redis学习系列（六）--redis基础跳跃表的构造_redis跳表构造方法-优快云博客

本文介绍了Redis中使用的跳跃表数据结构，详细解释了其组成部分包括zskiplist和zskiplistNode，以及如何通过前进指针和跨度实现高效查找。

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看redis的时候第一次看见跳跃表这种数据结构，说实话，之前还真不知道有这么个结构，哎。。。。。。。。。。。

跳跃表其实也是一个链表，不过子啊实现上有点区别，也是由一个一个的Node组成，zskiplistNode是组成，zsklist则是保存了跳跃表节点的相关信息，例如节点的数量，头结点指针，尾节点指针。

$digraph { rankdir = LR; node [shape = record, width = "0.5"]; // l [label = " <header> header | <tail> tail | level \n 5 | length \n 3 "]; subgraph cluster_nodes { style = invisible; header [label = " <l32> L32 | ... | <l5> L5 | <l4> L4 | <l3> L3 | <l2> L2 | <l1> L1 "]; bw_null [label = "NULL", shape = plaintext]; level_null [label = "NULL", shape = plaintext]; A [label = " <l4> L4 | <l3> L3 | <l2> L2 | <l1> L1 | <backward> BW | 1.0 | o1 "]; B [label = " <l2> L2 | <l1> L1 | <backward> BW | 2.0 | o2 "]; C [label = " <l5> L5 | <l4> L4 | <l3> L3 | <l2> L2 | <l1> L1 | <backward> BW | 3.0 | o3 "]; } subgraph cluster_nulls { style = invisible; n1 [label = "NULL", shape = plaintext]; n2 [label = "NULL", shape = plaintext]; n3 [label = "NULL", shape = plaintext]; n4 [label = "NULL", shape = plaintext]; n5 [label = "NULL", shape = plaintext]; } // l:header -> header; l:tail -> C; header:l32 -> level_null [label = "0"]; header:l5 -> C:l5 [label = "3"]; header:l4 -> A:l4 [label = "1"]; header:l3 -> A:l3 [label = "1"]; header:l2 -> A:l2 [label = "1"]; header:l1 -> A:l1 [label = "1"]; A:l4 -> C:l4 [label = "2"]; A:l3 -> C:l3 [label = "2"]; A:l2 -> B:l2 [label = "1"]; A:l1 -> B:l1 [label = "1"]; B:l2 -> C:l2 [label = "1"]; B:l1 -> C:l1 [label = "1"]; C:l5 -> n5 [label = "0"]; C:l4 -> n4 [label = "0"]; C:l3 -> n3 [label = "0"]; C:l2 -> n2 [label = "0"]; C:l1 -> n1 [label = "0"]; bw_null -> A:backward -> B:backward -> C:backward [dir = back]; label = "\n 图 5-1 一个跳跃表";}$

zskiplist

最左边的就是zskiplist结构：

header: 指向表头节点

tail: 指向表尾节点

lever: 记录跳跃表内，层级最大的那个节点的层级（表头节点的层级不计算在内）

length：记录跳跃表的长度，就是跳跃表包含的节点的数量（表头节点不在计算内）

zskiplistNode

右边的是四个zskiplistNode结构：

层（lever）：节点中用L1，L2，L3等字样标记各个层，L1代表第一层，L2代表第二层，每层有两个属性：前进指针和跨度

前进指针：用于访问表尾方向的其他节点

跨度：记录了前进指针所指向节点和当前节点的距离

具体看上图：连线上带有数字的箭头就代表前进指针，而数字就是跨度

程序在从表头遍历想表尾遍历时，访问会沿着层的前进指针进行

后退指针：节点中用BW字样标记节点的后退指针，它指向位于当前节点的前一个节点，因此它在从表尾向表头遍历时使用。

分值：节点中保存的值 1.0 ,2.0, 3.0

成员对象：保存的成员对象，o1,o2,o3

zskiplistNode的构造

typedef struct zskiplistNode {

    // 后退指针
    struct zskiplistNode *backward;

    // 分值
    double score;

    // 成员对象
    robj *obj;

    // 层
    struct zskiplistLevel {

        // 前进指针
        struct zskiplistNode *forward;

        // 跨度
        unsigned int span;

    } level[];

} zskiplistNode;

层

跳跃表节点的lever数组可以包含多个元素，每个元素都包含一个指向其他节点的指针，程序就是通过这些指针实现快速访问

$digraph { label = "\n 图 5-2 带有不同层高的节点"; rankdir = LR; // node [shape = record]; n1 [label = " zskiplistNode | level[0] | backward | score | obj "]; n2 [label = " zskiplistNode | level[2] | level[1] | level[0] | backward | score | obj "]; n3 [label = " zskiplistNode | level[4] | level[3] | level[2] | level[1] | level[0] | backward | score | obj "]; // edge [style = invis]; n1 -> n2 -> n3;}$

前进指针：

每一层都会有一个前进指针，表名遍历的方向；

图5-3中用虚线表示出从表头向表尾的方向，遍历所有节点的路径。

1.迭代程序首先访问跳跃表的第一个节点（表头），然后从L4的前进指针移动到表中的第二个节点

2.在第二个节点时，程序沿着第二层的前进指针移动到表中的第三个节点

3.在第三个节点时，程序同样沿着第二层的前进指针移动到表中的第四个节点。

4.当程序在第四个节点的前进指针移动时，会碰到一个null，此时已经到达了跳跃表的表尾，

$digraph { rankdir = LR; node [shape = record, width = "0.5"]; // l [label = " <header> header | <tail> tail | level \n 5 | length \n 3 "]; subgraph cluster_nodes { style = invisible; header [label = " <l32> L32 | ... | <l5> L5 | <l4> L4 | <l3> L3 | <l2> L2 | <l1> L1 "]; bw_null [label = "NULL", shape = plaintext]; level_null [label = "NULL", shape = plaintext]; A [label = " <l4> L4 | <l3> L3 | <l2> L2 | <l1> L1 | <backward> BW | 1.0 | o1 "]; B [label = " <l2> L2 | <l1> L1 | <backward> BW | 2.0 | o2 "]; C [label = " <l5> L5 | <l4> L4 | <l3> L3 | <l2> L2 | <l1> L1 | <backward> BW | 3.0 | o3 "]; } subgraph cluster_nulls { style = invisible; n1 [label = "NULL", shape = plaintext]; n2 [label = "NULL", shape = plaintext]; n3 [label = "NULL", shape = plaintext]; n4 [label = "NULL", shape = plaintext]; n5 [label = "NULL", shape = plaintext]; } // l:header -> header [style = dashed]; l:tail -> C; header:l32 -> level_null [label = "0"]; header:l5 -> C:l5 [label = "3"]; header:l4 -> A:l4 [label = "1", style = dashed]; header:l3 -> A:l3 [label = "1"]; header:l2 -> A:l2 [label = "1"]; header:l1 -> A:l1 [label = "1"]; A:l4 -> C:l4 [label = "2"]; A:l3 -> C:l3 [label = "2"]; A:l2 -> B:l2 [label = "1", style = dashed]; A:l1 -> B:l1 [label = "1"]; B:l2 -> C:l2 [label = "1", style = dashed]; B:l1 -> C:l1 [label = "1"]; C:l5 -> n5 [label = "0"]; C:l4 -> n4 [label = "0"]; C:l3 -> n3 [label = "0"]; C:l2 -> n2 [label = "0", style = dashed]; C:l1 -> n1 [label = "0"]; bw_null -> A:backward -> B:backward -> C:backward [dir = back]; label = "\n 图 5-3 遍历整个跳跃表";}$

跨度

层的跨度用于记录两个节点之间的距离

1.两个节点之间的跨度越大，说明它们相隔的越远

2.指向null的所有前进指针的跨度为0，因为它们没有连上任何节点。

跨度是计算排位的：排位是指在查找某个节点的过程中，将沿途访问过的所有层的跨度累计起来，得到的结果就是跳跃表中的排位。

下图例子中，虚线标注了在跳跃表中查找分值为3.0，成员对象为o3的节点时，沿途经历的层，由图可知只经历过一层，跨度为3

$digraph { rankdir = LR; node [shape = record, width = "0.5"]; // l [label = " <header> header | <tail> tail | level \n 5 | length \n 3 "]; subgraph cluster_nodes { style = invisible; header [label = " <l32> L32 | ... | <l5> L5 | <l4> L4 | <l3> L3 | <l2> L2 | <l1> L1 "]; bw_null [label = "NULL", shape = plaintext]; level_null [label = "NULL", shape = plaintext]; A [label = " <l4> L4 | <l3> L3 | <l2> L2 | <l1> L1 | <backward> BW | 1.0 | o1 "]; B [label = " <l2> L2 | <l1> L1 | <backward> BW | 2.0 | o2 "]; C [label = " <l5> L5 | <l4> L4 | <l3> L3 | <l2> L2 | <l1> L1 | <backward> BW | 3.0 | o3 "]; } subgraph cluster_nulls { style = invisible; n1 [label = "NULL", shape = plaintext]; n2 [label = "NULL", shape = plaintext]; n3 [label = "NULL", shape = plaintext]; n4 [label = "NULL", shape = plaintext]; n5 [label = "NULL", shape = plaintext]; } // l:header -> header [style = dashed]; l:tail -> C; header:l32 -> level_null [label = "0"]; header:l5 -> C:l5 [label = "3", style = dashed]; header:l4 -> A:l4 [label = "1"]; header:l3 -> A:l3 [label = "1"]; header:l2 -> A:l2 [label = "1"]; header:l1 -> A:l1 [label = "1"]; A:l4 -> C:l4 [label = "2"]; A:l3 -> C:l3 [label = "2"]; A:l2 -> B:l2 [label = "1"]; A:l1 -> B:l1 [label = "1"]; B:l2 -> C:l2 [label = "1"]; B:l1 -> C:l1 [label = "1"]; C:l5 -> n5 [label = "0"]; C:l4 -> n4 [label = "0"]; C:l3 -> n3 [label = "0"]; C:l2 -> n2 [label = "0"]; C:l1 -> n1 [label = "0"]; bw_null -> A:backward -> B:backward -> C:backward [dir = back]; label = "\n 图 5-4 计算节点的排位";}$

后退指针

bw属性，用于表尾向表头访问节点时，与前进指针不同的是，每个节点只有一个后退指针，所以每次只能后退一个节点

图5-6表示的是从tail节点从表尾向表头遍历，直至遇到null节点，结束

$digraph { rankdir = LR; node [shape = record, width = "0.5"]; // l [label = " <header> header | <tail> tail | level \n 5 | length \n 3 "]; subgraph cluster_nodes { style = invisible; header [label = " <l32> L32 | ... | <l5> L5 | <l4> L4 | <l3> L3 | <l2> L2 | <l1> L1 "]; bw_null [label = "NULL", shape = plaintext]; level_null [label = "NULL", shape = plaintext]; A [label = " <l4> L4 | <l3> L3 | <l2> L2 | <l1> L1 | <backward> BW | 1.0 | o1 "]; B [label = " <l2> L2 | <l1> L1 | <backward> BW | 2.0 | o2 "]; C [label = " <l5> L5 | <l4> L4 | <l3> L3 | <l2> L2 | <l1> L1 | <backward> BW | 3.0 | o3 "]; } subgraph cluster_nulls { style = invisible; n1 [label = "NULL", shape = plaintext]; n2 [label = "NULL", shape = plaintext]; n3 [label = "NULL", shape = plaintext]; n4 [label = "NULL", shape = plaintext]; n5 [label = "NULL", shape = plaintext]; } // l:header -> header; l:tail -> C [style = dashed]; header:l32 -> level_null [label = "0"]; header:l5 -> C:l5 [label = "3"]; header:l4 -> A:l4 [label = "1"]; header:l3 -> A:l3 [label = "1"]; header:l2 -> A:l2 [label = "1"]; header:l1 -> A:l1 [label = "1"]; A:l4 -> C:l4 [label = "2"]; A:l3 -> C:l3 [label = "2"]; A:l2 -> B:l2 [label = "1"]; A:l1 -> B:l1 [label = "1"]; B:l2 -> C:l2 [label = "1"]; B:l1 -> C:l1 [label = "1"]; C:l5 -> n5 [label = "0"]; C:l4 -> n4 [label = "0"]; C:l3 -> n3 [label = "0"]; C:l2 -> n2 [label = "0"]; C:l1 -> n1 [label = "0"]; bw_null -> A:backward -> B:backward -> C:backward [dir = back, style = dashed]; label = "\n 图 5-6 从表尾向表头方向遍历跳跃表";}$

分值和成员

节点的分值都是按照分值大小从小到大来排序

节点的成员对象是一个指针，指向一个字符串对象，而字符串保存着一个SDS值。

在同一个跳跃表中，保存的成员对象是唯一的，但是分值确实可以相同的，分值相同的节点将按照在字典序中来排序，成员对象小的节点排在前面。

$digraph { rankdir = LR; node [shape = record, width = "0.5"]; // l [label = " <header> header | <tail> tail | level \n 5 | length \n 3 "]; subgraph cluster_nodes { style = invisible; header [label = " <l32> L32 | ... | <l5> L5 | <l4> L4 | <l3> L3 | <l2> L2 | <l1> L1 "]; bw_null [label = "NULL", shape = plaintext]; level_null [label = "NULL", shape = plaintext]; A [label = " <l4> L4 | <l3> L3 | <l2> L2 | <l1> L1 | <backward> BW | 10086.0 | o1 "]; B [label = " <l2> L2 | <l1> L1 | <backward> BW | 10086.0 | o2 "]; C [label = " <l5> L5 | <l4> L4 | <l3> L3 | <l2> L2 | <l1> L1 | <backward> BW | 10086.0 | o3 "]; } subgraph cluster_nulls { style = invisible; n1 [label = "NULL", shape = plaintext]; n2 [label = "NULL", shape = plaintext]; n3 [label = "NULL", shape = plaintext]; n4 [label = "NULL", shape = plaintext]; n5 [label = "NULL", shape = plaintext]; } // l:header -> header; l:tail -> C; header:l32 -> level_null [label = "0"]; header:l5 -> C:l5 [label = "3"]; header:l4 -> A:l4 [label = "1"]; header:l3 -> A:l3 [label = "1"]; header:l2 -> A:l2 [label = "1"]; header:l1 -> A:l1 [label = "1"]; A:l4 -> C:l4 [label = "2"]; A:l3 -> C:l3 [label = "2"]; A:l2 -> B:l2 [label = "1"]; A:l1 -> B:l1 [label = "1"]; B:l2 -> C:l2 [label = "1"]; B:l1 -> C:l1 [label = "1"]; C:l5 -> n5 [label = "0"]; C:l4 -> n4 [label = "0"]; C:l3 -> n3 [label = "0"]; C:l2 -> n2 [label = "0"]; C:l1 -> n1 [label = "0"]; bw_null -> A:backward -> B:backward -> C:backward [dir = back]; label = "\n 图 5-7 三个带有相同分值的跳跃表节点";}$

zskiplist 结构的定义如下：

typedef struct zskiplist {

    // 表头节点和表尾节点
    struct zskiplistNode *header, *tail;

    // 表中节点的数量
    unsigned long length;

    // 表中层数最大的节点的层数
    int level;

} zskiplist

头指针和尾指针定位表头和表尾，时间复杂度为O（1）