dict, hash

dict： 

　　dictKey -- > dictVal

example:

     dictEntry *dictFind(dict *d, const void *key)
     Key is like a index which to find the real entry.

how to find:

　　depend on the construction of dict.

       if dict is constructed by arr then we get arr[i]. key is not integer usual and here the array is a map.

　　if dict is constructed by hash table, then we get hash table value.

 

redis code dictFind:

　　

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;

typedef struct dictht {
    dictEntry **table;
    unsigned long size;
    unsigned long sizemask;
    unsigned long used;
} dictht;

　　

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;
}

　　key -> hashValue -> &hashMask -> hashIndex -> hashTable[hashIndex] (hashTableEleList[idx]) -> hashTableEleList[idx][j]

　　dictKey ----------------------------------------------------------------------------------------------------------------------> dictVal

why hash:

   

     no  hash table , store m * n elems. 

　 hash, store m list head.

    seraching, storing...

 

    比如 dict 是个管理仓库的，仓库管理员按客人的姓名笔划来分架子。李一一来取货，dict算下李的在8号架子上，但是是8号架上的哪个柜子，要一个个来查看，只能根据姓名来一一对比了,最终找到客人的柜子，那个大的东西，肯定不好拿了，直接告诉李一一你的柜子地址就好了，让李一一去自己去对这个柜子中的东西操作。