redis相关知识(六)
1.7.1.3 SkipList(跳跃表)操作1) An empty SkipList
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2) Finding an element with key x
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12345678910111213p=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>n )
1.7.2 redis SkipList 实现 /* ZSETs use a specialized version of Skiplists */
12345678910111213141516171819202122232425262728293031323334353637383940414243typedef 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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