Redis源码阅读-Dict哈希字典

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。

、

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);
}