1.7.1.3 SkipList(跳跃表)操作
1) An empty SkipList
2) Finding an element with key x
1 2 3 4 5 6 7 8 9 10 11 12 13 | 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 – inserting 119. k=2
If k is larger than the current number of levels, add new levels (and update top)
Example – inser(119) when k=4
Determining k
k – 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 */
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 | 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; |
