redis的基本数据类型之一"list"的底层编码从"ziplist"和"LinkedList"[3.2版本之前]升级到"由ziplist组成的LinkedList"[3.2版本及以后]再升级到"由listpack组成的LinkedList"[7.2.2版本]。

本篇文章介绍"LinkedList"
更多有关链表的知识可以参考博客"数组链表专题"

adlist

adlist.h - A generic doubly linked list implementation

adlist 是一个通用的双向链表

listNode

首先看一下链表节点是如何定义的

typedef struct listNode {
    struct listNode *prev;//指向前一个节点的指针
    struct listNode *next;//指向后一个节点的指针
    void *value;
} listNode;

list

链表的定义

typedef struct list {
    listNode *head;//头节点
    listNode *tail;//尾节点
    /*
    以下是一些指向"实现特定功能的函数"的指针
    */
    void *(*dup)(void *ptr);//dup指向传入参数类型和返回值类型都是"void*"的函数
    void (*free)(void *ptr);//dup指向传入参数类型和返回值类型都是"void*"的函数
    int (*match)(void *ptr, void *key);//dup指向传入参数类型是两个"void *"和返回值类型都是"int*"的函数
    unsigned long len;//记录list中的元素个数
} list;

• 获取list的元素个数的时间复杂度为O(1)因为链表的定义中包含记录节点个数的字段len

macros[宏定义]

来看看都定义了哪些宏方法。

//获取长度，由此可以看出对于OBJ_ENCODING_LINKEDLIST 编码方式的list来说，获取长度的时间复杂度为O(1)
#define listLength(l) ((l)->len)
//获取头节点
#define listFirst(l) ((l)->head)
//获取尾节点
#define listLast(l) ((l)->tail)
//获取当前节点的前一个节点
#define listPrevNode(n) ((n)->prev)
//获取当前节点的下一个节点
#define listNextNode(n) ((n)->next)
//获取节点指向的value值
#define listNodeValue(n) ((n)->value)

listCreate

//创建一个list
list *listCreate(void)
{
    struct list *list;
	//分配内存失败
    if ((list = zmalloc(sizeof(*list))) == NULL)
        return NULL;
    //分配内存成功
    //初始化
    list->head = list->tail = NULL;
    list->len = 0;
    list->dup = NULL;
    list->free = NULL;
    list->match = NULL;
    return list;
}

listInitNode

//初始化链表节点，将前后指针置为空，value值置为对应的value值
void listInitNode(listNode *node, void *value) {
    node->prev = NULL;
    node->next = NULL;
    node->value = value;
}

listEmpty

//清空list，只是将节点从链表中移除而不销毁链表
/* Remove all the elements from the list without destroying the list itself. */
void listEmpty(list *list)
{
    unsigned long len;
    listNode *current, *next;

    current = list->head;
    len = list->len;
    //逐一释放每个节点
    while(len--) {
    	//先保存下一个节点
        next = current->next;
        //释放当前节点value指向的内存
        if (list->free) list->free(current->value);
        //释放当前节点
        zfree(current);
        current = next;
    }
    //头尾节点设置为NULL，len设置为0
    list->head = list->tail = NULL;
    list->len = 0;
}

listRelease

/* Free the whole list.
 *
 * This function can't fail. */
void listRelease(list *list)
{
    listEmpty(list);
    //释放list本身
    zfree(list);
}

listAddNodeHead

list *listAddNodeHead(list *list, void *value)
{
    listNode *node;
    //为新结点分配内存
    if ((node = zmalloc(sizeof(*node))) == NULL)
        return NULL;
    node->value = value;
    //从头部插入list
    listLinkNodeHead(list, node);
    return list;
}

listLinkNodeHead

void listLinkNodeHead(list* list, listNode *node) {
    if (list->len == 0) {
    //第一次插入节点
        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;
    }
    //元素个数加1
    list->len++;
}

listAddNodeTail

list *listAddNodeTail(list *list, void *value)
{
    listNode *node;
	//为新节点分配内存
    if ((node = zmalloc(sizeof(*node))) == NULL)
        return NULL;
    node->value = value;
    listLinkNodeTail(list, node);
    return list;
}

listLinkNodeTail

void listLinkNodeTail(list *list, listNode *node) {
    if (list->len == 0) {
    //第一次插入节点
        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;
    }
    //元素个数加1
    list->len++;
}

listInsertNode

//如果after为1则在old_node之后插入新节点
//反之在old_node之前插入新节点

list *listInsertNode(list *list, listNode *old_node, void *value, int after) {
    listNode *node;
    //为node分配内存
    if ((node = zmalloc(sizeof(*node))) == NULL)
        return NULL;
    node->value = value;
    //在old_node之后插入新节点
    if (after) {
        node->prev = old_node;
        node->next = old_node->next;
        //如果old_node是尾节点，插入之后要修改tail
        if (list->tail == old_node) {
            list->tail = node;
        }
    } else {
    //在old_node之前插入新节点
        node->next = old_node;
        node->prev = old_node->prev;
        //如果old_node是头节点，插入之后要修改head
        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;
}

listDelNode

void listDelNode(list *list, listNode *node)
{
    listUnlinkNode(list, node);
    //移除node并释放内存
    if (list->free) list->free(node->value);
    zfree(node);
}

listUlinkNode

//从list中移除node
void listUnlinkNode(list *list, listNode *node) {
	//node->prev不为空说明node不是第一个元素
    if (node->prev)
        node->prev->next = node->next;
    else
    	//node是第一个元素，删除之后需要修改head节点
        list->head = node->next;
    //node不是最后一个元素
    if (node->next)
        node->next->prev = node->prev;
    else
    //node是最后一个元素，删除之后需要修改tail节点
        list->tail = node->prev;
	
	//从list中移除node
    node->next = NULL;
    node->prev = NULL;
	//元素个数减1
    list->len--;
}

listIndex

/*
寻找list中第index个元素，正序遍历index从0开始，0代表head
倒序遍历从-1开始，-1指向tail
*/
listNode *listIndex(list *list, long index) {
    listNode *n;
	//如果index<0先将其转换为正数
    if (index < 0) {
    	//这里减一是因为，从尾节点找到第一个元素无需移动只需返回尾节点即可；找到第二个元素只需从尾节点向前移动一次即可
        index = (-index)-1;
        n = list->tail;
        while(index-- && n) n = n->prev;
    } else {
        n = list->head;
        while(index-- && n) n = n->next;
    }
    return n;
}

redis3.2.100quicklist

在redis3.2.100版本中quicklist是由“ziplist”组成的"linkedlist"

quicklist.c - A doubly linked list of ziplists

看一下它的结构

/* quicklistNode is a 32 byte struct describing a ziplist for a quicklist.
 * We use bit fields keep the quicklistNode at 32 bytes.
 * count: 16 bits, max 65536 (max zl bytes is 65k, so max count actually < 32k).
 * encoding: 2 bits, RAW=1, LZF=2.
 * container: 2 bits, NONE=1, ZIPLIST=2.
 * recompress: 1 bit, bool, true if node is temporarry decompressed for usage.
 * attempted_compress: 1 bit, boolean, used for verifying during testing.
 * extra: 12 bits, free for future use; pads out the remainder of 32 bits */
/*
quicklistnode是“由ziplist组成的linkedlist”的节点，占有32bytes
*/
typedef struct quicklistNode {
    struct quicklistNode *prev;
    struct quicklistNode *next;
    unsigned char *zl;
    unsigned int sz;             /* ziplist size in bytes ziplist占用的bytes*/
    unsigned int count : 16;     /* count of items in ziplist  ziplist里的entry数量*/
    unsigned int encoding : 2;   /* RAW==1 or LZF==2 */
    unsigned int container : 2;  /* NONE==1 or ZIPLIST==2 */
    unsigned int recompress : 1; /* was this node previous compressed? */
    unsigned int attempted_compress : 1; /* node can't compress; too small */
    unsigned int extra : 10; /* more bits to steal for future usage */
} quicklistNode;

/* quicklist is a 32 byte struct (on 64-bit systems) describing a quicklist.
 * 'count' is the number of total entries.
 * 'len' is the number of quicklist nodes.
 * 'compress' is: -1 if compression disabled, otherwise it's the number
 *                of quicklistNodes to leave uncompressed at ends of quicklist.
 * 'fill' is the user-requested (or default) fill factor. */
typedef struct quicklist {
    quicklistNode *head;
    quicklistNode *tail;
    unsigned long count;        /* total count of all entries in all ziplists 所有ziplist中的所有entry总和，也就是linkedlist中总的元素个数 */
    unsigned int len;           /* number of quicklistNodes  quicklistnode的个数*/
    int fill : 16;              /* fill factor for individual nodes */
    unsigned int compress : 16; /* depth of end nodes not to compress;0=off */
} quicklist;

redis7.2.2quicklist

在7.2.2版本中quicklist是由“listpack组成的”

quicklist.c - A doubly linked list of listpacks

看一下它的结构

/* quicklistNode is a 32 byte struct describing a listpack for a quicklist.
 * We use bit fields keep the quicklistNode at 32 bytes.
 * count: 16 bits, max 65536 (max lp bytes is 65k, so max count actually < 32k).
 * encoding: 2 bits, RAW=1, LZF=2.
 * container: 2 bits, PLAIN=1 (a single item as char array), PACKED=2 (listpack with multiple items).
 * recompress: 1 bit, bool, true if node is temporary decompressed for usage.
 * attempted_compress: 1 bit, boolean, used for verifying during testing.
 * extra: 10 bits, free for future use; pads out the remainder of 32 bits */
typedef struct quicklistNode {
    struct quicklistNode *prev;
    struct quicklistNode *next;
    unsigned char *entry;
    size_t sz;             /* entry size in bytes  listpack占用bytes*/
    unsigned int count : 16;     /* count of items in listpack  listpack中的元素个数*/
    unsigned int encoding : 2;   /* RAW==1 or LZF==2 */
    unsigned int container : 2;  /* PLAIN==1 or PACKED==2 */
    unsigned int recompress : 1; /* was this node previous compressed? */
    unsigned int attempted_compress : 1; /* node can't compress; too small */
    unsigned int dont_compress : 1; /* prevent compression of entry that will be used later */
    unsigned int extra : 9; /* more bits to steal for future usage */
} quicklistNode;

/* quicklist is a 40 byte struct (on 64-bit systems) describing a quicklist.
 * 'count' is the number of total entries.
 * 'len' is the number of quicklist nodes.
 * 'compress' is: 0 if compression disabled, otherwise it's the number
 *                of quicklistNodes to leave uncompressed at ends of quicklist.
 * 'fill' is the user-requested (or default) fill factor.
 * 'bookmarks are an optional feature that is used by realloc this struct,
 *      so that they don't consume memory when not used. */
typedef struct quicklist {
    quicklistNode *head;
    quicklistNode *tail;
    unsigned long count;        /* total count of all entries in all listpacks  linkedlist的元素总数*/
    unsigned long len;          /* number of quicklistNodes quicklistnode的数量 */
    signed int fill : QL_FILL_BITS;       /* fill factor for individual nodes */
    unsigned int compress : QL_COMP_BITS; /* depth of end nodes not to compress;0=off */
    unsigned int bookmark_count: QL_BM_BITS;
    quicklistBookmark bookmarks[];
} quicklist;

总结

• 不管是ziplist还是listpack构成的quicklist获取元素总数的时间复杂度为都为O(1)。