Redis的列表对象底层所使用的数据结构其中之一就是list。
list
Redis的list是一个双端链表，其由3部分构成：链表节点、链表迭代器、链表。这一设计思想和STL的list是一样的，STL的list也是由这三部分组成。需要特别说明的是Redis用C语言实现了list的迭代器，比较巧妙，下面就来分析list源码。
list节点
节点的值为void*类型，从而可以保存不同类型的值，甚至是另一种类型的对象。
// 双端链表的节点
typedef struct listNode {
struct listNode *prev; // 指向上一个节点
struct listNode *next; // 指向下一个节点
void *value; // 指向节点的值, void*类型，使得节点可以保存不同类型的值
} listNode;
list迭代器
c语言实现c++中的迭代器；双端链表的迭代器，方便了遍历链表的操作；根据direction，可设置为前向/反向迭代器
typedef struct listIter {
listNode *next; // 指向迭代器方向上下一个链表结点
int direction; // AL_START_HEAD=0：从头部往尾部方向移动；AL_START_TAIL=1：往尾部往头部方向移动
} listIter;
其中direction的取值有：
/* Directions for iterators */
// 迭代器方向的宏定义
#define AL_START_HEAD 0
#define AL_START_TAIL 1
list
与一般设计类似，list中有指向头尾节点的指针，以及链表节点数量的计数。不同的是，由于链表节点为void*类型，被设计为可以存储不同类型的数据，甚至是另一种类型的对象，所以添加了与节点相关的3个函数，作用分别是复制、释放、比较节点的值。
// 双端链表
typedef struct list {
listNode *head; // 指向链表头节点
listNode *tail; // 指向链表尾节点
void *(*dup)(void *ptr); // 复制链表节点所保存的值
void (*free)(void *ptr); // 释放链表节点所保存的值
int (*match)(void *ptr, void *key); // 节点值比较函数
unsigned long len; // 链表的节点数目
} list;
list的操作函数
Redis用宏定义实现了一些复杂度为O(1)的链表操作，以提高list操作的效率。
/* Functions implemented as macros */
// 通过宏来实现一些O(1)时间复杂度的函数
#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)
list的源码比较好理解，本人对其已经做了详细的注释，就不仔细介绍了，下面附上源码及注释。list相关的文件有两个：adlist.h, adlist.c
adlist.h
#ifndef __ADLIST_H__
#define __ADLIST_H__ /* Node, List, and Iterator are the only data structures used currently. */ // redis的链表为双端链表
// 节点的值为void*类型，从而可以保存不同类型的值
// 结合dup,free,match函数实现链表的多态 // 双端链表的节点
typedef struct listNode {
struct listNode *prev; // 指向上一个节点
struct listNode *next; // 指向下一个节点
void *value; // 指向节点的值, void*类型，使得节点可以保存不同类型的值
} listNode; // c语言实现c++中的迭代器！！！
// 双端链表的迭代器，方便了遍历链表的操作
// 根据direction，可设置为前向/反向迭代器
typedef struct listIter {
listNode *next; // 指向迭代器方向上下一个链表结点
int direction; // AL_START_HEAD=0：从头部往尾部方向移动；AL_START_TAIL=1：往尾部往头部方向移动
} listIter; // 双端链表
typedef struct list {
listNode *head; // 指向链表头节点
listNode *tail; // 指向链表尾节点
void *(*dup)(void *ptr); // 复制链表节点所保存的值
void (*free)(void *ptr); // 释放链表节点所保存的值
int (*match)(void *ptr, void *key); // 节点值比较函数
unsigned long len; // 链表的节点数目
} list; /* Functions implemented as macros */
// 通过宏来实现一些O(1)时间复杂度的函数
#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) /* Prototypes */
// list数据结构相关的函数
// 具体含义见adlist.c
list *listCreate(void);
void listRelease(list *list);
list *listAddNodeHead(list *list, void *value);
list *listAddNodeTail(list *list, void *value);
list *listInsertNode(list *list, listNode *old_node, void *value, int after);
void listDelNode(list *list, listNode *node);
listIter *listGetIterator(list *list, int direction);
listNode *listNext(listIter *iter);
void listReleaseIterator(listIter *iter);
list *listDup(list *orig);
listNode *listSearchKey(list *list, void *key);
listNode *listIndex(list *list, long index);
void listRewind(list *list, listIter *li);
void listRewindTail(list *list, listIter *li);
void listRotate(list *list); /* Directions for iterators */
// 迭代器方向的宏定义
#define AL_START_HEAD 0
#define AL_START_TAIL 1 #endif /* __ADLIST_H__ */
adlist.c
/* adlist.c - A generic doubly linked list implementation
*/ #include <stdlib.h>
#include "adlist.h"
#include "zmalloc.h" /* 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. */ // 创建一个链表
// 返回值：list/NULL
list *listCreate(void)
{
struct list *list; if ((list = zmalloc(sizeof(*list))) == NULL) // 为链表分配内存
return NULL;
// 初始化链表结构体的成员
list->head = list->tail = NULL;
list->len = ;
list->dup = NULL;
list->free = NULL;
list->match = NULL;
return list; // 返回为新链表分配的内存的起始地址
} /* Free the whole list.
*
* This function can't fail. */ // 释放链表及链表节点
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); // 释放链表节点的值
zfree(current); // 释放链表节点
current = next;
}
zfree(list); // 释放链表
} /* 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. */ // 从双端链表的头部插入新节点
// 返回值：list/NULL
list *listAddNodeHead(list *list, void *value)
{
listNode *node; if ((node = zmalloc(sizeof(*node))) == NULL)
return NULL;
node->value = value;
if (list->len == ) { // 原链表为一空链表
list->head = list->tail = node;
node->prev = node->next = NULL;
} else {
// 插入到双端链表的头结点之前
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. */ // 从双端链表的尾部插入新节点
// 返回值：list/NULL
list *listAddNodeTail(list *list, void *value)
{
listNode *node; if ((node = zmalloc(sizeof(*node))) == NULL)
return NULL;
node->value = value;
if (list->len == ) {
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;
} // 在链表list的节点old_node的前或后插入新节点
// after为0，则在old_node之前插入；否则，在old_node之后插入
// 返回值：list/NULL
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) { // old_node之后插入
node->prev = old_node;
node->next = old_node->next;
if (list->tail == old_node) {
list->tail = node;
}
} else { // old_node之前插入
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. */ // 删除链表list中节点node
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. */ // 返回链表的迭代器
// 返回值：list/NULL
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; // 设置为前向迭代器
else
iter->next = list->tail; // 设置为反向迭代器
iter->direction = direction;
return iter;
} /* Release the iterator memory */ // 释放迭代器的内存
void listReleaseIterator(listIter *iter) {
zfree(iter);
} /* Create an iterator in the list private iterator structure */ // 回绕迭代器到链表头部
void listRewind(list *list, listIter *li) {
li->next = list->head;
li->direction = AL_START_HEAD;
} // 回绕迭代器到链表尾部
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));
* }
*
* */ // 返回迭代器所指向的元素，并将迭代器往其方向上移动一步
// 返回值：指向当前节点的指针/NULL
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. */ // 复制输入链表
// list*/NULL
list *listDup(list *orig)
{
list *copy;
listIter iter;
listNode *node; if ((copy = listCreate()) == NULL) // 创建新链表
return NULL;
copy->dup = orig->dup;
copy->free = orig->free;
copy->match = orig->match;
listRewind(orig, &iter); // 回绕迭代器到链表头部
while((node = listNext(&iter)) != NULL) { // 遍历原链表，顺序取出节点
void *value; if (copy->dup) {
value = copy->dup(node->value); // 通过list.dup函数复制节点值
if (value == NULL) {
listRelease(copy); // 出错释放链表
return NULL;
}
} else
value = node->value;
if (listAddNodeTail(copy, value) == NULL) { // 从新链表尾部插入值
listRelease(copy); // 出错释放链表
return NULL;
}
}
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. */ // 返回链表中节点值与key相匹配的节点
// listNode*/NULL
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)) { // 调用list.match函数对节点值进行比较
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. */ // 返回给定索引位置的节点
// index=0，返回头结点
// index < 0，则从尾部开始返回，index = -1，返回尾部节点
listNode *listIndex(list *list, long index) {
listNode *n; if (index < ) {
index = (-index)-;
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. */ // 将尾部节点弹出，插入到链表头节点之前，成为新的表头节点
void listRotate(list *list) {
listNode *tail = list->tail; if (listLength(list) <= ) 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;
}
（全文完）