redis相關知識(六)

1.7.1.3 SkipList(跳躍表)操作

1) An empty SkipList

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2) Finding an element with key x

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p=topWhile(1){while (p->next->key < x ) p=p->next;If (p->down == NULL ) return p->nextp=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

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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)

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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 實現

/* 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;

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