深入redis内部--字典实现
redis的字典定义和实现在dict.h和dict.c文件中。1.字典结构
typedef struct dict {
dictType *type;//定义了字典需要的函数
void *privdata;
dictht ht; //哈希表结构
int rehashidx; //下一个需要扩容的字典编号，若rehashidx == -1 则不会进行重新散列。
int iterators; //当前正在运行的迭代器数目
} dict;
其中涉及到数据结构，如下所示：
1.1 字典类型，包含了一系列字典所需要用到的函数
typedef struct dictType {
unsigned int (*hashFunction)(const void *key); //hash函数
void *(*keyDup)(void *privdata, const void *key); //键复制
void *(*valDup)(void *privdata, const void *obj); //值复制
int (*keyCompare)(void *privdata, const void *key1, const void *key2);//key比较
void (*keyDestructor)(void *privdata, void *key); //key析构
void (*valDestructor)(void *privdata, void *obj); //value析构
} dictType;
1.2 哈希表结构，每个字典有两个哈希表。当哈希表扩容时实现散列。
typedef struct dictht {
dictEntry **table;
unsigned long size; //桶的大小，是2的指数
unsigned long sizemask; //sizemask=size-1，方便取模（i%sizemask 开放链地址法处理hash冲突）。
unsigned long used; //哈希表中的记录数
} dictht;
1.3 dictEntry为字典的条目，其定义如下：
typedef struct dictEntry {
void *key; // 键
union { //值的共用体
void *val;
uint64_t u64;
int64_t s64;
} v;
struct dictEntry *next;
} dictEntry;
2. 字典的遍历--字典遍历器
typedef struct dictIterator {
dict *d;
int table, index, safe;
dictEntry *entry, *nextEntry;
long long fingerprint; /* unsafe iterator fingerprint for misuse detection */
} dictIterator;
注意：当safe=1时，该遍历器是安全的，即字典可以在遍历的同时执行dictAdd, dictFind, 和别的函数。否则遍历器是不安全的，遍历时只能执行dictNext()。
迭代器提供了遍历字典中所有元素的方法，通过dicGetIterator()获得迭代器后，使用dictNext(dictIterator *)获得下一个元素。遍历的过程，先从ht开始，依次从第一个桶table开始遍历桶中的元素，然后遍历table,'*** ,table,若正在扩容，则会继续遍历ht中的桶。遍历桶中元素时，依次访问链表中的每一个元素。
3.宏定义函数
#define dictFreeVal(d, entry) \
if ((d)->type->valDestructor) \
(d)->type->valDestructor((d)->privdata, (entry)->v.val)
#define dictSetVal(d, entry, _val_) do { \
if ((d)->type->valDup) \
entry->v.val = (d)->type->valDup((d)->privdata, _val_); \
else \
entry->v.val = (_val_); \
} while(0)
#define dictSetSignedIntegerVal(entry, _val_) \
do { entry->v.s64 = _val_; } while(0)
#define dictSetUnsignedIntegerVal(entry, _val_) \
do { entry->v.u64 = _val_; } while(0)
#define dictFreeKey(d, entry) \
if ((d)->type->keyDestructor) \
(d)->type->keyDestructor((d)->privdata, (entry)->key)
#define dictSetKey(d, entry, _key_) do { \
if ((d)->type->keyDup) \
entry->key = (d)->type->keyDup((d)->privdata, _key_); \
else \
entry->key = (_key_); \
} while(0)
#define dictCompareKeys(d, key1, key2) \
(((d)->type->keyCompare) ? \
(d)->type->keyCompare((d)->privdata, key1, key2) : \
(key1) == (key2))
#define dictHashKey(d, key) (d)->type->hashFunction(key)
#define dictGetKey(he) ((he)->key)
#define dictGetVal(he) ((he)->v.val)
#define dictGetSignedIntegerVal(he) ((he)->v.s64)
#define dictGetUnsignedIntegerVal(he) ((he)->v.u64)
#define dictSlots(d) ((d)->ht.size+(d)->ht.size)
#define dictSize(d) ((d)->ht.used+(d)->ht.used)
#define dictIsRehashing(ht) ((ht)->rehashidx != -1)
4. 字典提供的api，有字典的创建，增加、删除、修改记录，还有迭代器（前面已经介绍）和自动扩容（下面介绍）。
dict *dictCreate(dictType *type, void *privDataPtr);
int dictExpand(dict *d, unsigned long size);
int dictAdd(dict *d, void *key, void *val);
dictEntry *dictAddRaw(dict *d, void *key);
int dictReplace(dict *d, void *key, void *val);
dictEntry *dictReplaceRaw(dict *d, void *key);
int dictDelete(dict *d, const void *key);
int dictDeleteNoFree(dict *d, const void *key);
void dictRelease(dict *d);
dictEntry * dictFind(dict *d, const void *key);
void *dictFetchValue(dict *d, const void *key);
int dictResize(dict *d);
dictIterator *dictGetIterator(dict *d);
dictIterator *dictGetSafeIterator(dict *d);
dictEntry *dictNext(dictIterator *iter);
void dictReleaseIterator(dictIterator *iter);
dictEntry *dictGetRandomKey(dict *d);
void dictPrintStats(dict *d);
unsigned int dictGenHashFunction(const void *key, int len);
unsigned int dictGenCaseHashFunction(const unsigned char *buf, int len);
void dictEmpty(dict *d);
void dictEnableResize(void);
void dictDisableResize(void);
int dictRehash(dict *d, int n);
int dictRehashMilliseconds(dict *d, int ms);
void dictSetHashFunctionSeed(unsigned int initval);
unsigned int dictGetHashFunctionSeed(void);
5.外部定义变量
/* 哈希表类型*/
extern dictType dictTypeHeapStringCopyKey;
extern dictType dictTypeHeapStrings;
extern dictType dictTypeHeapStringCopyKeyValue;
6. 自动扩容
Redis使用标识dict_can_resize来记录字典是否可以扩容，可以使用dictEnableResize()方法和dictDisableResize()来改变此标识。使用dictResize()来扩容，但需要首先判断是否允许扩容及是否正在扩容。若可以扩容，则调用dictExpand()扩容，然后调用dictRehashMilliseconds()启动扩容，并指定扩容过程中记录的copy速度。请看程序：
6.1 dictResize()
/* Resize the table to the minimal size that contains all the elements,
* but with the invariant of a USED/BUCKETS ratio near to ht.used;
if (minimal < DICT_HT_INITIAL_SIZE)
minimal = DICT_HT_INITIAL_SIZE;
return dictExpand(d, minimal);
}
6.2 dictExpand()
/* Expand or create the hash table */
int dictExpand(dict *d, unsigned long size)
{
dictht n; /* the new hash table */
unsigned long realsize = _dictNextPower(size);
/* the size is invalid if it is smaller than the number of
* elements already inside the hash table */
if (dictIsRehashing(d) || d->ht.used > size)
return DICT_ERR;
/* Allocate the new hash table and initialize all pointers to NULL */
n.size = realsize;
n.sizemask = realsize-1;
n.table = zcalloc(realsize*sizeof(dictEntry*));
n.used = 0;
/* Is this the first initialization? If so it's not really a rehashing
* we just set the first hash table so that it can accept keys. */
if (d->ht.table == NULL) {
d->ht = n;
return DICT_OK;
}
/* Prepare a second hash table for incremental rehashing */
d->ht = n;
d->rehashidx = 0;
return DICT_OK;
}
6.3
/* Rehash for an amount of time between ms milliseconds and ms+1 milliseconds */
int dictRehashMilliseconds(dict *d, int ms) {
long long start = timeInMilliseconds();
int rehashes = 0;
while(dictRehash(d,100)) {
rehashes += 100;
if (timeInMilliseconds()-start > ms) break;
}
return rehashes;
}
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