redis源码分析 -- 链表结构

redis源码分析 - 链表结构

redis 中链表结构如下

typedef struct listNode {
struct listNode *prev;  //指向前驱指针
struct listNode *next;  //指向后继指针
void *value;
} listNode; //节点

typedef struct list {
listNode *head;     //头结点
listNode *tail;     //尾节点
void *(*dup)(void *ptr);
void (*free)(void *ptr);
int (*match)(void *ptr, void *key);
unsigned long len;
} list;     //链表

redis 的链表为双向链表，每一个链表结点同时具有一个指向前驱和指向其后继节点的指针，多个节点就组成了一个双向链表。

list 结构为链表提供了头指针 head 和尾指针 tail，以及链表长度计数器 len，free 和 match 则用于实现多态链表所需的类型特定函数:

dup 用于复制节点所保存的值

free 用于释放节点所保存的值

match 用于比对节点所保存的值与另一个输入值是否相等

创建链表

/* 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;
}

创建链表，链表没有节点，头尾指针均指向

NULL

从链表头或者链表尾插入新节点

/* 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) {   //链表为空
list->head = list->tail = node;
node->prev = node->next = NULL;
} else {    //从头插入，头节点的 prev 指向 NULL
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 {    //从尾插入，尾指针的 next 指向 NULL
node->prev = list->tail;
node->next = NULL;
list->tail->next = node;
list->tail = node;
}
list->len++;
return list;
}

在指定节点处添加新节点

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) {    //在 old_node 节点后插入节点
node->prev = old_node;
node->next = old_node->next;
if (list->tail == old_node) {
list->tail = node;
}
} else {    //在old_node节点前插入节点
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;
}

旋转链表，将链表尾节点插入到头部

/* 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 */
// 将尾节点分离出来
list->tail = tail->prev;
list->tail->next = NULL;
/* Move it as head */
// 在头部插入尾节点
list->head->prev = tail;
tail->prev = NULL;
tail->next = list->head;
list->head = tail;
}

释放链表

/* 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);
}

redis 双向链表的特性：

链表节点带有 prev 和 next 指针，获取某节点前驱或者后继节点的时间复杂度为 O(1)

无环：链表头节点的 prev 指针和尾节点的 next 指针均指向 NULL，即从头遍历或者从尾遍历都是以 NULL 结束链表遍历，不会出现环。

头尾指针使得获取链表头尾指针的时间复杂度为 O(1)

list 的 长度计数器 len，使得获取链表长度的时间复杂度为 O(1)

链表结点通过

void*

来保存节点值，可以接受任何类型的数据，同时，free，match 和 dup 三个属性为节点值设置类型特定函数，使得链表可以用于保存各种不同类型的值。