zskiplist *zslCreate(void) {
    int j;
    zskiplist *zsl;

    zsl = zmalloc(sizeof(*zsl));	//申请空间
    zsl->level = 1;								//初始化层级为1
    zsl->length = 0;							//初始化长度为0
    zsl->header = zslCreateNode(ZSKIPLIST_MAXLEVEL,0,NULL);		//创建header节点，并且层数为ZSKIPLIST_MAXLEVEL
    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;
}

/* Create a skiplist node with the specified number of levels.
 * The SDS string 'ele' is referenced by the node after the call. */
zskiplistNode *zslCreateNode(int level, double score, sds ele) {
    zskiplistNode *zn =
        zmalloc(sizeof(*zn)+level*sizeof(struct zskiplistLevel));
    zn->score = score;
    zn->ele = ele;
    return zn;
}

/* 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 int 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];		//（1）
        while (x->level[i].forward &&						//（2）
                (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;										//（3）
    }
    /* 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) {								//（4）
        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;	//（5）
        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]);		//（6）
        update[i]->level[i].span = (rank[0] - rank[i]) + 1;
    }

    /* increment span for untouched levels */
    for (i = level; i < zsl->level; i++) {					//（7）
        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;
}