redis源码解析（四）双向链表

你的名字 2022-05-15 14:38 173阅读 0赞

###   本文分析redis里的双向链表的基础结构以及基础函数 ###

### 1. 基本结构 ###

redis的双向链表主要实现了以下三个结构体：  
（1）链表节点，包括两个指针指向前后节点，一个指针指向节点的值  
（2）迭代器，用于遍历  
（3）链表结构体，包括头、尾、复制释放遍历函数、链表长度。

/* Node, List, and Iterator are the only data structures used currently. */
    /* 经典的双向链表节点定义： * 两个指针指向前后节点，一个指针指向值 */
    typedef struct listNode { 
        struct listNode *prev;
        struct listNode *next;
        void *value;
    } listNode;
    
    /*类似于c++11的迭代器，仅指向next，direction注明方向*/
    typedef struct listIter { 
        listNode *next;
        int direction;
    } listIter;
    
    /* 链表结构体，包括头尾节点、长度和几个函数 * 复制、释放、查找函数 */
    typedef struct list { 
        listNode *head;
        listNode *tail;
        void *(*dup)(void *ptr);
        void (*free)(void *ptr);
        int (*match)(void *ptr, void *key);
        unsigned long len;
    } list;

### 2. 基本函数 ###

链表的基本函数无非是新建、插入、删除、遍历、复制，代码如下所示

/* 创建新链表，赋值为NULL * Create a new list. The created list can be freed with * AlFreeList(), but private value of every node need to be freed * by the user before to call AlFreeList(). * * On error, NULL is returned. Otherwise the pointer to the new list. */
    list *listCreate(void)
    { 
        struct list *list;
    
        if ((list = zmalloc(sizeof(*list))) == NULL)
            return NULL;
        list->head = list->tail = NULL;
        list->len = 0;
        list->dup = NULL;
        list->free = NULL;
        list->match = NULL;
        return list;
    }
    
    /* 将链表清空，并赋值为NULL * Remove all the elements from the list without destroying the list itself. */
    void listEmpty(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;
        }
        list->head = list->tail = NULL;
        list->len = 0;
    }
    
    /* 释放链表 * Free the whole list. * * This function can't fail. */
    void listRelease(list *list)
    { 
        listEmpty(list);
        zfree(list);
    }
    
    /* 为链表添加新的一个头结点，值为value * 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;
    }
    
    /* 为链表添加新的一个尾结点，值为value * 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;
    }
    
    /*在old_node之后/前插入节点，由after控制*/
    list *listInsertNode(list *list, listNode *old_node, void *value, int after) 
    { 
        listNode *node;
    
        if ((node = zmalloc(sizeof(*node))) == NULL)
            return NULL;
        node->value = value;
    
    	/* after大于0则在old_node之后插入 * 否则在old_node之前插入 */
        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;
    }
    
    /* 从链表中删除节点 * 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)
    { 
    	/*如果前面有节点，则让前面节点指向后面节点*/
        if (node->prev)
            node->prev->next = node->next;
        else
            list->head = node->next;
    	
    	/*如果有后面节点，则后面节点的prev指向前面节点*/
        if (node->next)
            node->next->prev = node->prev;
        else
            list->tail = node->prev;
    
    	/*释放节点并减少len*/
        if (list->free) list->free(node->value);
        zfree(node);
        list->len--;
    }
    
    /* 类似于STL的迭代器，根据direction确定指向头或者尾 * 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;
    }
    
    /* 释放资源 * Release the iterator memory */
    void listReleaseIterator(listIter *iter) { 
        zfree(iter);
    }
    
    /* 使用li作为list的头部迭代器 * Create an iterator in the list private iterator structure */
    void listRewind(list *list, listIter *li) { 
        li->next = list->head;
        li->direction = AL_START_HEAD;
    }
    
    /*使用li作为list的尾部迭代器*/
    void listRewindTail(list *list, listIter *li) { 
        li->next = list->tail;
        li->direction = AL_START_TAIL;
    }
    
    /* 返回迭代器的下一个元素 * 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;
    }
    
    /* 复制链表 * 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;
        listRewind(orig, &iter);
        while((node = listNext(&iter)) != NULL) { 
            void *value;
    
            if (copy->dup) { 
                value = copy->dup(node->value);
                if (value == NULL) { 
                    listRelease(copy);
                    return NULL;
                }
            } else
                value = node->value;
            if (listAddNodeTail(copy, value) == NULL) { 
                listRelease(copy);
                return NULL;
            }
        }
        return copy;
    }
    
    /* 寻找关键字key所在链表 * 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;
    
        listRewind(list, &iter);
        while((node = listNext(&iter)) != NULL) { 
            if (list->match) { 
                if (list->match(node->value, key)) { 
                    return node;
                }
            } else { 
                if (key == node->value) { 
                    return node;
                }
            }
        }
        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;
    }
    
    /* 双向链表的旋转十分简单：删除尾节点，插入头部即可 * 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;
    }
    
    /* 在链表l的尾部插入链表o * Add all the elements of the list 'o' at the end of the * list 'l'. The list 'other' remains empty but otherwise valid. */
    void listJoin(list *l, list *o) { 
        if (o->head)
            o->head->prev = l->tail;
    
        if (l->tail)
            l->tail->next = o->head;
        else
            l->head = o->head;
    
        if (o->tail) l->tail = o->tail;
        l->len += o->len;
    
        /* Setup other as an empty list. */
        o->head = o->tail = NULL;
        o->len = 0;
    }