0. 前言
Redis
的跳表在面试过程中,经常被问到,这里通过源码深入研究一个其实现原理。
1. 结构源码
// 跳表中的每一个节点的结构
typedef struct zskiplistNode {
// 成员对象,sds 结构
sds ele;
// 分值
double score;
// 后退指针,指向当前节点的前一个指针
struct zskiplistNode *backward;
struct zskiplistLevel {
// 前进指向
struct zskiplistNode *forward;
// 前进指针指向的节点和当前节点的距离
unsigned long span;
} level[];
} zskiplistNode;
// 跳表对象
typedef struct zskiplist {
// header 表头
// tail 表尾
struct zskiplistNode *header, *tail;
// 跳表长度(不包含表头节点)
unsigned long length;
// 非表头节点的最大层
int level;
} zskiplist;
2. 创建
/* 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;
}
/* Create a new skiplist. */
zskiplist *zslCreate(void) {
int j;
zskiplist *zsl;
zsl = zmalloc(sizeof(*zsl));
zsl->level = 1;
zsl->length = 0;
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;
}
3. 插入
/* 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) {
// update 用来记录需要进行更新的节点
zskiplistNode *update[ZSKIPLIST_MAXLEVEL], *x;
// rank 用来记录每层到新元素插入位置的距离
unsigned int rank[ZSKIPLIST_MAXLEVEL];
int i, level;
serverAssert(!isnan(score));
x = zsl->header;
// 找到插入的位置
// 从最高层向下寻找
for (i = zsl->level-1; i >= 0; i--) {
// 最高层 rank 初始化为 0,下面的层初始化为上一层的结果
rank[i] = i == (zsl->level-1) ? 0 : rank[i+1];
// 如果当前节点的分值小于插入分值,或者分值相等,但插入的对象大于当前节点的对象,继续向后进行查找
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;
}
// 随机生成一个层数(1~32)
level = zslRandomLevel();
if (level > zsl->level) {
// 初始化空白 rank level 比原始的层数大的层
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);
// 将前面记录需要修改的节点指向新节点新节点指向原来的下个节点
for (i = 0; i < level; i++) {
// 新插入的元素一定是 update 的下一个节点
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 */
// 计算 span
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 */
// 未接触的节点的span 值也需要加一
for (i = level; i < zsl->level; i++) {
update[i]->level[i].span++;
}
// 设置新节点的后退节点
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;
}
4. 删除
/* Internal function used by zslDelete, zslDeleteRangeByScore and
* zslDeleteRangeByRank. */
void zslDeleteNode(zskiplist *zsl, zskiplistNode *x, zskiplistNode **update) {
int i;
// 所有和被删除节点x有关的节点的指针,解除它们之间的关系
for (i = 0; i < zsl->level; i++) {
if (update[i]->level[i].forward == x) {
update[i]->level[i].span += x->level[i].span - 1;
update[i]->level[i].forward = x->level[i].forward;
} else {
update[i]->level[i].span -= 1;
}
}
// 更新被删除节点 x 的前进和后退指针
if (x->level[0].forward) {
x->level[0].forward->backward = x->backward;
} else {
zsl->tail = x->backward;
}
// 更新跳跃表最大层数(只有被删除节点是跳跃表最高的节点时才执行)
while(zsl->level > 1 && zsl->header->level[zsl->level-1].forward == NULL)
zsl->level--;
zsl->length--;
}
5. 获取
/* Find the first node that is contained in the specified range.
* Returns NULL when no element is contained in the range. */
zskiplistNode *zslFirstInRange(zskiplist *zsl, zrangespec *range) {
zskiplistNode *x;
int i;
/* If everything is out of range, return early. */
if (!zslIsInRange(zsl,range)) return NULL;
x = zsl->header;
for (i = zsl->level-1; i >= 0; i--) {
/* Go forward while *OUT* of range. */
// 检测节点是否小于range中的最小项,如果是,则继续向后找
while (x->level[i].forward &&
!zslValueGteMin(x->level[i].forward->score,range))
x = x->level[i].forward;
}
/* This is an inner range, so the next node cannot be NULL. */
x = x->level[0].forward;
serverAssert(x != NULL);
/* Check if score <= max. */
if (!zslValueLteMax(x->score,range)) return NULL;
return x;
}