OpenRtsp源码剖析之hashtable解析

最新推荐文章于 2021-01-21 20:57:18 发布

luxingqiang110

最新推荐文章于 2021-01-21 20:57:18 发布

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分类专栏： c/c++ 文章标签： table null delete class

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本文详细解析了OpenRTSP项目中Hashtable的具体实现原理及操作过程，包括其内部结构、添加元素方法和自动扩容机制等核心内容。

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OpenRtsp源码剖析：hashtable

这是主要保存数据value和key的结构体：

    class TableEntry {       
    public:
       TableEntry* fNext;
       char const* key;
       void* value;
    };

这是操作hashtable的类，里面包括一系列操作函数，下面只列出主要的数据成员：

     TableEntry** fBuckets; // pointer to bucket array
     TableEntry* fStaticBuckets[SMALL_HASH_TABLE_SIZE];// used for small tables
     unsigned fNumBuckets;  //桶的个数，初始化时等于SMALL_HASH_TABLE_SIZE
     unsigned fNumEntries;   //数据entry的个数
     unsigned fRebuildSize;  //当fNumEntries的个数大于等于它时就要重建rebuild，初始化时等于3*SMALL_HASH_TABLE_SIZE
     int fKeyType;  //key的类型

下面具体介绍几个主要的操作：

主要的设计模型：

添加成员部分源代码：

   void* BasicHashTable::Add(char const* key, void* value) {
   void* oldValue;
   unsigned index;
   TableEntry* entry = lookupKey(key, index);
   if (entry != NULL) {
      // There's already an item with this key
      oldValue = entry->value;
     } else {
      // There's no existing entry; create a new one:
      entry = insertNewEntry(index, key);
      oldValue = NULL;
     }
   entry->value = value;

   // If the table has become too large, rebuild it with more buckets:
   if (fNumEntries >= fRebuildSize) rebuild();

   return oldValue;
  }

对应的图：

当添加到一定数量的时候重建：

部分源代码：

    void BasicHashTable::rebuild() {
      // Remember the existing table size:
    unsigned oldSize = fNumBuckets;
    TableEntry** oldBuckets = fBuckets;

    // Create the new sized table:
    fNumBuckets *= 4;
    fBuckets = new TableEntry*[fNumBuckets];
    for (unsigned i = 0; i < fNumBuckets; ++i) {
      fBuckets[i] = NULL;
    }
    fRebuildSize *= 4;
    fDownShift -= 2;
    fMask = (fMask<<2)|0x3;

    // Rehash the existing entries into the new table:
    for (TableEntry** oldChainPtr = oldBuckets; oldSize > 0;
         --oldSize, ++oldChainPtr) {
      for (TableEntry* hPtr = *oldChainPtr; hPtr != NULL;
	   hPtr = *oldChainPtr) {
        *oldChainPtr = hPtr->fNext;

        unsigned index = hashIndexFromKey(hPtr->key);

        hPtr->fNext = fBuckets[index];
        fBuckets[index] = hPtr;
      }
    }

    // Free the old bucket array, if it was dynamically allocated:
    if (oldBuckets != fStaticBuckets) delete[] oldBuckets;
  }

重建图模型：