redis相关知识(六)

1.7.1.3 SkipList(跳跃表)操作

1) An empty SkipList

2) Finding an element with key x

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p=top While(1) { while (p->next->key < x ) p=p->next; If (p->down == NULL ) return p->next p=p->down ; }

Observe that we return x, if exists, or succ(x) if x is not in the SkipList

3) Inserting new element X

Determine k the number of levels in which x participates (explained later)

Do find(x), and insert x to the appropriate places in the lowest k levels. (after the elements at which the search path turns down or terminates)

Example �C inserting 119. k=2

If k is larger than the current number of levels, add new levels (and update top)

Example �C inser(119) when k=4

Determining k

k �C the number of levels at which an element x participate.

Use a random function OurRnd() ― returns 1 or 0 (True/False) with equal probability.

k=1 ;

While( OurRnd() ) k++ ;

Deleteing a key x

Find x in all the levels it participates, and delete it using the standard ‘delete from a linked list’ method.

If one or more of the upper levels are empty, remove them (and update top)

Facts about SkipList

The expected number of levels is O( log n )

(here n is the numer of elements)

The expected time for insert/delete/find is O( log n )

The expected size (number of cells) is O(n )

1.7.2 redis SkipList 实现

/* ZSETs use a specialized version of Skiplists */

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typedef struct zskiplistNode { robj *obj; 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; typedef struct zset { dict *dict; zskiplist *zsl; } zset;