nginx advanced data structure source code analysis (1) ----- doubly linked list

typedef struct ngx_queue_s  ngx_queue_t;

struct ngx_queue_s {
    ngx_queue_t  *prev;
    ngx_queue_t  *next;
};


#define ngx_queue_init(q)                                                     \初始化，为空，都指向容器结构体
    (q)->prev = q;                                                            \
    (q)->next = q


#define ngx_queue_empty(h)                                                    \是否为空
    (h == (h)->prev)


#define ngx_queue_insert_head(h, x)                                           \插入链表容器头部
    (x)->next = (h)->next;                                                    \
    (x)->next->prev = x;                                                      \
    (x)->prev = h;                                                            \
    (h)->next = x


#define ngx_queue_insert_after   ngx_queue_insert_head


#define ngx_queue_insert_tail(h, x)                                           \插入链表容器尾部
    (x)->prev = (h)->prev;                                                    \
    (x)->prev->next = x;                                                      \
    (x)->next = h;                                                            \
    (h)->prev = x


#define ngx_queue_head(h)                                                     \返回第一个结构体指针
    (h)->next


#define ngx_queue_last(h)                                                     \返回最后一个结构体指针
    (h)->prev


#define ngx_queue_sentinel(h)                                                 \返回容器结构体指针
    (h)


#define ngx_queue_next(q)                                                     \返回q的下一个元素
    (q)->next


#define ngx_queue_prev(q)                                                     \返回q的上一个元素
    (q)->prev


#if (NGX_DEBUG)

#define ngx_queue_remove(x)                                                   \
    (x)->next->prev = (x)->prev;                                              \
    (x)->prev->next = (x)->next;                                              \
    (x)->prev = NULL;                                                         \
    (x)->next = NULL

#else

#define ngx_queue_remove(x)                                                   \删除x
    (x)->next->prev = (x)->prev;                                              \
    (x)->prev->next = (x)->next

#endif


#define ngx_queue_split(h, q, n)                                              \拆分成两个链表
    (n)->prev = (h)->prev;                                                    \
    (n)->prev->next = n;                                                      \
    (n)->next = q;                                                            \
    (h)->prev = (q)->prev;                                                    \
    (h)->prev->next = h;                                                      \
    (q)->prev = n;


#define ngx_queue_add(h, n)                                                   \合并链表
    (h)->prev->next = (n)->next;                                              \
    (n)->next->prev = (h)->prev;                                              \
    (h)->prev = (n)->prev;                                                    \
    (h)->prev->next = h;


#define ngx_queue_data(q, type, link)                                         \返回q所属结构体地址
    (type *) ((u_char *) q - offsetof(type, link))

ngx_queue_t *
ngx_queue_middle(ngx_queue_t *queue)                       //返回链表中心元素
{
    ngx_queue_t  *middle, *next;

    middle = ngx_queue_head(queue);

    if (middle == ngx_queue_last(queue)) {               //特殊情况也要单独判断，如果只有一个，则返回这个元素
        return middle;
    }

    next = ngx_queue_head(queue);

    for ( ;; ) {
        middle = ngx_queue_next(middle);                 //一个指针走一步，另外一个指针走两步

        next = ngx_queue_next(next);

        if (next == ngx_queue_last(queue)) {              //next每走一步都要判断一下的
            return middle;
        }

        next = ngx_queue_next(next);

        if (next == ngx_queue_last(queue)) {               //这也要判断，考虑真全面啊
            return middle;
        }
    }
}


/* the stable insertion sort */

void
ngx_queue_sort(ngx_queue_t *queue,
    ngx_int_t (*cmp)(const ngx_queue_t *, const ngx_queue_t *))   //用插入法排序
{
    ngx_queue_t  *q, *prev, *next;

    q = ngx_queue_head(queue);

    if (q == ngx_queue_last(queue)) {    //只有一个节点就不用排了
        return;
    }

    for (q = ngx_queue_next(q); q != ngx_queue_sentinel(queue); q = next) {

        prev = ngx_queue_prev(q);  //分别保存q的前后节点
        next = ngx_queue_next(q);

        ngx_queue_remove(q);

        do {
            if (cmp(prev, q) <= 0) {  //若prev小于，这个循环结束
                break;
            }

            prev = ngx_queue_prev(prev);//否则跟前面一个元素比

        } while (prev != ngx_queue_sentinel(queue));//这个为结束条件，prev为链表结构体结束

        ngx_queue_insert_after(prev, q);//插入q
    }
}