Redis源码-数据结构之Adlist双端链表

Redis的Adlist实现了数据结构中的双端链表，整个结构如下：

链表节点定义：

typedef struct listNode {
    struct listNode *prev;
    struct listNode *next;
    void *value;
} listNode;

链表定义：

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;

其中的三个函数指针先不用管，后面遇到了再看具体是干什么的，另外还实现了一个迭代器，有点c++的味道在里面

typedef struct listIter {
    listNode *next;
    int direction;
} listIter;

链表三要素，创建，插入，和删除

list *listCreate(void)
{
    struct list *list;

    if ((list = malloc(sizeof(*list))) == NULL)
        return NULL;
    list->head = list->tail = NULL;
    list->len = 0;
    list->dup = NULL;
    list->free = NULL;
    list->match = NULL;
    return list;
}

插入分为从头部插入和尾部插入，源代码实现头部都有非常清晰的注释，告诉这个函数的一些细节，作者很是用心：

list *listAddNodeHead(list *list, void *value)
{
    listNode *node;

    if ((node = malloc(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;
}

释放内存

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);
        free(current);
        current = next;
    }
    free(list);
}

迭代器的创建，以后可以效仿这种做法，迭代器分方向：

/* 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 = malloc(sizeof(*iter))) == NULL) return NULL;
    if (direction == AL_START_HEAD)
        iter->next = list->head;
    else
        iter->next = list->tail;
    iter->direction = direction;
    return iter;
}