2.NSDictionary

参考 https://www.jianshu.com/p/2fd9680b46f6
哈希概念:哈希表的本质是一个数组，数组中每一个元素称为一个箱子(bin)，箱子中存放的是键值对。
简单了解总结，
1.NSMapTable 结构体里面包含一个map数组，数组里面存储
2.NSConcreteMapTable
将键和值添加到表中。<br />
*如果表中已存在等号，则替换其映射值
*使用新的，无需更换密钥本身。<br

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Nsdictionary:::::::

//根据key来  存储位置bucket种位置，
GSIMapBucketForKey(GSIMapTable map, GSIMapKey key)
{
  return GSIMapPickBucket(GSI_MAP_HASH(map, key),
    map->buckets, map->bucketCount);
}
GSIMapPickBucket(unsigned hash, GSIMapBucket buckets, uintptr_t bucketCount)
{
  return buckets + hash % bucketCount;
}
//Bucket 里面存储nodeCount 和fristNode
struct  _GSIMapBucket {
  uintptr_t nodeCount;  /* Number of nodes in bucket.   */
  GSIMapNode    firstNode;  /* The linked list of nodes.    */
};

//将键和值添加到表中。<br />
  *如果表中已存在等号，则替换其映射值
  *使用新的，无需更换密钥本身。<br />
  *如果key等于表的notAKeyMarker字段
  * NSMapTableKeyCallBacks，引发NSInvalidArgumentException。

void NSMapInsert(NSMapTable *table, const void *key, const void *value)
{
  if (table == nil)
    {
      [NSException raise: NSInvalidArgumentException
          format: @"Attempt to place key-value in null table"];
    }
  if (object_getClass(table) == concreteClass)
    {
      GSIMapTable   t = (GSIMapTable)table;
      GSIMapNode    n;

      if (t->legacy == YES)
    {
      if (key == t->cb.old.k.notAKeyMarker)
        {
          [NSException raise: NSInvalidArgumentException
                  format: @"Attempt to place notAKeyMarker in map"];
        }
    }
      else if (key == 0)
    {
      [NSException raise: NSInvalidArgumentException
              format: @"Attempt to place nil key in map"];
    }
      n = GSIMapNodeForKey(t, (GSIMapKey)key);
      if (n == 0)
    {
      GSIMapAddPair(t, (GSIMapKey)key, (GSIMapVal)value);
      t->version++;
    }
      else if (n->value.ptr != value)
    {
      GSIMapVal tmp = n->value;

      n->value = (GSIMapVal)value;
      GSI_MAP_RETAIN_VAL(t, n->value);
      GSI_MAP_RELEASE_VAL(t, tmp);
      t->version++;
    }
    }
  else
    {
      [table setObject: (id)value forKey: (id)key];
    }
}