redis 5.0.7 源码阅读——字典dict

redis中字典相关的文件为：dict.h与dict.c

与其说是一个字典，道不如说是一个哈希表。

一、数据结构

dictEntry

 1 typedef struct dictEntry {
 2     void *key;
 3     union {
 4         void *val;
 5         uint64_t u64;
 6         int64_t s64;
 7         double d;
 8     } v;
 9     struct dictEntry *next;
10 } dictEntry;

dictEntry是一个kv对的单向链表，其中v是一个联合体，支持数字，或者是指向一块内存的指针。

 1 /*
 2 +---------------+
 3 |void *key      |
 4 +---------------+
 5 |union{...} v   |
 6 +---------------+
 7 |dictEntry *next|---+
 8 +---------------+   |
 9                     |
10 +---------------+ <-+
11 |void *key      |
12 +---------------+
13 |union{...} v   |
14 +---------------+
15 |dictEntry *next|
16 +---------------+
17 */

为了节约篇幅，后续用以下结构表示：

1 /*
2 +---+  +---+
3 |K|V|->|K|V|->NULL
4 +---+  +---+
5 */

dictht

 1 typedef struct dictht {
 2     dictEntry **table;
 3     unsigned long size;
 4     /*
 5         这样写可能更容易理解
 6         const unsigned long size = 4;
 7         dictEntry *table[size];
 8     */
 9     
10 
11     unsigned long sizemask;
12     //sizemask，始终为size-1
13 
14     unsigned long used;
15     //当前总dictEntry数量
16 } dictht;

dictht是一个hash table，整体结构大致为：

 1 /*
 2 +----------------------+   +---> +-----------------+  +---+
 3 |dictEntry **table     |---+     |dictEntry *bucket|->|K|V|->NULL
 4 +----------------------+         +-----------------+  +---+
 5 |unsigned long size = 4|         |dictEntry *bucket|->NULL
 6 +----------------------+         +-----------------+
 7 |unsigned long sizemask|         |dictEntry *bucket|->NULL
 8 +----------------------+         +-----------------+
 9 |unsigned long used    |         |dictEntry *bucket|->NULL
10 +----------------------+         +-----------------+ 
11 */

其中，table指向大小为sizeof(dictEntry*) * size的一片内存空间，每个dictEntry*可以视为一个bucket，每个bucket下挂着一个dictEntry单向链表。

size的值始终为2的位数，而sizemask的值始终为size-1，其作用是决定kv对要挂在哪个bucket上。

举个例子，size=4时，sizemask=3，其二进制为 0011，若通过hash函数计算出来key对应的hash值hash_value为5，二进制为0101，则通过位运算 sizemask & hash_value = 0011 & 0101 = 0001，十进制为1，则将会挂在idx = 1的bucket上。

dictType

1 typedef struct dictType {
2     uint64_t (*hashFunction)(const void *key);
3     void *(*keyDup)(void *privdata, const void *key);
4     void *(*valDup)(void *privdata, const void *obj);
5     int (*keyCompare)(void *privdata, const void *key1, const void *key2);
6     void (*keyDestructor)(void *privdata, void *key);
7     void (*valDestructor)(void *privdata, void *obj);
8 } dictType;

dictType用于自定义一些操作的方法，如拷贝key、拷贝value、销毁key、销毁value，比较key，以及hash函数。

dict

1 typedef struct dict {
2     dictType *type;
3     void *privdata;
4     dictht ht[2];
5     long rehashidx; /* rehashing not in progress if rehashidx == -1 */
6     unsigned long iterators; /* number of iterators currently running */
7 } dict;

之前提到的dictType与dictht都是dict的成员变量。除此之外，还有privdata，是在创建dict的时候调用者传入，用于特定操作时回传给函数的。如：

 1 #define dictFreeVal(d, entry) \
 2     if ((d)->type->valDestructor) \
 3         (d)->type->valDestructor((d)->privdata, (entry)->v.val)
 4 
 5 #define dictSetVal(d, entry, _val_) do { \
 6     if ((d)->type->valDup) \
 7         (entry)->v.val = (d)->type->valDup((d)->privdata, _val_); \
 8     else \
 9         (entry)->v.val = (_val_); \
10 } while(0)
11 
12 #define dictSetSignedIntegerVal(entry, _val_) \
13     do { (entry)->v.s64 = _val_; } while(0)
14 
15 #define dictSetUnsignedIntegerVal(entry, _val_) \
16     do { (entry)->v.u64 = _val_; } while(0)
17 
18 #define dictSetDoubleVal(entry, _val_) \
19     do { (entry)->v.d = _val_; } while(0)
20 
21 #define dictFreeKey(d, entry) \
22     if ((d)->type->keyDestructor) \
23         (d)->type->keyDestructor((d)->privdata, (entry)->key)
24 
25 #define dictSetKey(d, entry, _key_) do { \
26     if ((d)->type->keyDup) \
27         (entry)->key = (d)->type->keyDup((d)->privdata, _key_); \
28     else \
29         (entry)->key = (_key_); \
30 } while(0)
31 
32 #define dictCompareKeys(d, key1, key2) \
33     (((d)->type->keyCompare) ? \
34         (d)->type->keyCompare((d)->privdata, key1, key2) : \
35         (key1) == (key2))

rehashidx，是与ht[2]配合实现渐进式rehash操作的。若使用一步到位的方式，当key的数量非常大的时候，rehashing期间，是会卡死所有操作的。

iterators，是用于记录当前使用的安全迭代器数量，与rehashing操作有关。

整体结构如下：

 1 /*
 2 +---------+    /+-----------+   +-->+----------+  +---+
 3 |dictType*|   / |dictEntry**|---+   |dictEntry*|->|K|V|->NULL
 4 +---------+  /  +-----------+       +----------+  +---+
 5 |privdata | /   |size       |       |dictEntry*|->NULL
 6 +---------+/    +-----------+       +----------+
 7 |ht[2]    |     |sizemask   |       |dictEntry*|->NULL
 8 +---------+\    +-----------+       +----------+
 9 |rehashidx| \   |used       |       |dictEntry*|->NULL
10 +---------+  \  +-----------+       +----------+ 
11 |iterators|   \ 
12 +---------+    \+-----------+
13                 |dictEntry**|-->NULL
14                 +-----------+
15                 |size       |
16                 +-----------+
17                 |sizemask   |
18                 +-----------+
19                 |used       |
20                 +-----------+
21 */

二、创建

 1 static void _dictReset(dictht *ht)
 2 {
 3     ht->table = NULL;
 4     ht->size = 0;
 5     ht->sizemask = 0;
 6     ht->used = 0;
 7 }
 8 
 9 int _dictInit(dict *d, dictType *type,
10         void *privDataPtr)
11 {
12     _dictReset(&d->ht[0]);
13     _dictReset(&d->ht[1]);
14     d->type = type;
15     d->privdata = privDataPtr;
16     d->rehashidx = -1;
17     d->iterators = 0;
18     return DICT_OK;
19 }
20 
21 dict *dictCreate(dictType *type,
22         void *privDataPtr)
23 {
24     dict *d = zmalloc(sizeof(*d));
25 
26     _dictInit(d,type,privDataPtr);
27     return d;
28 }

可以调用dictCreate创建一个空的dict，它会分配好dict的空间，并初始化所有成员变量。在这里把privdata传入并保存。搜了一下整个redis源码的dictCreate调用，看到传入的值全为NULL。目前的理解暂时不清楚这个变量是什么时候赋值的。初始化后的dict结构如下：

 1 /*
 2 +------------+    /+-----------+   
 3 |dictType*   |   / |dictEntry**|-->NULL
 4 +------------+  /  +-----------+   
 5 |privdata    | /   |size=0     |   
 6 +------------+/    +-----------+   
 7 |ht[2]       |     |sizemask=0 |   
 8 +------------+\    +-----------+   
 9 |rehashidx=-1| \   |used=0     |   
10 +------------+  \  +-----------+   
11 |iterators=0 |   \ 
12 +------------+    \+-----------+
13                    |dictEntry**|-->NULL
14                    +-----------+
15                    |size=0     |
16                    +-----------+
17                    |sizemask=0 |
18                    +-----------+
19                    |used=0     |
20                    +-----------+
21 */

刚创建好的dict是存不了任何数据的，其两个hash table的size都为0。这里先说明一下resize操作：

 1 #define DICT_HT_INITIAL_SIZE     4
 2 
 3 static unsigned long _dictNextPower(unsigned long size)
 4 {
 5     unsigned long i = DICT_HT_INITIAL_SIZE;
 6 
 7     if (size >= LONG_MAX) return LONG_MAX + 1LU;
 8     while(1) {
 9         if (i >= size)
10             return i;
11         i *= 2;
12     }
13 }
14 
15 /* Expand or create the hash table */
16 int dictExpand(dict *d, unsigned long size)
17 {
18     /* the size is invalid if it is smaller than the number of
19      * elements already inside the hash table */
20     if (dictIsRehashing(d) || d->ht[0].used > size)
21         return DICT_ERR;
22 
23     dictht n; /* the new hash table */
24     unsigned long realsize = _dictNextPower(size);
25 
26     /* Rehashing to the same table size is not useful. */
27     if (realsize == d->ht[0].size) return DICT_ERR;
28 
29     /* Allocate the new hash table and initialize all pointers to NULL */
30     n.size = realsize;
31     n.sizemask = realsize-1;
32     n.table = zcalloc(realsize*sizeof(dictEntry*));
33     n.used = 0;
34 
35     /* Is this the first initialization? If so it's not really a rehashing
36      * we just set the first hash table so that it can accept keys. */
37     if (d->ht[0].table == NULL) {
38         d->ht[0] = n;
39         return DICT_OK;
40     }
41 
42     /* Prepare a second hash table for incremental rehashing */
43     d->ht[1] = n;
44     d->rehashidx = 0;
45     return DICT_OK;
46 }
47 
48 int dictResize(dict *d)
49 {
50     int minimal;
51 
52     if (!dict_can_resize || dictIsRehashing(d)) return DICT_ERR;
53     minimal = d->ht[0].used;
54     if (minimal < DICT_HT_INITIAL_SIZE)
55         minimal = DICT_HT_INITIAL_SIZE;
56     return dictExpand(d, minimal);
57 }

_dictNextPower用于获取当前要分配给hash table的size，得到的值一定是2的倍数，初始值为4。

dictExpand，从源码注释上看，它是为了扩容hash table，或者创建一个。它不允许与rehashing操作同时进行，也不能强制缩容。在使用_dictNextPower得到需要的size之后，它先是使用一个临时变量n去分配空间，然后进行判断，若ht[0].table的值为NULL，则认为是刚create出来的dict，直接把n赋值给ht[0]，否则给ht[1]，并开始rehashing操作。

dictResize操作就不用多说了。

三、rehashing操作

若有这样一个dict，假设K1、K2、K3、K4计算出来的hash值分别为0、5、2、7，使用sizemask计算出来的idx分别为0、1、2、3

 1 /*
 2                                                       +----+
 3                                                    +->|K1|V|->NULL
 4 +------------+    /+-----------+  +->+----------+ /   +----+
 5 |dictType*   |   / |dictEntry**|--+  |dictEntry*|/    +----+
 6 +------------+  /  +-----------+     +----------+ +-->|K2|V|->NULL
 7 |privdata    | /   |size=4     |     |dictEntry*|/    +----+
 8 +------------+/    +-----------+     +----------+
 9 |ht[2]       |     |sizemask=3 |     |dictEntry*|\    +----+
10 +------------+\    +-----------+     +----------+ +-->|K3|V|->NULL
11 |rehashidx=-1| \   |used=4     |     |dictEntry*|\    +----+
12 +------------+  \  +-----------+     +----------+ \   +----+
13 |iterators=0 |   \                                 +->|K4|V|->NULL
14 +------------+    \+-----------+                      +----+
15                    |dictEntry**|-->NULL
16                    +-----------+
17                    |size=0     |
18                    +-----------+
19                    |sizemask=0 |
20                    +-----------+
21                    |used=0     |
22                    +-----------+
23 */

 1 static int _dictExpandIfNeeded(dict *d)
 2 {
 3     /* Incremental rehashing already in progress. Return. */
 4     if (dictIsRehashing(d)) return DICT_OK;
 5 
 6     /* If the hash table is empty expand it to the initial size. */
 7     if (d->ht[0].size == 0) return dictExpand(d, DICT_HT_INITIAL_SIZE);
 8 
 9     /* If we reached the 1:1 ratio, and we are allowed to resize the hash
10      * table (global setting) or we should avoid it but the ratio between
11      * elements/buckets is over the "safe" threshold, we resize doubling
12      * the number of buckets. */
13     if (d->ht[0].used >= d->ht[0].size &&
14         (dict_can_resize ||
15          d->ht[0].used/d->ht[0].size > dict_force_resize_ratio))
16     {
17         return dictExpand(d, d->ht[0].used*2);
18     }
19     return DICT_OK;
20 }

通过函数_dictExpandIfNeeded，可知当used >= size且dict_can_resize == TRUE的时候，需要调用dictExpand进入rehashing状态。dict_can_resize默认为1

1 static int dict_can_resize = 1;
2 static unsigned int dict_force_resize_ratio = 5;

需要的size为当前used * 2，即为8。调用dictExpand之后的结构：

 1 /*
 2                                                        +----+
 3                                                     +->|K1|V|->NULL
 4                                    +->+----------+ /   +----+
 5                                    |  |dictEntry*|/    +----+
 6                                    |  +----------+ +-->|K2|V|->NULL
 7                                    |  |dictEntry*|/    +----+
 8  +------------+    /+-----------+  |  +----------+
 9  |dictType*   |   / |dictEntry**|--+  |dictEntry*|\    +----+
10  +------------+  /  +-----------+     +----------+ +-->|K3|V|->NULL
11  |privdata    | /   |size=4     |     |dictEntry*|\    +----+
12  +------------+/    +-----------+     +----------+ \   +----+
13  |ht[2]       |     |sizemask=3 |                   +->|K4|V|->NULL
14  +------------+\    +-----------+                      +----+
15  |rehashidx=0 | \   |used=4     | 
16  +------------+  \  +-----------+  
17  |iterators=0 |   \                
18  +------------+    \+-----------+  +->+----------+
19                     |dictEntry**|--+  |dictEntry*|->NULL
20                     +-----------+     +----------+
21                     |size=8     |     |dictEntry*|->NULL
22                     +-----------+     +----------+
23                     |sizemask=7 |     |dictEntry*|->NULL
24                     +-----------+     +----------+
25                     |used=0     |     |dictEntry*|->NULL
26                     +-----------+     +----------+
27                                       |dictEntry*|->NULL
28                                       +----------+
29                                       |dictEntry*|->NULL
30                                       +----------+
31                                       |dictEntry*|->NULL
32                                       +----------+
33                                       |dictEntry*|->NULL
34                                       +----------+
35 */

根据rehashing操作

 1 int dictRehash(dict *d, int n) {
 2     int empty_visits = n*10; /* Max number of empty buckets to visit. */
 3     if (!dictIsRehashing(d)) return 0;
 4 
 5     while(n-- && d->ht[0].used != 0) {
 6         dictEntry *de, *nextde;
 7 
 8         /* Note that rehashidx can't overflow as we are sure there are more
 9          * elements because ht[0].used != 0 */
10         assert(d->ht[0].size > (unsigned long)d->rehashidx);
11         while(d->ht[0].table[d->rehashidx] == NULL) {
12             d->rehashidx++;
13             if (--empty_visits == 0) return 1;
14         }
15         de = d->ht[0].table[d->rehashidx];
16         /* Move all the keys in this bucket from the old to the new hash HT */
17         while(de) {
18             uint64_t h;
19 
20             nextde = de->next;
21             /* Get the index in the new hash table */
22             h = dictHashKey(d, de->key) & d->ht[1].sizemask;
23             de->next = d->ht[1].table[h];
24             d->ht[1].table[h] = de;
25             d->ht[0].used--;
26             d->ht[1].used++;
27             de = nextde;
28         }
29         d->ht[0].table[d->rehashidx] = NULL;
30         d->rehashidx++;
31     }
32 
33     /* Check if we already rehashed the whole table... */
34     if (d->ht[0].used == 0) {
35         zfree(d->ht[0].table);
36         d->ht[0] = d->ht[1];
37         _dictReset(&d->ht[1]);
38         d->rehashidx = -1;
39         return 0;
40     }
41 
42     /* More to rehash... */
43     return 1;
44 }

rehashing操作将会把ht[0]里，rehashidx的值对应的bucket下的所有dictEntry，移至ht[1]，之后对rehashidx进行自增处理。当ht[0]->used为0时，认为ht[0]的所有dictEntry已经移至ht[1]，此时return 0，否则 return 1，告诉调用者，还需要继续进行rehashing操作。同时，rehashing时允许最多跳过10n的空bucket，就要退出流程。假设传入的n=1，即只进行一次rehashing操作，转换至完成之后的结构：

 1 /*
 2                                                        
 3                                                     +->NULL
 4                                    +->+----------+ /   
 5                                    |  |dictEntry*|/    +----+
 6                                    |  +----------+ +-->|K2|V|->NULL
 7                                    |  |dictEntry*|/    +----+
 8  +------------+    /+-----------+  |  +----------+
 9  |dictType*   |   / |dictEntry**|--+  |dictEntry*|\    +----+
10  +------------+  /  +-----------+     +----------+ +-->|K3|V|->NULL
11  |privdata    | /   |size=4     |     |dictEntry*|\    +----+
12  +------------+/    +-----------+     +----------+ \   +----+
13  |ht[2]       |     |sizemask=3 |                   +->|K4|V|->NULL
14  +------------+\    +-----------+                      +----+
15  |rehashidx=1 | \   |used=3     | 
16  +------------+  \  +-----------+  
17  |iterators=0 |   \                
18  +------------+    \+-----------+  +->+----------+   +----+
19                     |dictEntry**|--+  |dictEntry*|-->|K1|V|->NULL
20                     +-----------+     +----------+   +----+ 
21                     |size=8     |     |dictEntry*|->NULL
22                     +-----------+     +----------+
23                     |sizemask=7 |     |dictEntry*|->NULL
24                     +-----------+     +----------+
25                     |used=1     |     |dictEntry*|->NULL
26                     +-----------+     +----------+
27                                       |dictEntry*|->NULL
28                                       +----------+
29                                       |dictEntry*|->NULL
30                                       +----------+
31                                       |dictEntry*|->NULL
32                                       +----------+
33                                       |dictEntry*|->NULL
34                                       +----------+
35 */

所有节点移完时：

 1 /*
 2                                                        
 3                                                   
 4                                    +->+----------+
 5                                    |  |dictEntry*|->NULL
 6                                    |  +----------+
 7                                    |  |dictEntry*|->NULL    
 8  +------------+    /+-----------+  |  +----------+
 9  |dictType*   |   / |dictEntry**|--+  |dictEntry*|->NULL    
10  +------------+  /  +-----------+     +----------+
11  |privdata    | /   |size=4     |     |dictEntry*|->NULL    
12  +------------+/    +-----------+     +----------+ 
13  |ht[2]       |     |sizemask=3 |                  
14  +------------+\    +-----------+                      
15  |rehashidx=4 | \   |used=0     | 
16  +------------+  \  +-----------+  
17  |iterators=0 |   \                
18  +------------+    \+-----------+  +->+----------+   +----+
19                     |dictEntry**|--+  |dictEntry*|-->|K1|V|->NULL
20                     +-----------+     +----------+   +----+ 
21                     |size=8     |     |dictEntry*|->NULL
22                     +-----------+     +----------+   +----+
23                     |sizemask=7 |     |dictEntry*|-->|K3|V|->NULL
24                     +-----------+     +----------+   +----+
25                     |used=4     |     |dictEntry*|->NULL
26                     +-----------+     +----------+
27                                       |dictEntry*|->NULL
28                                       +----------+   +----+
29                                       |dictEntry*|-->|K2|V|->NULL
30                                       +----------+   +----+
31                                       |dictEntry*|->NULL
32                                       +----------+   +----+
33                                       |dictEntry*|-->|K4|V|->NULL
34                                       +----------+   +----+ 
35 */

此时ht[0]->used为0，释放原ht[0]的hash table，把ht[1]赋值给ht[0]，并设置ht[1] = NULL，最后重置rehashidx=-1，rehashing操作结束

 1 /*
 2  +------------+    /+-----------+   +-->+----------+   +----+
 3  |dictType*   |   / |dictEntry**|---+   |dictEntry*|-->|K1|V|->NULL
 4  +------------+  /  +-----------+       +----------+   +----+
 5  |privdata    | /   |size=8     |       |dictEntry*|->NULL
 6  +------------+/    +-----------+       +----------+   +----+
 7  |ht[2]       |     |sizemask=7 |       |dictEntry*|-->|K3|V|->NULL
 8  +------------+\    +-----------+       +----------+   +----+
 9  |rehashidx=-1| \   |used=4     |       |dictEntry*|->NULL
10  +------------+  \  +-----------+       +----------+
11  |iterators=0 |   \                     |dictEntry*|->NULL
12  +------------+    \+-----------+       +----------+   +----+
13                     |dictEntry**|->NULL |dictEntry*|-->|K2|V|->NULL
14                     +-----------+       +----------+   +----+
15                     |size=0     |       |dictEntry*|->NULL
16                     +-----------+       +----------+   +----+
17                     |sizemask=0 |       |dictEntry*|-->|K4|V|->NULL
18                     +-----------+       +----------+   +----+
19                     |used=0     |     
20                     +-----------+     
21 */

rehashing操作的触发共有两种方式

1、定时操作

 1 long long timeInMilliseconds(void) {
 2     struct timeval tv;
 3 
 4     gettimeofday(&tv,NULL);
 5     return (((long long)tv.tv_sec)*1000)+(tv.tv_usec/1000);
 6 }
 7 
 8 /* Rehash for an amount of time between ms milliseconds and ms+1 milliseconds */
 9 int dictRehashMilliseconds(dict *d, int ms) {
10     long long start = timeInMilliseconds();
11     int rehashes = 0;
12 
13     while(dictRehash(d,100)) {
14         rehashes += 100;
15         if (timeInMilliseconds()-start > ms) break;
16     }
17     return rehashes;
18 }

外部传入一个毫秒时间，在这时间内循环执行rehashing，每次执行100次。

2、操作时触发

1 static void _dictRehashStep(dict *d) {
2     if (d->iterators == 0) dictRehash(d,1);
3 }

在插入、删除、查找等操作时，顺带执行一次rehashing操作。值得注意的是，如果存在安全的迭代器，即d->iterators != 0，则不会进行rehashing操作

三、插入

获取可插入新节点的bucket idx的方法：

 1 static long _dictKeyIndex(dict *d, const void *key, uint64_t hash, dictEntry **existing)
 2 {
 3     unsigned long idx, table;
 4     dictEntry *he;
 5     if (existing) *existing = NULL;
 6 
 7     /* Expand the hash table if needed */
 8     if (_dictExpandIfNeeded(d) == DICT_ERR)
 9         return -1;
10     for (table = 0; table <= 1; table++) {
11         idx = hash & d->ht[table].sizemask;
12         /* Search if this slot does not already contain the given key */
13         he = d->ht[table].table[idx];
14         while(he) {
15             if (key==he->key || dictCompareKeys(d, key, he->key)) {
16                 if (existing) *existing = he;
17                 return -1;
18             }
19             he = he->next;
20         }
21         if (!dictIsRehashing(d)) break;
22     }
23     return idx;
24 }

此方法在进行查找idx之前，先进行一次判断，是否需要rehashing操作。而后进行查找。idx的值就是通过hash函数计算出来的hash_value与sizemask做位运算的结果，然后遍历此idx对应的bucket，若已存在相同的key，则认为不可插入，并把对应的dictEntry用传入的二级指针的方式传出，供调用者使用。若不存在，则需要判断是否正在进行rehashing操作。若在，则会对ht[1]做一次相同的操作。最终可以得到一个idx值，或传出一个dictEntry。

由于rehashing期间，将会把ht[0]的所有dictEntry依次转移至ht[1]，为了防止新插入的dictEntry落到ht[0]已完成rehashing操作的bucket上，在rehashing期间，返回的可插入的idx一定是属于ht[1]的。

插入方法：

 1 dictEntry *dictAddRaw(dict *d, void *key, dictEntry **existing)
 2 {
 3     long index;
 4     dictEntry *entry;
 5     dictht *ht;
 6 
 7     if (dictIsRehashing(d)) _dictRehashStep(d);
 8 
 9     /* Get the index of the new element, or -1 if
10      * the element already exists. */
11     if ((index = _dictKeyIndex(d, key, dictHashKey(d,key), existing)) == -1)
12         return NULL;
13 
14     /* Allocate the memory and store the new entry.
15      * Insert the element in top, with the assumption that in a database
16      * system it is more likely that recently added entries are accessed
17      * more frequently. */
18     ht = dictIsRehashing(d) ? &d->ht[1] : &d->ht[0];
19     entry = zmalloc(sizeof(*entry));
20     entry->next = ht->table[index];
21     ht->table[index] = entry;
22     ht->used++;
23 
24     /* Set the hash entry fields. */
25     dictSetKey(d, entry, key);
26     return entry;
27 }

若不存在相同key，则插入，否则，传出dictEntry的指针。插入时，由于没有记录每个dictEntry链表的尾指针，所以使用头插法，可以节约插入时的时间消耗。

dictAddRaw做为最终插入的方法，被多个方法所调用：

 1 //若不存在，则插入，否则报错
 2 int dictAdd(dict *d, void *key, void *val)
 3 {
 4     dictEntry *entry = dictAddRaw(d,key,NULL);
 5 
 6     if (!entry) return DICT_ERR;
 7     dictSetVal(d, entry, val);
 8     return DICT_OK;
 9 }
10 
11 //若存在，则替换value，否则插入
12 int dictReplace(dict *d, void *key, void *val)
13 {
14     dictEntry *entry, *existing, auxentry;
15     entry = dictAddRaw(d,key,&existing);
16     if (entry) {
17         dictSetVal(d, entry, val);
18         return 1;
19     }
20     auxentry = *existing;
21     dictSetVal(d, existing, val);
22     dictFreeVal(d, &auxentry);
23     return 0;
24 }
25 
26 //若存在，则返回对应dictEntry，否则插入后返回新的dictEntry
27 dictEntry *dictAddOrFind(dict *d, void *key) {
28     dictEntry *entry, *existing;
29     entry = dictAddRaw(d,key,&existing);
30     return entry ? entry : existing;
31 }

对于一个刚刚create的dict：

 1 /*
 2                                                                            
 3 +------------+    /+-----------+  
 4 |dictType*   |   / |dictEntry**|-->NULL
 5 +------------+  /  +-----------+     
 6 |privdata    | /   |size=0     |     
 7 +------------+/    +-----------+     
 8 |ht[2]       |     |sizemask=0 |     
 9 +------------+\    +-----------+     
10 |rehashidx=-1| \   |used=0     |     
11 +------------+  \  +-----------+     
12 |iterators=0 |   \                   
13 +------------+    \+-----------+                      
14                    |dictEntry**|-->NULL
15                    +-----------+
16                    |size=0     |
17                    +-----------+
18                    |sizemask=0 |
19                    +-----------+
20                    |used=0     |
21                    +-----------+
22 */

假设K1、K2、K3、K4计算出来的hash值分别为0、5、2、7，使用sizemask计算出来的idx分别为0、1、2、3

现调用dictAdd方法进行插入

1、插入K1

执行完dictAddRaw中的_dictKeyIndex里的_dictExpandIfNeeded：

 1 /*
 2                                                       
 3                                                    +-->NULL
 4 +------------+    /+-----------+  +->+----------+ /   
 5 |dictType*   |   / |dictEntry**|--+  |dictEntry*|/    
 6 +------------+  /  +-----------+     +----------+ +--->NULL
 7 |privdata    | /   |size=4     |     |dictEntry*|/    
 8 +------------+/    +-----------+     +----------+
 9 |ht[2]       |     |sizemask=3 |     |dictEntry*|\    
10 +------------+\    +-----------+     +----------+ +--->NULL
11 |rehashidx=-1| \   |used=0     |     |dictEntry*|\    
12 +------------+  \  +-----------+     +----------+ \   
13 |iterators=0 |   \                                 +-->NULL
14 +------------+    \+-----------+                      
15                    |dictEntry**|-->NULL
16                    +-----------+
17                    |size=0     |
18                    +-----------+
19                    |sizemask=0 |
20                    +-----------+
21                    |used=0     |
22                    +-----------+
23 */

同时得到其在ht[0]的idx = 0，且不在rehashing操作中，于是直接插入

 1 /*
 2                                                       +----+
 3                                                    +->|K1|V|->NULL
 4 +------------+    /+-----------+  +->+----------+ /   +----+
 5 |dictType*   |   / |dictEntry**|--+  |dictEntry*|/    
 6 +------------+  /  +-----------+     +----------+ +--->NULL
 7 |privdata    | /   |size=4     |     |dictEntry*|/    
 8 +------------+/    +-----------+     +----------+
 9 |ht[2]       |     |sizemask=3 |     |dictEntry*|\    
10 +------------+\    +-----------+     +----------+ +--->NULL
11 |rehashidx=-1| \   |used=1     |     |dictEntry*|\    
12 +------------+  \  +-----------+     +----------+ \   
13 |iterators=0 |   \                                 +-->NULL
14 +------------+    \+-----------+                      
15                    |dictEntry**|-->NULL
16                    +-----------+
17                    |size=0     |
18                    +-----------+
19                    |sizemask=0 |
20                    +-----------+
21                    |used=0     |
22                    +-----------+
23 */

2、插入K2、K3、K4后：

 1 /*
 2                                                       +----+
 3                                                    +->|K1|V|->NULL
 4 +------------+    /+-----------+  +->+----------+ /   +----+
 5 |dictType*   |   / |dictEntry**|--+  |dictEntry*|/    +-----
 6 +------------+  /  +-----------+     +----------+ +-->|K2|V|->NULL
 7 |privdata    | /   |size=4     |     |dictEntry*|/    +----+
 8 +------------+/    +-----------+     +----------+
 9 |ht[2]       |     |sizemask=3 |     |dictEntry*|\    +----+
10 +------------+\    +-----------+     +----------+ +-->|K3|V|->NULL
11 |rehashidx=-1| \   |used=4     |     |dictEntry*|\    +----+
12 +------------+  \  +-----------+     +----------+ \   +----+
13 |iterators=0 |   \                                 +->|K4|V|->NULL
14 +------------+    \+-----------+                      +----+
15                    |dictEntry**|-->NULL
16                    +-----------+
17                    |size=0     |
18                    +-----------+
19                    |sizemask=0 |
20                    +-----------+
21                    |used=0     |
22                    +-----------+
23 */

3、此时若有一个K5，计算出来的hash值为8，则：

i.因此刻不在rehashing操作，所以不用做处理

ii.执行完dictAddRaw中的_dictKeyIndex里的_dictExpandIfNeeded：

 1 /*
 2                                                        +----+
 3                                                     +->|K1|V|->NULL
 4                                    +->+----------+ /   +----+
 5                                    |  |dictEntry*|/    +----+
 6                                    |  +----------+ +-->|K2|V|->NULL
 7                                    |  |dictEntry*|/    +----+
 8  +------------+    /+-----------+  |  +----------+
 9  |dictType*   |   / |dictEntry**|--+  |dictEntry*|\    +----+
10  +------------+  /  +-----------+     +----------+ +-->|K3|V|->NULL
11  |privdata    | /   |size=4     |     |dictEntry*|\    +----+
12  +------------+/    +-----------+     +----------+ \   +----+
13  |ht[2]       |     |sizemask=3 |                   +->|K4|V|->NULL
14  +------------+\    +-----------+                      +----+
15  |rehashidx=0 | \   |used=4     | 
16  +------------+  \  +-----------+  
17  |iterators=0 |   \                
18  +------------+    \+-----------+  +->+----------+   
19                     |dictEntry**|--+  |dictEntry*|->NULL
20                     +-----------+     +----------+   
21                     |size=8     |     |dictEntry*|->NULL
22                     +-----------+     +----------+
23                     |sizemask=7 |     |dictEntry*|->NULL
24                     +-----------+     +----------+
25                     |used=0     |     |dictEntry*|->NULL
26                     +-----------+     +----------+
27                                       |dictEntry*|->NULL
28                                       +----------+
29                                       |dictEntry*|->NULL
30                                       +----------+
31                                       |dictEntry*|->NULL
32                                       +----------+
33                                       |dictEntry*|->NULL
34                                       +----------+
35 */

同时得到其在ht[1]的idx=0

iii.插入

 1 /*
 2                                                        +----+
 3                                                     +->|K1|V|->NULL
 4                                    +->+----------+ /   +----+
 5                                    |  |dictEntry*|/    +----+
 6                                    |  +----------+ +-->|K2|V|->NULL
 7                                    |  |dictEntry*|/    +----+
 8  +------------+    /+-----------+  |  +----------+
 9  |dictType*   |   / |dictEntry**|--+  |dictEntry*|\    +----+
10  +------------+  /  +-----------+     +----------+ +-->|K3|V|->NULL
11  |privdata    | /   |size=4     |     |dictEntry*|\    +----+
12  +------------+/    +-----------+     +----------+ \   +----+
13  |ht[2]       |     |sizemask=3 |                   +->|K4|V|->NULL
14  +------------+\    +-----------+                      +----+
15  |rehashidx=0 | \   |used=4     | 
16  +------------+  \  +-----------+  
17  |iterators=0 |   \                
18  +------------+    \+-----------+  +->+----------+   +----+
19                     |dictEntry**|--+  |dictEntry*|-->|K5|V|->NULL
20                     +-----------+     +----------+   +----+ 
21                     |size=8     |     |dictEntry*|->NULL
22                     +-----------+     +----------+
23                     |sizemask=7 |     |dictEntry*|->NULL
24                     +-----------+     +----------+
25                     |used=1     |     |dictEntry*|->NULL
26                     +-----------+     +----------+
27                                       |dictEntry*|->NULL
28                                       +----------+
29                                       |dictEntry*|->NULL
30                                       +----------+
31                                       |dictEntry*|->NULL
32                                       +----------+
33                                       |dictEntry*|->NULL
34                                       +----------+
35 */

4、此时若有一个K6，计算出来的hash值为16，则：

i.此时已处理rehashing操作，执行一步：

 1 /*
 2                                                        
 3                                                     +-->NULL
 4                                    +->+----------+ /   
 5                                    |  |dictEntry*|/    +----+
 6                                    |  +----------+ +-->|K2|V|->NULL
 7                                    |  |dictEntry*|/    +----+
 8  +------------+    /+-----------+  |  +----------+
 9  |dictType*   |   / |dictEntry**|--+  |dictEntry*|\    +----+
10  +------------+  /  +-----------+     +----------+ +-->|K3|V|->NULL
11  |privdata    | /   |size=4     |     |dictEntry*|\    +----+
12  +------------+/    +-----------+     +----------+ \   +----+
13  |ht[2]       |     |sizemask=3 |                   +->|K4|V|->NULL
14  +------------+\    +-----------+                      +----+
15  |rehashidx=1 | \   |used=3     | 
16  +------------+  \  +-----------+  
17  |iterators=0 |   \                
18  +------------+    \+-----------+  +->+----------+   +----+  +----+
19                     |dictEntry**|--+  |dictEntry*|-->|K1|V|->|K5|V|->NULL
20                     +-----------+     +----------+   +----+  +----+
21                     |size=8     |     |dictEntry*|->NULL
22                     +-----------+     +----------+
23                     |sizemask=7 |     |dictEntry*|->NULL
24                     +-----------+     +----------+
25                     |used=2     |     |dictEntry*|->NULL
26                     +-----------+     +----------+
27                                       |dictEntry*|->NULL
28                                       +----------+
29                                       |dictEntry*|->NULL
30                                       +----------+
31                                       |dictEntry*|->NULL
32                                       +----------+
33                                       |dictEntry*|->NULL
34                                       +----------+
35 */

ii.执行完dictAddRaw中的_dictKeyIndex里的_dictExpandIfNeeded，因已在进行rehashing，所以不做任何处理，只返回其在ht[1]的idx 0

iii.头插法将K6插入

 1 /*
 2                                                        
 3                                                     +-->NULL
 4                                    +->+----------+ /   
 5                                    |  |dictEntry*|/    +----+
 6                                    |  +----------+ +-->|K2|V|->NULL
 7                                    |  |dictEntry*|/    +----+
 8  +------------+    /+-----------+  |  +----------+
 9  |dictType*   |   / |dictEntry**|--+  |dictEntry*|\    +----+
10  +------------+  /  +-----------+     +----------+ +-->|K3|V|->NULL
11  |privdata    | /   |size=4     |     |dictEntry*|\    +----+
12  +------------+/    +-----------+     +----------+ \   +----+
13  |ht[2]       |     |sizemask=3 |                   +->|K4|V|->NULL
14  +------------+\    +-----------+                      +----+
15  |rehashidx=1 | \   |used=3     | 
16  +------------+  \  +-----------+  
17  |iterators=0 |   \                
18  +------------+    \+-----------+  +->+----------+   +----+  +----+  +----+
19                     |dictEntry**|--+  |dictEntry*|-->|K6|V|->|K1|V|->|K5|V|->NULL
20                     +-----------+     +----------+   +----+  +----+  +----+
21                     |size=8     |     |dictEntry*|->NULL
22                     +-----------+     +----------+
23                     |sizemask=7 |     |dictEntry*|->NULL
24                     +-----------+     +----------+
25                     |used=3     |     |dictEntry*|->NULL
26                     +-----------+     +----------+
27                                       |dictEntry*|->NULL
28                                       +----------+
29                                       |dictEntry*|->NULL
30                                       +----------+
31                                       |dictEntry*|->NULL
32                                       +----------+
33                                       |dictEntry*|->NULL
34                                       +----------+
35 */

以上为正常插入时的情况，key已存在，或是调用另外两个方法的情况与之大同小异，不再做过多叙述。

四、查找

 1 dictEntry *dictFind(dict *d, const void *key)
 2 {
 3     dictEntry *he;
 4     uint64_t h, idx, table;
 5 
 6     if (d->ht[0].used + d->ht[1].used == 0) return NULL; /* dict is empty */
 7     if (dictIsRehashing(d)) _dictRehashStep(d);
 8     h = dictHashKey(d, key);
 9     for (table = 0; table <= 1; table++) {
10         idx = h & d->ht[table].sizemask;
11         he = d->ht[table].table[idx];
12         while(he) {
13             if (key==he->key || dictCompareKeys(d, key, he->key))
14                 return he;
15             he = he->next;
16         }
17         if (!dictIsRehashing(d)) return NULL;
18     }
19     return NULL;
20 }

同样，若在rehashing期间，则执行一次。首先在ht[0]里查找，计算出hash值对应ht[0]的idx，取得其bucket，然后遍历之，找到与指定key相同的dictEntry。若ht[0]中找不到指定的key，且正在进行rehashing操作，则去ht[1]以相同方式也查找一次。

redis额外提供一个，根据key只获取其value的方法：

1 void *dictFetchValue(dict *d, const void *key) {
2     dictEntry *he;
3 
4     he = dictFind(d,key);
5     return he ? dictGetVal(he) : NULL;
6 }

key不存在时返回NULL

五、删除

删除方法：

 1 static dictEntry *dictGenericDelete(dict *d, const void *key, int nofree) {
 2     uint64_t h, idx;
 3     dictEntry *he, *prevHe;
 4     int table;
 5 
 6     if (d->ht[0].used == 0 && d->ht[1].used == 0) return NULL;
 7 
 8     if (dictIsRehashing(d)) _dictRehashStep(d);
 9     h = dictHashKey(d, key);
10 
11     for (table = 0; table <= 1; table++) {
12         idx = h & d->ht[table].sizemask;
13         he = d->ht[table].table[idx];
14         prevHe = NULL;
15         while(he) {
16             if (key==he->key || dictCompareKeys(d, key, he->key)) {
17                 /* Unlink the element from the list */
18                 if (prevHe)
19                     prevHe->next = he->next;
20                 else
21                     d->ht[table].table[idx] = he->next;
22                 if (!nofree) {
23                     dictFreeKey(d, he);
24                     dictFreeVal(d, he);
25                     zfree(he);
26                 }
27                 d->ht[table].used--;
28                 return he;
29             }
30             prevHe = he;
31             he = he->next;
32         }
33         if (!dictIsRehashing(d)) break;
34     }
35     return NULL; /* not found */
36 }

查找方式与dictFind相同。找到之后，由调用者指定是否要销毁此dictEntry，若不销毁，则要把对应指针传出来，给外部使用。

此方法被两个接口所调用：

1 int dictDelete(dict *ht, const void *key) {
2     return dictGenericDelete(ht,key,0) ? DICT_OK : DICT_ERR;
3 }
4 
5 dictEntry *dictUnlink(dict *ht, const void *key) {
6     return dictGenericDelete(ht,key,1);
7 }

dictDelete就不用多说了，直接删除对应dictEntry。关于为什么需要dictUnlink，源码的注释上写道，如果有某种操作，需要先查找指定key对应的dictEntry，然后对其做点操作，接着就直接删除，在没有dictUnlink的时候，需要这样：

1 entry = dictFind(...);
2 // Do something with entry
3 dictDelete(dictionary,entry);

实际需要查找两次。而在有dictUnlink的情况下：

1 entry = dictUnlink(dictionary,entry);
2 // Do something with entry
3 dictFreeUnlinkedEntry(entry);

只需要一次查找，配合专门的删除操作，即可。

1 void dictFreeUnlinkedEntry(dict *d, dictEntry *he) {
2     if (he == NULL) return;
3     dictFreeKey(d, he);
4     dictFreeVal(d, he);
5     zfree(he);
6 }

六、销毁

清空一个hash table的方法

 1 int _dictClear(dict *d, dictht *ht, void(callback)(void *)) {
 2     unsigned long i;
 3 
 4     /* Free all the elements */
 5     for (i = 0; i < ht->size && ht->used > 0; i++) {
 6         dictEntry *he, *nextHe;
 7 
 8         if (callback && (i & 65535) == 0) callback(d->privdata);
 9 
10         if ((he = ht->table[i]) == NULL) continue;
11         while(he) {
12             nextHe = he->next;
13             dictFreeKey(d, he);
14             dictFreeVal(d, he);
15             zfree(he);
16             ht->used--;
17             he = nextHe;
18         }
19     }
20     /* Free the table and the allocated cache structure */
21     zfree(ht->table);
22     /* Re-initialize the table */
23     _dictReset(ht);
24     return DICT_OK; /* never fails */
25 }

两层循环，分别遍历所有bucket与单bucket里所有dictEntry进行释放。关于这里的 (i&65535) == 0的判断，_dictClear方法仅在相同文件的方法dictEmpty与dictRelease调用

 1 void dictRelease(dict *d)
 2 {
 3     _dictClear(d,&d->ht[0],NULL);
 4     _dictClear(d,&d->ht[1],NULL);
 5     zfree(d);
 6 }
 7 
 8 void dictEmpty(dict *d, void(callback)(void*)) {
 9     _dictClear(d,&d->ht[0],callback);
10     _dictClear(d,&d->ht[1],callback);
11     d->rehashidx = -1;
12     d->iterators = 0;
13 }

dictRelease不用多说，传入的callback为NULL。而dictEmpty，搜索redis源码所有文件的调用，

ccx@ccx:~/Desktop/redis-5.0.7/redis-5.0.7/src$ grep dictEmpty * -r          
blocked.c:    dictEmpty(c->bpop.keys,NULL);
db.c:            dictEmpty(server.db[j].dict,callback);
db.c:            dictEmpty(server.db[j].expires,callback);
dict.c:void dictEmpty(dict *d, void(callback)(void*)) {
dict.h:void dictEmpty(dict *d, void(callback)(void*));
replication.c:    dictEmpty(server.repl_scriptcache_dict,NULL);
sentinel.c:    dictEmpty(server.commands,NULL);

仅db.c里传了callback进来，对应的方法为

1 long long emptyDb(int dbnum, int flags, void(callback)(void*));

继续搜索：

ccx@ccx:~/Desktop/redis-5.0.7/redis-5.0.7/src$ grep emptyDb * -r
cluster.c:        emptyDb(-1,EMPTYDB_NO_FLAGS,NULL);
db.c:long long emptyDb(int dbnum, int flags, void(callback)(void*)) {
db.c:            emptyDbAsync(&server.db[j]);
db.c:/* Return the set of flags to use for the emptyDb() call for FLUSHALL*/
db.c:    server.dirty += emptyDb(c->db->id,flags,NULL);
db.c:    server.dirty += emptyDb(-1,flags,NULL);
debug.c:        emptyDb(-1,EMPTYDB_NO_FLAGS,NULL);
debug.c:        emptyDb(-1,EMPTYDB_NO_FLAGS,NULL);
lazyfree.c:void emptyDbAsync(redisDb *db) {
replication.c: * data with emptyDb(), and while we load the new data received as an
replication.c:/* Callback used by emptyDb() while flushing away old data to load*/
replication.c:        emptyDb(
server.h:long long emptyDb(int dbnum, int flags, void(callback)(void*));
server.h:void emptyDbAsync(redisDb *db);

真正调用的地方传入的也是NULL，并不知道是拿来做什么用的...

ps：用grep查找只是方便cv过来....

七、迭代器操作

数据结构：

1 typedef struct dictIterator {
2     dict *d;
3     long index;
4     int table, safe;
5     dictEntry *entry, *nextEntry;
6     /* unsafe iterator fingerprint for misuse detection. */
7     long long fingerprint;
8 } dictIterator;

根据源码注释可知，如果是个安全的迭代器，即safe == 1，则在迭代中可以调用dictAdd、dictFind等方法，否则只能调用dictNext。

index表示，ht[table]对应的bucket的idx。

获取迭代器：

 1 dictIterator *dictGetIterator(dict *d)
 2 {
 3     dictIterator *iter = zmalloc(sizeof(*iter));
 4 
 5     iter->d = d;
 6     iter->table = 0;
 7     iter->index = -1;
 8     iter->safe = 0;
 9     iter->entry = NULL;
10     iter->nextEntry = NULL;
11     return iter;
12 }
13 
14 dictIterator *dictGetSafeIterator(dict *d) {
15     dictIterator *i = dictGetIterator(d);
16 
17     i->safe = 1;
18     return i;
19 }

刚获取的迭代器并不指向具体哪个dictEntry

next操作：

 1 dictEntry *dictNext(dictIterator *iter)
 2 {
 3     while (1) {
 4         if (iter->entry == NULL) {
 5             dictht *ht = &iter->d->ht[iter->table];
 6             if (iter->index == -1 && iter->table == 0) {
 7                 if (iter->safe)
 8                     iter->d->iterators++;
 9                 else
10                     iter->fingerprint = dictFingerprint(iter->d);
11             }
12             iter->index++;
13             if (iter->index >= (long) ht->size) {
14                 if (dictIsRehashing(iter->d) && iter->table == 0) {
15                     iter->table++;
16                     iter->index = 0;
17                     ht = &iter->d->ht[1];
18                 } else {
19                     break;
20                 }
21             }
22             iter->entry = ht->table[iter->index];
23         } else {
24             iter->entry = iter->nextEntry;
25         }
26         if (iter->entry) {
27             /* We need to save the 'next' here, the iterator user
28              * may delete the entry we are returning. */
29             iter->nextEntry = iter->entry->next;
30             return iter->entry;
31         }
32     }
33     return NULL;
34 }

对于一个新的迭代器，首次调用时，会根据是否安全，做不同操作。安全的迭代器会给dict里的计数器+1，不安全的将会记录本字典的指纹。之后会遍历ht[0]，取到第一个非NULL的dictEntry。当ht[0]遍历完且取不到非NULL的dictEntry时，如果正在进行rehashing操作，则会去ht[1]里找。

如：

 1 /*
 2 
 3      +-------------------------+
 4 +----|dict *                   |
 5 |    +-------------------------+
 6 |    |long index               |
 7 |    +-------------------------+
 8 |    |int table                |
 9 |    +-------------------------+
10 |    |int safe                 |
11 |    +-------------------------+
12 |    |dictEntry *entry         |->NULL
13 |    +-------------------------+
14 |    |dictEntry *entrynextEntry|->NULL
15 |    +-------------------------+
16 |    |long long fingerprint    |
17 |    +-------------------------+
18 |
19 |
20 |                                                      
21 |                                                       +-->NULL
22 |                                      +->+----------+ /   
23 |                                      |  |dictEntry*|/    +----+
24 |                                      |  +----------+ +-->|K2|V|->NULL
25 |                                      |  |dictEntry*|/    +----+
26 +--->+------------+    /+-----------+  |  +----------+
27      |dictType*   |   / |dictEntry**|--+  |dictEntry*|\    +----+
28      +------------+  /  +-----------+     +----------+ +-->|K3|V|->NULL
29      |privdata    | /   |size=4     |     |dictEntry*|\    +----+
30      +------------+/    +-----------+     +----------+ \   +----+
31      |ht[2]       |     |sizemask=3 |                   +->|K4|V|->NULL
32      +------------+\    +-----------+                      +----+
33      |rehashidx=1 | \   |used=3     | 
34      +------------+  \  +-----------+  
35      |iterators=0 |   \                
36      +------------+    \+-----------+  +->+----------+   +----+  +----+
37                         |dictEntry**|--+  |dictEntry*|-->|K1|V|->|K5|V|->NULL
38                         +-----------+     +----------+   +----+  +----+
39                         |size=8     |     |dictEntry*|->NULL
40                         +-----------+     +----------+
41                         |sizemask=7 |     |dictEntry*|->NULL
42                         +-----------+     +----------+
43                         |used=3     |     |dictEntry*|->NULL
44                         +-----------+     +----------+
45                                           |dictEntry*|->NULL
46                                           +----------+
47                                           |dictEntry*|->NULL
48                                           +----------+
49                                           |dictEntry*|->NULL
50                                           +----------+
51                                           |dictEntry*|->NULL
52                                           +----------+
53 */

遍历顺序为，K2，K3，K4，K1，K5。

迭代器销毁：

 1 void dictReleaseIterator(dictIterator *iter)
 2 {
 3     if (!(iter->index == -1 && iter->table == 0)) {
 4         if (iter->safe)
 5             iter->d->iterators--;
 6         else
 7             assert(iter->fingerprint == dictFingerprint(iter->d));
 8     }
 9     zfree(iter);
10 }

与首次执行next操作相对应，若为safe的迭代器，要给dict的计算减1，否则要校验期间dict的指纹是否发生了变化。

指纹的计算：

 1 long long dictFingerprint(dict *d) {
 2     long long integers[6], hash = 0;
 3     int j;
 4 
 5     integers[0] = (long) d->ht[0].table;
 6     integers[1] = d->ht[0].size;
 7     integers[2] = d->ht[0].used;
 8     integers[3] = (long) d->ht[1].table;
 9     integers[4] = d->ht[1].size;
10     integers[5] = d->ht[1].used;
11 
12     /* We hash N integers by summing every successive integer with the integer
13      * hashing of the previous sum. Basically:
14      *
15      * Result = hash(hash(hash(int1)+int2)+int3) ...
16      *
17      * This way the same set of integers in a different order will (likely) hash
18      * to a different number. */
19     for (j = 0; j < 6; j++) {
20         hash += integers[j];
21         /* For the hashing step we use Tomas Wang's 64 bit integer hash. */
22         hash = (~hash) + (hash << 21); // hash = (hash << 21) - hash - 1;
23         hash = hash ^ (hash >> 24);
24         hash = (hash + (hash << 3)) + (hash << 8); // hash * 265
25         hash = hash ^ (hash >> 14);
26         hash = (hash + (hash << 2)) + (hash << 4); // hash * 21
27         hash = hash ^ (hash >> 28);
28         hash = hash + (hash << 31);
29     }
30     return hash;
31 }

对于不安全的迭代器，在迭代过程中，不允许操作任何修改dict的操作，是只读的，不会发生迭代器失效的问题。对于安全的迭代器，在进行操作本节点的时候，redis中记录了当前迭代的bucket idx，以及当前dictEntry的next节点。如果只是add操作，即使是用了头插法把新dictEntry插在本节点之前，对迭代器本身并没有影响。如果是delete了本节点，迭代器中还记录了next节点的位置，调用next时直接取就好。如果next为空，则可以认为当前bucket遍历完了，取下一个bucket就行了。当然，如果在add/delete等操作的时候，进行了rehashing操作，那么当前迭代器里记录的next，在rehashing之后，可能就不是当前节点新位置的next了。所以在使用安全迭代器的时候，禁止了rehashing操作。

八、其它操作

dict还支持其它的一些操作。

1、随机获取一个key，可以用于一些随机操作的dictGetRandomKey

2、随机获取n个key：dictGetSomeKeys

3、scan操作

关于scan操作，redis采用了一个很巧妙的方法，保证了在开始scan时未删除的元素一定能遍历到，又能保证尽量少地重复遍历。

这里直接给个传送门，这里讲得很好：

https://blog.csdn.net/gqtcgq/article/details/50533336

redis 5.0.7 下载链接

http://download.redis.io/releases/redis-5.0.7.tar.gz

源码阅读顺序参考：

https://github.com/huangz1990/blog/blob/master/diary/2014/how-to-read-redis-source-code.rst