Nginx的雙向鏈表(ngx_queue_t)

        Nginx中的雙向鏈表(ngx_queue_t)可以高效的執行插入、刪除、合併等操作，在移動鏈表中的元素時只需要修改指針的指向，適合頻繁修改容器的場合。並且它還支持排序和鏈表間的合併功能。

源代碼

ngx_queue.h文件

#include <ngx_config.h>
#include <ngx_core.h>


#ifndef _NGX_QUEUE_H_INCLUDED_
#define _NGX_QUEUE_H_INCLUDED_


typedef struct ngx_queue_s  ngx_queue_t;

/// nginx中的雙向鏈表
struct ngx_queue_s {
    ngx_queue_t  *prev;
    ngx_queue_t  *next;
};

/// q的prev和next都指向自己，是一個空鏈表
#define ngx_queue_init(q)                                                     \
    (q)->prev = q;                                                            \
    (q)->next = q


#define ngx_queue_empty(h)                                                    \
    (h == (h)->prev)

// 在頭部插入一個節點x
#define ngx_queue_insert_head(h, x)                                           \
    (x)->next = (h)->next;                                                    \
    (x)->next->prev = x;                                                      \
    (x)->prev = h;                                                            \
    (h)->next = x


#define ngx_queue_insert_after   ngx_queue_insert_head

// 在尾部插入一個節點x
#define ngx_queue_insert_tail(h, x)                                           \
    (x)->prev = (h)->prev;                                                    \
    (x)->prev->next = x;                                                      \
    (x)->next = h;                                                            \
    (h)->prev = x


#define ngx_queue_head(h)                                                     \
    (h)->next


#define ngx_queue_last(h)                                                     \
    (h)->prev


#define ngx_queue_sentinel(h)                                                 \
    (h)


#define ngx_queue_next(q)                                                     \
    (q)->next


#define ngx_queue_prev(q)                                                     \
    (q)->prev


#if (NGX_DEBUG)
// 	刪除一個節點，不會釋放該節點內存
#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)                                                   \
    (x)->next->prev = (x)->prev;                                              \
    (x)->prev->next = (x)->next

#endif


#define ngx_queue_split(h, q, n)                                              \
    (n)->prev = (h)->prev;                                                    \
    (n)->prev->next = n;                                                      \
    (n)->next = q;                                                            \
    (h)->prev = (q)->prev;                                                    \
    (h)->prev->next = h;                                                      \
    (q)->prev = n;

// 合併鏈表，將n鏈表添加到h鏈表的末尾
#define ngx_queue_add(h, n)                                                   \
    (h)->prev->next = (n)->next;                                              \
    (n)->next->prev = (h)->prev;                                              \
    (h)->prev = (n)->prev;                                                    \
    (h)->prev->next = h;

// 返回q元素所屬類型結構體的地址
#define ngx_queue_data(q, type, link)                                         \
    (type *) ((u_char *) q - offsetof(type, link))


ngx_queue_t *ngx_queue_middle(ngx_queue_t *queue);
void ngx_queue_sort(ngx_queue_t *queue,
    ngx_int_t (*cmp)(const ngx_queue_t *, const ngx_queue_t *));


#endif /* _NGX_QUEUE_H_INCLUDED_ */

ngx_queue.c文件

#include <ngx_config.h>
#include <ngx_core.h>


/*
 * find the middle queue element if the queue has odd number of elements
 * or the first element of the queue's second part otherwise
 * 返回鏈表的中心節點，思想爲兩個節點，一個節點一次移動一步，另一個節點一次移動2次，
 * 當一次移動2次的節點到達鏈表尾部時，一次移動一步的節點正好是中間結點
 */

ngx_queue_t *
ngx_queue_middle(ngx_queue_t *queue)
{
    ngx_queue_t  *middle, *next;

    middle = ngx_queue_head(queue);

    if (middle == ngx_queue_last(queue)) {
        return middle;
    }

    next = ngx_queue_head(queue);

    for ( ;; ) {
        middle = ngx_queue_next(middle);

        next = ngx_queue_next(next);

        if (next == ngx_queue_last(queue)) {
            return middle;
        }

        next = ngx_queue_next(next);

        if (next == ngx_queue_last(queue)) {
            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 *))
{
    ngx_queue_t  *q, *prev, *next;

	// queue爲循環隊列
    q = ngx_queue_head(queue);

    if (q == ngx_queue_last(queue)) {
        return;
    }

	// 該隊列排序的思想 類似於插入排序
    for (q = ngx_queue_next(q); q != ngx_queue_sentinel(queue); q = next) {

        prev = ngx_queue_prev(q);
        next = ngx_queue_next(q);

        ngx_queue_remove(q);

        do {
            if (cmp(prev, q) <= 0) {
                break;
            }

            prev = ngx_queue_prev(prev);

        } while (prev != ngx_queue_sentinel(queue));

        ngx_queue_insert_after(prev, q);
    }
}