作为zset的底层实现之一(另一个是dict),今天来扯扯zskiplist(跳跃表)这个东西,至于zset是怎么利用这两个东西的,咱们之后再表,这次只是说说跳跃表的实现。
跳跃表是一种有序的数据结构,它通过在每个节点中存放多个指向其他节点的指针(正向的指针>=1个,反向的指针就一个)来达到快速访问节点的目的。在redis当中,其代码定义如下:
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typedef struct zskiplistNode {
sds ele;
double score;
struct zskiplistNode *backward;
struct zskiplistLevel {
struct zskiplistNode *forward;
unsigned int span;
} level[];
} zskiplistNode;
typedef struct zskiplist {
struct zskiplistNode *header, tail;
unsigned long length;
int level;
} zskiplist;
#define ZSKIPLIST_MAXLEVEL 32 / Should be enough for 2^32 elements /
#define ZSKIPLIST_P 0.25 / Skiplist P = 1/4 */
先来说说zskiplistNode结构:
字段名称 含义
ele 当前节点存储的元素
score 当前节点存储的元素的得分
level[] 当前节点向前遍历时的指针数组,数组中的每个元素包含两个属性:1). 可以到达的节点指针forward;2). 该节点与当前节点的距离span,span用于计算forward节点在当前zskiplist当中的排名,而非用于遍历。
backward 当前节点向后遍历时的指针。注意这里不是数组,也就是说反向遍历只能从后往前一个一个来~~~
之后是zskiplist结构
字段名称 含义
header 指向跳跃表的头结点的指针
tail 指向跳跃表的尾节点的指针
length 跳跃表的长度,即当前跳跃表包含的节点数目(头结点不包含在内)
level 跳跃表中,level数组中元素最多的那个节点的level数组的元素个数(头结点不包含在内)
zskiplist结构当中需要多说一下的就是为什么length和level字段没有包括头结点?原因在于当创建一个zskiplist的时候,会直接把头结点设置成最大level,以便后续操作,其创建代码如下所示:
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/* 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;
}
至于对zskiplist的操作方面,个人觉得还是得推插入和删除了
由于zskiplist当中不允许存在相同的sds,这一保障性工作交由调用插入操作的调用者来保证~
插入的时候需要注意的就是它是以level从大到小的顺序来遍历并更新各个指针的,详情见代码:
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/* 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;
// 确定每一个level当中由那一个节点来指向新插入的节点,并记录span
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;
}
/ 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();
// 如果超出level,则更新span
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;
}
// 创建并插入节点
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;
}
zskiplistNode创建的时候会随机生成一个介于1和32之间的数来决定level数组的大小,这个数的生成遵循“越大的数出现的概率越小”这一原则,代码如下: -
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int zslRandomLevel(void) {
int level = 1;
while ((random()&0xFFFF) < (ZSKIPLIST_P * 0xFFFF))
level += 1;
return (level<ZSKIPLIST_MAXLEVEL) ? level : ZSKIPLIST_MAXLEVEL;
}
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节点的删除工作就比较的中规中矩了
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/* Internal function used by zslDelete, zslDeleteByScore and zslDeleteByRank */
void zslDeleteNode(zskiplist *zsl, zskiplistNode *x, zskiplistNode **update) {
int i;
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;
}
}
if (x->level[0].forward) {
x->level[0].forward->backward = x->backward;
} else {
zsl->tail = x->backward;
}
while(zsl->level > 1 && zsl->header->level[zsl->level-1].forward == NULL)
zsl->level–;
zsl->length–;
}