zskiplistNode *zslInsert(zskiplist *zsl, double score, robj *obj) {
zskiplistNode *update[ZSKIPLIST_MAXLEVEL], *x;
//记录每层小于score,或者等于score但相应的字符串小的节点,插入节点的更新路径(每一层的上一个节点),插入节点需要修改每一层这个节点的上一个节点的信息(跨度),
unsigned int rank[ZSKIPLIST_MAXLEVEL];
//记录每一层插入节点的上一个节点在skiplist中的排名
int i, level;
//索引,层数
serverAssert(!isnan(score));
x = zsl->header;
//x用于迭代zskiplistNode
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 &&//短路判断
(x->level[i].forward->score < score ||
(x->level[i].forward->score == score &&
compareStringObjects(x->level[i].forward->obj,obj) < 0))) {
rank[i] += x->level[i].span;
//层越往下,越接近score,排名也在增大
x = x->level[i].forward;
}
update[i] = x;
//当前层的最后一个小于score,或者等于score但相应的字符串小的节点
}
/* we assume the key is not already inside, since we allow duplicated
* scores, and the re-insertion of score and redis object should never
* happen since the caller of zslInsert() should test in the hash table
* if the element is already inside or not. */
level = zslRandomLevel();
//给插入节点一个随机层数(1/4)的上升几率
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;
//高出部分,只有头结点有forward指针
}
zsl->level = level;
}
x = zslCreateNode(level,score,obj);
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 */
//更新跨度,需要用的rank数组,x在i层上一个节点的排名
x->level[i].span = update[i]->level[i].span - (rank[0] - rank[i]);
update[i]->level[i].span = (rank[0] - rank[i]) + 1;
//很明显了,i层的上一个节点到x的跨度等于 i层的上一个节点到0层的上一个节点的跨度再加1(0层上一个节点到x的跨度)
}
/* 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;
}
跳跃表主要用在有序集合中,和字典一起为zset提供常数查找,有序插入O(logN). 下面分别是有序集合,跳跃表和跳跃表节点的结构
在 redis3.0 中 c_zset.c 文件跳跃表插入节点的实现,ZSKIPLIST_MAXLEVEL层(32层)