2.redis学习笔记：redis List底层数据实现（通用双端链表）

野性酷女 2022-08-17 15:09 47阅读 0赞

# redis List底层数据实现 #

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redis列表使用两种数据结构左为底层实现：  
1.**双端链表**  
2.**压缩列表**

今天我们来介绍redis中的双端链表，在前边的数据结构章节中已经介绍了通用双端链表的实现，在redis数据库中，双端链表还被很多内部模块所应用：  
1.事务模块使用双端链表依序保存输入的命令；  
2.服务器模块使用双端链表来保存多个客户端；  
3.订阅/发送模块使用双端链表来保存订阅模式的多个客户端；  
4.事件模块使用双端链表来保存时间事件（time event）；

在redis的双端链表中，数据结构可以分为两个部分，一个是**控制信息**，一个是**链表节点信息**。

关于redis双端链表的定义和实现在其根目录**src/adlist.c**和**src/adlist.h**上进行定义和实现。

//链表控制信息
    typedef struct list {
        listNode *head;    //双端链表的头部节点
        listNode *tail;    //双端链表的尾部节点
        void *(*dup)(void *ptr);    //链表节点数据域拷贝
        void (*free)(void *ptr);    //链表节点数据域释放
        int (*match)(void *ptr, void *key);    //链表节点数据域匹配
        unsigned long len;    //双端链表长度
    } list;

//链表节点信息
    typedef struct listNode {
        struct listNode *prev;    //前驱节点
        struct listNode *next;    //后继节点
        void *value;              //数据域
    } listNode;
    
    //迭代器的迭代方向
    #define AL_START_HEAD 0
    #define AL_START_TAIL 1

值的一提的是链表的遍历操作采用的是迭代器操作，关于迭代器的定义如下所示：

//迭代器定义
    typedef struct listIter {
        listNode *next;
        int direction;
    } listIter;

不过比较其他开源库的迭代器设计，redis的迭代器设计并不算优雅，在博客<5.数据结构之通用动态数组>章节中，我们介绍了acl库中迭代器的操作，有兴趣的同学可以参考一下。

redis把指针的操作封装成了宏的类型，方便了程序员的使用，不过如果你习惯指针写法也可以采用指针的方式：

//宏定义操作
    #define listLength(l) ((l)->len)
    #define listFirst(l) ((l)->head)
    #define listLast(l) ((l)->tail)
    #define listPrevNode(n) ((n)->prev)
    #define listNextNode(n) ((n)->next)
    #define listNodeValue(n) ((n)->value)
    
    #define listSetDupMethod(l,m) ((l)->dup = (m))
    #define listSetFreeMethod(l,m) ((l)->free = (m))
    #define listSetMatchMethod(l,m) ((l)->match = (m))
    
    #define listGetDupMethod(l) ((l)->dup)
    #define listGetFree(l) ((l)->free)
    #define listGetMatchMethod(l) ((l)->match)

--------------------

## redis通用双端链表接口 ##

//接口声明
    list *listCreate(void)                        ;    //链表的创建
    void listRelease(list *list)                  ;    //链表的释放
    list *listAddNodeHead(list *list, void *value);    //链表的头部添加
    list *listAddNodeTail(list *list, void *value);    //链表的尾部添加
    list *listInsertNode(list *list, listNode *old_node, void *value, int after);    //链表的节点插入
    void     listDelNode(list *list, listNode *node)        ;    //链表节点删除
    listIter *listGetIterator(list *list, int direction)    ;    //初始化链表迭代器（可以通过direction调整初始化方向，头部或者尾部）
    listNode *listNext(listIter *iter)                      ;    //链表的下一个节点(有direction确定是向前遍历还是向后遍历)
    void     listReleaseIterator(listIter *iter)            ;    //链表迭代器的释放
    list     *listDup(list *orig)                           ;    //链表的拷贝
    listNode *listSearchKey(list *list, void *key)          ;   //链表的查找
    listNode *listIndex(list *list, long index)             ;    //找到下标为index的链表节点
    void     listRewind(list *list, listIter *li)           ;   //让迭代器指向头部
    void     listRewindTail(list *list, listIter *li)       ;    //让迭代器指向尾部
    void     listRotate(list *list)                         ;    //取出链表的表尾节点，并且插入到头部

--------------------

## redis双端链表的接口实现 ##

//adlist.c包含的头部文件
    #include <stdlib.h>
    #include "adlist.h"
    #include "zmalloc.h"   //这个内存配置器需要好好了解，其根据系统的内存配置器进行动态的选取，可以是tcmalloc（google）或者是jemalloc（freeBSD）

接下来我们介绍redis双端链表接口的具体实现：

//adlist.c
    list *listCreate(void)
    {
        struct list *list;
    
        if ((list = zmalloc(sizeof(*list))) == NULL)
            return NULL;
        //各个变量的初始化
        list->head = list->tail = NULL;
        list->len = 0;
        list->dup = NULL;
        list->free = NULL;
        list->match = NULL;
        return list;
    }
    
    /* Free the whole list. * * This function can't fail. */
    void listRelease(list *list)
    {
        unsigned long len;
        listNode *current, *next;
    
        current = list->head;
        len = list->len;
        while(len--) {
            next = current->next;
            if (list->free){
                list->free(current->value);
            }
            zfree(current);
            current = next;
        }
        zfree(list);
    }
    
    /* Add a new node to the list, to head, containing the specified 'value' * pointer as value. * * On error, NULL is returned and no operation is performed (i.e. the * list remains unaltered). * On success the 'list' pointer you pass to the function is returned. */
    list *listAddNodeHead(list *list, void *value)
    {
        listNode *node;
    
        if ((node = zmalloc(sizeof(*node))) == NULL)
            return NULL;
        node->value = value;
        if (list->len == 0) {
            list->head = list->tail = node;
            node->prev = node->next = NULL;
        } else {
            node->prev = NULL;
            node->next = list->head;
            list->head->prev = node;
            list->head = node;
        }
        list->len++;
        return list;
    }
    
    /* Add a new node to the list, to tail, containing the specified 'value' * pointer as value. * * On error, NULL is returned and no operation is performed (i.e. the * list remains unaltered). * On success the 'list' pointer you pass to the function is returned. */
    list *listAddNodeTail(list *list, void *value)
    {
        listNode *node;
    
        if ((node = zmalloc(sizeof(*node))) == NULL)
            return NULL;
        node->value = value;
        if (list->len == 0) {
            list->head = list->tail = node;
            node->prev = node->next = NULL;
        } else {
            node->prev = list->tail;
            node->next = NULL;
            list->tail->next = node;
            list->tail = node;
        }
        list->len++;
        return list;
    }
    
    list *listInsertNode(list *list, listNode *old_node, void *value, int after) {
        listNode *node;
    
        if ((node = zmalloc(sizeof(*node))) == NULL)
            return NULL;
        node->value = value;
        if (after) {
            node->prev = old_node;
            node->next = old_node->next;
            if (list->tail == old_node) {
                list->tail = node;
            }
        } else {
            node->next = old_node;
            node->prev = old_node->prev;
            if (list->head == old_node) {
                list->head = node;
            }
        }
        if (node->prev != NULL) {
            node->prev->next = node;
        }
        if (node->next != NULL) {
            node->next->prev = node;
        }
        list->len++;
        return list;
    }
    
    /* Remove the specified node from the specified list. * It's up to the caller to free the private value of the node. * * This function can't fail. */
    void listDelNode(list *list, listNode *node)
    {
        if (node->prev)
            node->prev->next = node->next;
        else
            list->head = node->next;
        if (node->next)
            node->next->prev = node->prev;
        else
            list->tail = node->prev;
        if (list->free){
            list->free(node->value);
        }
        zfree(node);
        list->len--;
    }
    
    /* Returns a list iterator 'iter'. After the initialization every * call to listNext() will return the next element of the list. * * This function can't fail. */
    listIter *listGetIterator(list *list, int direction)
    {
        listIter *iter;
    
        if ((iter = zmalloc(sizeof(*iter))) == NULL) return NULL;
        if (direction == AL_START_HEAD){
            iter->next = list->head;
        }
        else{
            iter->next = list->tail;
        }
        iter->direction = direction;
        return iter;
    }
    
    /* Release the iterator memory */
    void listReleaseIterator(listIter *iter) {
        zfree(iter);
    }
    
    /* Create an iterator in the list private iterator structure */
    void listRewind(list *list, listIter *li) {
        li->next = list->head;
        li->direction = AL_START_HEAD;
    }
    
    void listRewindTail(list *list, listIter *li) {
        li->next = list->tail;
        li->direction = AL_START_TAIL;
    }
    
    /* Return the next element of an iterator. * It's valid to remove the currently returned element using * listDelNode(), but not to remove other elements. * * The function returns a pointer to the next element of the list, * or NULL if there are no more elements, so the classical usage patter * is: * * iter = listGetIterator(list,<direction>); * while ((node = listNext(iter)) != NULL) { * doSomethingWith(listNodeValue(node)); * } * * */
    listNode *listNext(listIter *iter)
    {
        listNode *current = iter->next;
    
        if (current != NULL) {
            if (iter->direction == AL_START_HEAD)
                iter->next = current->next;
            else
                iter->next = current->prev;
        }
        return current;
    }
    
    /* Duplicate the whole list. On out of memory NULL is returned. * On success a copy of the original list is returned. * * The 'Dup' method set with listSetDupMethod() function is used * to copy the node value. Otherwise the same pointer value of * the original node is used as value of the copied node. * * The original list both on success or error is never modified. */
    list *listDup(list *orig)
    {
        list *copy;
        listIter *iter;
        listNode *node;
    
        if ((copy = listCreate()) == NULL)
            return NULL;
        copy->dup = orig->dup;
        copy->free = orig->free;
        copy->match = orig->match;
        iter = listGetIterator(orig, AL_START_HEAD);
        while((node = listNext(iter)) != NULL) {
            void *value;
    
            if (copy->dup) {
                value = copy->dup(node->value);
                if (value == NULL) {
                    listRelease(copy);
                    listReleaseIterator(iter);
                    return NULL;
                }
            } else
                value = node->value;
            if (listAddNodeTail(copy, value) == NULL) {
                listRelease(copy);
                listReleaseIterator(iter);
                return NULL;
            }
        }
        listReleaseIterator(iter);
        return copy;
    }
    
    /* Search the list for a node matching a given key. * The match is performed using the 'match' method * set with listSetMatchMethod(). If no 'match' method * is set, the 'value' pointer of every node is directly * compared with the 'key' pointer. * * On success the first matching node pointer is returned * (search starts from head). If no matching node exists * NULL is returned. */
    listNode *listSearchKey(list *list, void *key)
    {
        listIter *iter;
        listNode *node;
    
        iter = listGetIterator(list, AL_START_HEAD);
        while((node = listNext(iter)) != NULL) {
            if (list->match) {
                if (list->match(node->value, key)) {
                    listReleaseIterator(iter);
                    return node;
                }
            } else {
                if (key == node->value) {
                    listReleaseIterator(iter);
                    return node;
                }
            }
        }
        listReleaseIterator(iter);
        return NULL;
    }
    
    /* Return the element at the specified zero-based index * where 0 is the head, 1 is the element next to head * and so on. Negative integers are used in order to count * from the tail, -1 is the last element, -2 the penultimate * and so on. If the index is out of range NULL is returned. */
    listNode *listIndex(list *list, long index) {
        listNode *n;
    
        if (index < 0) {
            index = (-index)-1;
            n = list->tail;
            while(index-- && n) n = n->prev;
        } else {
            n = list->head;
            while(index-- && n) n = n->next;
        }
        return n;
    }
    
    /* Rotate the list removing the tail node and inserting it to the head. */
    void listRotate(list *list) {
        listNode *tail = list->tail;
    
        if (listLength(list) <= 1) return;
    
        /* Detach current tail */
        list->tail = tail->prev;
        list->tail->next = NULL;
        /* Move it as head */
        list->head->prev = tail;
        tail->prev = NULL;
        tail->next = list->head;
        list->head = tail;
    }

--------------------

小结：  
可以看到redis的通用双端链表实现非常的简洁明了，其难点在于要把数据类型从结构的设计中剥离出来，也就是采用void \*这样的指针。在接下来的章节中我们将会继续介绍redis的使用，敬请期待。