Dict和Java中的HashMap很相似，都是数组开链法解决冲突。
但是Redis为了高性能， 有很多比较微妙的方法，例如 数组的大小总是2的倍数，初始大小是4。
rehash并不是一次就执行完，而是分多次执行。每次执行一部分。其中rehashidx表示现在hash到哪一个桶啦，-1表示现在并没有rehash.
dict包含两个dicttable， 编号为0,1, dictht0是直接存储哈希表的地方， dictht1在rehash中用到，当rehashidx不为-1时， 查找key，同时在dictht1和dictht0中查找。
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数据结构
typedef struct dictEntry { void *key; union { void *val; uint64_t u64; int64_t s64; double d; } v; struct dictEntry *next;} dictEntry;typedef struct dictType { unsigned int (*hashFunction)(const void *key); void *(*keyDup)(void *privdata, const void *key); void *(*valDup)(void *privdata, const void *obj); int (*keyCompare)(void *privdata, const void *key1, const void *key2); void (*keyDestructor)(void *privdata, void *key); void (*valDestructor)(void *privdata, void *obj);} dictType;/* This is our hash table structure. Every dictionary has two of this as we * implement incremental rehashing, for the old to the new table. */typedef struct dictht { dictEntry **table; unsigned long size; unsigned long sizemask; unsigned long used;} dictht;typedef struct dict { dictType *type; void *privdata; dictht ht[2]; long rehashidx; /* rehashing not in progress if rehashidx == -1 */ int iterators; /* number of iterators currently running */} dict;/* If safe is set to 1 this is a safe iterator, that means, you can call * dictAdd, dictFind, and other functions against the dictionary even while * iterating. Otherwise it is a non safe iterator, and only dictNext() * should be called while iterating. */typedef struct dictIterator { dict *d; long index; int table, safe; dictEntry *entry, *nextEntry; /* unsafe iterator fingerprint for misuse detection. */ long long fingerprint;} dictIterator;typedef void (dictScanFunction)(void *privdata, const dictEntry *de);
查找key
dictEntry *dictFind(dict *d, const void *key){ dictEntry *he; unsigned int h, idx, table; if (d->ht[0].size == 0) return NULL; /* We don't have a table at all */ if (dictIsRehashing(d)) _dictRehashStep(d); h = dictHashKey(d, key); for (table = 0; table <= 1; table++) { idx = h & d->ht[table].sizemask; he = d->ht[table].table[idx]; while(he) { if (dictCompareKeys(d, key, he->key)) return he; he = he->next; } if (!dictIsRehashing(d)) return NULL; } return NULL;}
redis的rehash是增量rehash，每次rehash一部分
rehash过程：
1. 从 dictht0的table 0到----N-1查找不为NULL的位置（非空桶）
2. 对该位置的链表进行处理， hash到dictht 1的table 1中。
rehash的函数，设置了n参数，表示要处理的非空桶的个数，但是在函数内部设置了最多访问10*n个空桶。
int dictRehash(dict *d, int n) { int empty_visits = n*10; /* Max number of empty buckets to visit. */ if (!dictIsRehashing(d)) return 0; while(n-- && d->ht[0].used != 0) { dictEntry *de, *nextde; /* Note that rehashidx can't overflow as we are sure there are more * elements because ht[0].used != 0 */ assert(d->ht[0].size > (unsigned long)d->rehashidx); while(d->ht[0].table[d->rehashidx] == NULL) { d->rehashidx++; if (--empty_visits == 0) return 1; } de = d->ht[0].table[d->rehashidx]; /* Move all the keys in this bucket from the old to the new hash HT */ while(de) { unsigned int h; nextde = de->next; /* Get the index in the new hash table */ h = dictHashKey(d, de->key) & d->ht[1].sizemask; de->next = d->ht[1].table[h]; d->ht[1].table[h] = de; d->ht[0].used--; d->ht[1].used++; de = nextde; } d->ht[0].table[d->rehashidx] = NULL; d->rehashidx++; } /* Check if we already rehashed the whole table... */ if (d->ht[0].used == 0) { zfree(d->ht[0].table); d->ht[0] = d->ht[1]; _dictReset(&d->ht[1]); d->rehashidx = -1; return 0; } /* More to rehash... */ return 1;}
和adlist一样，dict也有迭代器
迭代方法如下：
dictEntry *dictNext(dictIterator *iter){ //对表的桶进行遍历，直到找到一个非空桶，返回 while (1) { if (iter->entry == NULL) { dictht *ht = &iter->d->ht[iter->table]; if (iter->index == -1 && iter->table == 0) { if (iter->safe) iter->d->iterators++; else iter->fingerprint = dictFingerprint(iter->d);//对dict进行指纹 } iter->index++; //如果迭代到表的最后一个桶，就判断要不要迭代第二个表 if (iter->index >= (long) ht->size) { if (dictIsRehashing(iter->d) && iter->table == 0) { iter->table++; iter->index = 0; ht = &iter->d->ht[1]; } else { break; } } iter->entry = ht->table[iter->index]; } else { iter->entry = iter->nextEntry; } if (iter->entry) { /* We need to save the 'next' here, the iterator user * may delete the entry we are returning. */ iter->nextEntry = iter->entry->next; return iter->entry; } } return NULL;} Dict的API如下：
/* API *//* 字典创建， type参数制定各类对字典的自定义函数，会初始化dictht, dict */dict *dictCreate(dictType *type, void *privDataPtr);int dictExpand(dict *d, unsigned long size);/* 添加键值对，内部调用addRaw和setvalue ，如果已经存在，返回NULL*/int dictAdd(dict *d, void *key, void *val);/* 添加键 ，如果已经存在，返回NULL*/dictEntry *dictAddRaw(dict *d, void *key);/* 添加一个key,如果存在，直接设置value,设置key的value */int dictReplace(dict *d, void *key, void *val);/* 添加一个key,如果存在，直接返回 */dictEntry *dictReplaceRaw(dict *d, void *key);/* 删除一个节点，需要free那个节点 */int dictDelete(dict *d, const void *key);/* 删除一个节点，不需要free那个节点 */int dictDeleteNoFree(dict *d, const void *key);/* 删除dict*/void dictRelease(dict *d);/* 查找key*/dictEntry * dictFind(dict *d, const void *key);/* 查找key的value*/void *dictFetchValue(dict *d, const void *key);/* 将dict的size设置和元素数量一样，但是符合2的倍数*/int dictResize(dict *d);dictIterator *dictGetIterator(dict *d);dictIterator *dictGetSafeIterator(dict *d);dictEntry *dictNext(dictIterator *iter);void dictReleaseIterator(dictIterator *iter);dictEntry *dictGetRandomKey(dict *d);unsigned int dictGetSomeKeys(dict *d, dictEntry **des, unsigned int count);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(callback)(void*));void dictEnableResize(void);void dictDisableResize(void);int dictRehash(dict *d, int n);/* rehash，设置一个最长时间*/int dictRehashMilliseconds(dict *d, int ms);void dictSetHashFunctionSeed(unsigned int initval);unsigned int dictGetHashFunctionSeed(void);unsigned long dictScan(dict *d, unsigned long v, dictScanFunction *fn, void *privdata);
上面的API很多函数内部都会判断当前是不是还在rehash状态，如果是，就rehash一步。
在rehash前，会判断是不是有迭代器存在，如果有迭代器存在，就不rehash
static void _dictRehashStep(dict *d) {
if (d->iterators == 0) dictRehash(d,1);}