Live555源码阅读笔记：哈希表的实现(C++)_业界新闻

发布时间:2024-07-29 18:42

阅读量:0

😁博客主页😁：🚀https://blog.csdn.net/wkd_007🚀
🤑博客内容🤑：🍭嵌入式开发、Linux、C语言、C++、数据结构、音视频🍭
🤣本文内容🤣：🍭介绍哈希表的实现 🍭
😎金句分享😎：🍭你不能选择最好的，但最好的会来选择你——泰戈尔🍭
⏰发布时间⏰： 2024-07-24 23:44:05

本文未经允许，不得转发！！！

🎄一、概述

本文介绍Live555源码的哈希表实现，如果对哈希表不了解的，可以看这篇文章：https://blog.csdn.net/wkd_007/article/details/140655297

哈希表是存储 键值对 的一个数据结构。一个哈希表应该要提供插入元素、删除元素、查询元素的操作，而具体的哈希表实现还会出现哈希函数、处理哈希冲突、键值对相关结构体、比对键值等，我们看哈希表代码时需要格外注意这些实现。

下图是Live555哈希表相关类的关系图：
1、HashTable 是个基类，规范哈希表的接口；
2、BasicHashTable 是哈希表的具体实现；
3、TableEntry 是存储键值对的，同时它还有个指针指向自己，说明这里应该是使用链地址法来处理哈希冲突的。

Live555哈希表相关代码主要在这4个文件：HashTable.hh、HashTable.cpp、BasicHashTable.hh、BasicHashTable.cpp。主要涉及两个类，下面介绍具体实现。

🎄二、HashTable 类

HashTable 类声明：

class HashTable { public:   virtual ~HashTable();      // The following must be implemented by a particular   // implementation (subclass):   static HashTable* create(int keyType);      virtual void* Add(char const* key, void* value) = 0;  // Returns the old value if different, otherwise 0   virtual Boolean Remove(char const* key) = 0;   virtual void* Lookup(char const* key) const = 0;   virtual unsigned numEntries() const = 0;	// Returns 0 if not found   Boolean IsEmpty() const { return numEntries() == 0; }      // Used to iterate through the members of the table:   class Iterator {   public:     // The following must be implemented by a particular     // implementation (subclass):     static Iterator* create(HashTable const& hashTable);          virtual ~Iterator();          virtual void* next(char const*& key) = 0; // returns 0 if none        protected:     Iterator(); // abstract base class   };      // A shortcut that can be used to successively remove each of   // the entries in the table (e.g., so that their values can be   // deleted, if they happen to be pointers to allocated memory).   void* RemoveNext();      // Returns the first entry in the table.   // (This is useful for deleting each entry in the table, if the entry's destructor also removes itself from the table.)   void* getFirst();     protected:   HashTable(); // abstract base class };

首先，我们看到 HashTable 类声明了4个纯虚函数：

virtual void* Add(char const* key, void* value) = 0;// Returns the old value if different, otherwise 0 virtual Boolean Remove(char const* key) = 0; virtual void* Lookup(char const* key) const = 0; virtual unsigned numEntries() const = 0;

这说明 HashTable 类是个抽象基类，所以它不会实例化对象，同时很可能会使用 HashTable 类的指针或引用去管理其派生类对象。这四个函数对应了哈希表的插入、删除、查询、哈希值总个数 4个操作，这里也规定了子类必须按照这样的声明去实现这几个操作。

其次，注意到static HashTable* create(int keyType);，说明很大可能使用这个函数来创建哈希表，并且这里的返回值是 HashTable*，但HashTable是抽象类，不会有对象，这里的指针应该指向派生类 BasicHashTable 的对象。函数参数 keyType 说明创建哈希表需要指定键值类型，HashTable.hh给出了两个键值类型：

int const STRING_HASH_KEYS = 0; int const ONE_WORD_HASH_KEYS = 1;

再者，HashTable 类声明了一个迭代器类 Iterator ，并且要求传入哈希表引用，应该是用来遍历哈希表的。

最后就是 RemoveNext 操作删除一个哈希表元素，getFirst 操作获取第一个哈希表元素。

🎄三、BasicHashTable 类

BasicHashTable 类是哈希表的具体实现，它实现了哈希表的基本操作：插入、删除、查询。也实现了哈希函数、键值比对、哈希表扩容等函数，不过这些都是隐藏(private)的。

BasicHashTable 类声明如下：

class BasicHashTable: public HashTable { private: 	class TableEntry; // forward  public:   BasicHashTable(int keyType);   virtual ~BasicHashTable();    // Used to iterate through the members of the table:   class Iterator; friend class Iterator; // to make Sun's C++ compiler happy   class Iterator: public HashTable::Iterator {// 迭代器，用于迭代传入的哈希表 2024-07-22 23:43:05   public:     Iterator(BasicHashTable const& table);    private: // implementation of inherited pure virtual functions     void* next(char const*& key); // returns 0 if none    private:     BasicHashTable const& fTable;     unsigned fNextIndex; // index of next bucket to be enumerated after this     TableEntry* fNextEntry; // next entry in the current bucket   };  private: // implementation of inherited pure virtual functions   // 实现继承的纯虚函数，这里是 private 的，也就是说不能通过 BasicHashTable 对象直接调用，但可以通过父类指针或引用来多态调用 2024-07-24   virtual void* Add(char const* key, void* value);   // Returns the old value if different, otherwise 0   virtual Boolean Remove(char const* key);   virtual void* Lookup(char const* key) const;   // Returns 0 if not found   virtual unsigned numEntries() const;  private:   class TableEntry {   public:     TableEntry* fNext;     char const* key;     void* value;   };    TableEntry* lookupKey(char const* key, unsigned& index) const;     // returns entry matching "key", or NULL if none   Boolean keyMatches(char const* key1, char const* key2) const;     // used to implement "lookupKey()"    TableEntry* insertNewEntry(unsigned index, char const* key);     // creates a new entry, and inserts it in the table   void assignKey(TableEntry* entry, char const* key);     // used to implement "insertNewEntry()"    void deleteEntry(unsigned index, TableEntry* entry);   void deleteKey(TableEntry* entry);     // used to implement "deleteEntry()"    void rebuild(); // rebuilds the table as its size increases    unsigned hashIndexFromKey(char const* key) const;     // used to implement many of the routines above    unsigned randomIndex(uintptr_t i) const {     return (unsigned)(((i*1103515245) >> fDownShift) & fMask);   }  private:   TableEntry** fBuckets; // pointer to bucket array   TableEntry* fStaticBuckets[SMALL_HASH_TABLE_SIZE];// used for small tables   unsigned fNumBuckets, fNumEntries, fRebuildSize, fDownShift, fMask;   int fKeyType; };

首先，BasicHashTable类继承自HashTable类，并且实现了继承的全部纯虚函数，所以BasicHashTable类不是抽象类，可以实例化对象。BasicHashTable类有点特别，只提供了创建对象接口(BasicHashTable)、销毁对象接口(~BasicHashTable)，其他函数都是私有的，即使构造了对象，也没有公有接口可以使用，让用户只能通过基类(HashTable)指针或引用去使用基类声明为公有的几个纯虚函数，然后通过多态调用到 BasicHashTable 类的相应接口。

构造函数：

#define REBUILD_MULTIPLIER 3 BasicHashTable::BasicHashTable(int keyType)   : fBuckets(fStaticBuckets), fNumBuckets(SMALL_HASH_TABLE_SIZE),     fNumEntries(0), fRebuildSize(SMALL_HASH_TABLE_SIZE*REBUILD_MULTIPLIER),     fDownShift(28), fMask(0x3), fKeyType(keyType) {   for (unsigned i = 0; i < SMALL_HASH_TABLE_SIZE; ++i) {     fStaticBuckets[i] = NULL;   } }

哈希表最初创建时，只是用个包含3个元素的数组。

在阅读下面小节之前，先了解 BasicHashTable 存储键值对的结构体，除了key、value，还包含了一个 fNext 指针：

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

✨3.1 插入操作

BasicHashTable类插入操作通过 Add 函数实现，代码如下：

// 添加一个键值对到哈希表，2024-07-22 23:49:05 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; }

先查找 key 值是否已存在了，是的话就替换并返回旧的value值，不存在就插入新的键值对（insertNewEntry）。下面看看Add依赖的几个函数：lookupKey、hashIndexFromKey、keyMatches、insertNewEntry、assignKey、rebuild。

lookupKey 函数：根据key找到一个键值对(entry)，下标index

// 根据key找到一个键值对(entry),下标index，2024-07-22 23:36:55 BasicHashTable::TableEntry* BasicHashTable ::lookupKey(char const* key, unsigned& index) const {  TableEntry* entry;  index = hashIndexFromKey(key);   for (entry = fBuckets[index]; entry != NULL; entry = entry->fNext) {    if (keyMatches(key, entry->key)) break;  }   return entry; }

先计算出哈希地址（hashIndexFromKey），再比对键值，找到键值对 entry。

hashIndexFromKey函数：哈希函数，根据key生成哈希地址index

// 哈希函数，根据key生成一个下标 2024-07-22 23:07:25 unsigned BasicHashTable::hashIndexFromKey(char const* key) const {  unsigned result = 0;    if (fKeyType == STRING_HASH_KEYS) {    while (1) {      char c = *key++;      if (c == 0) break;      result += (result<<3) + (unsigned)c;    }    result &= fMask;    printf("result = %d\n",result);  } else if (fKeyType == ONE_WORD_HASH_KEYS) {    result = randomIndex((uintptr_t)key);	  } else {    unsigned* k = (unsigned*)key;    uintptr_t sum = 0;    for (int i = 0; i < fKeyType; ++i) {      sum += k[i];    }    result = randomIndex(sum);  }    return result; }

按照不同键值类型去生成哈希地址。

keyMatches 函数：比对两个键值是否相等，相等返回true。

// 比较两个key是否一致 2024-07-22 23:35:24 Boolean BasicHashTable ::keyMatches(char const* key1, char const* key2) const {  // The way we check the keys for a match depends upon their type:  if (fKeyType == STRING_HASH_KEYS) {    return (strcmp(key1, key2) == 0);  } else if (fKeyType == ONE_WORD_HASH_KEYS) {    return (key1 == key2);  } else {    unsigned* k1 = (unsigned*)key1;    unsigned* k2 = (unsigned*)key2;     for (int i = 0; i < fKeyType; ++i) {      if (k1[i] != k2[i]) return False; // keys differ    }    return True;  } }

insertNewEntry函数：生成一个entry, 插入到链表，并给entry->key赋值

// 生成一个entry, 插入到链表，并给entry->key赋值，2024-07-22 23:34:42 BasicHashTable::TableEntry* BasicHashTable ::insertNewEntry(unsigned index, char const* key) {  TableEntry* entry = new TableEntry();  entry->fNext = fBuckets[index];  fBuckets[index] = entry;   ++fNumEntries;  assignKey(entry, key);   return entry; }

这里使用头插法，将新的entry插入到 fBuckets[index] 的位置。

assignKey函数：根据键值类型、key值，创建一个 Key，并赋值到entry的key字段

// 根据键值类型、key值，创建一个 Key，并赋值到entry的key字段 2024-07-22 21:16:20 void BasicHashTable::assignKey(TableEntry* entry, char const* key) {  // The way we assign the key depends upon its type:  if (fKeyType == STRING_HASH_KEYS) {    entry->key = strDup(key);  } else if (fKeyType == ONE_WORD_HASH_KEYS) {    entry->key = key;  } else if (fKeyType > 0) {    unsigned* keyFrom = (unsigned*)key;    unsigned* keyTo = new unsigned[fKeyType];    for (int i = 0; i < fKeyType; ++i) keyTo[i] = keyFrom[i];     entry->key = (char const*)keyTo;  } }

rebuild 函数：重新建立一个哈希表

// 重新建立一个哈希表，2024-07-22 21:22:08 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; }

当哈希表键值对个数太多时，就会重建哈希表，避免因个数太多降低查询效率。

✨3.2 删除操作

BasicHashTable 的删除操作通过 Remove 函数实现，代码如下：

// 删除一个键值对(entry)，2024-07-22 23:49:08 Boolean BasicHashTable::Remove(char const* key) {   unsigned index;   TableEntry* entry = lookupKey(key, index);   if (entry == NULL) return False; // no such entry    deleteEntry(index, entry);    return True; }

1、先根据key找到键值对entry、index；
2、再删除键值对 (deleteEntry)。

deleteEntry 函数：删除指定下标的一个键值对(Entry)

// 删除指定下标的一个键值对(Entry)，2024-07-22 21:20:18 void BasicHashTable::deleteEntry(unsigned index, TableEntry* entry) {  TableEntry** ep = &fBuckets[index];	// 找到存在数组的位置   Boolean foundIt = False;  while (*ep != NULL) {  // 遍历链表    if (*ep == entry) {      foundIt = True;      *ep = entry->fNext;      break;    }    ep = &((*ep)->fNext);  }   if (!foundIt) { // shouldn't happen #ifdef DEBUG    fprintf(stderr, "BasicHashTable[%p]::deleteEntry(%d,%p): internal error - not found (first entry %p", this, index, entry, fBuckets[index]);    if (fBuckets[index] != NULL) fprintf(stderr, ", next entry %p", fBuckets[index]->fNext);    fprintf(stderr, ")\n"); #endif  }   --fNumEntries;  deleteKey(entry);  delete entry; }

1、先找到存在数组的位置；
2、遍历链表，找到键值对entry；
3、删除键值(deleteKey)；

deleteKey函数：删除一个键值对(Entry)的Key

// 删除一个键值对(Entry)的Key，2024-07-22 21:21:12 void BasicHashTable::deleteKey(TableEntry* entry) {  // The way we delete the key depends upon its type:  if (fKeyType == ONE_WORD_HASH_KEYS) {    entry->key = NULL;  } else {    delete[] (char*)entry->key;    entry->key = NULL;  } }

✨3.3 查询操作

BasicHashTable 的查询操作通过 Lookup 函数实现，代码如下：

// 查找，根据key找到value，2024-07-22 23:48:20 void* BasicHashTable::Lookup(char const* key) const {   unsigned index;   TableEntry* entry = lookupKey(key, index);   if (entry == NULL) return NULL; // no such entry    return entry->value; }

通过key找到键值对entry，然后将value返回。

✨3.4 查询哈希表元素个数

BasicHashTable 的查询元素个数通过 numEntries 函数实现，代码如下：

// 哈希表总的键值对(entry)个数，2024-07-22 23:47:35 unsigned BasicHashTable::numEntries() const {   return fNumEntries; }

🎄四、哈希表的使用

这个小节，写个示例来使用上面介绍的哈希表，完整源码也上传了。下载地址：https://download.csdn.net/download/wkd_007/89576315

// g++ *.cpp -o HashTest  #include <iostream> #include "BasicHashTable.hh" using namespace std;  void printHashTable(HashTable const& hashTable) { 	HashTable::Iterator *itor = HashTable::Iterator::create(hashTable); 	void* value = NULL; 	const char* pKey = NULL; 	while((value = itor->next(pKey)) != NULL) 	{ 		const char* value_str = (const char*)value; 		cout << "Key=" << pKey << ", Value=" << value_str << endl; 	} 	delete itor; }  int main() { 	HashTable *pHashTable = new BasicHashTable(STRING_HASH_KEYS); 	pHashTable->Add("testA",(void*)"testA"); 	pHashTable->Add("testB",(void*)"testB"); 	pHashTable->Add("testC",(void*)"testC"); 	pHashTable->Add("testD",(void*)"testD"); 	pHashTable->Add("testE",(void*)"testE"); 	pHashTable->Add("",(void*)"testF");		// 空字符串也可以  	const char *pTestA = (const char*)pHashTable->Lookup("testA"); 	cout << "testA=" << pTestA << endl; 	cout << endl;  	printHashTable(*pHashTable); 	cout << endl;  	pHashTable->Add("",(void*)"testG");		// 替换空字符串的值 	printHashTable(*pHashTable); 	cout << endl;  	delete pHashTable;  	return 0; }