Live555源码阅读笔记：哈希表的实现

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⏰发布时间⏰： 2024-07-24 23:44:05

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🎄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。

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🎄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 0virtual Boolean Remove(char const* key) = 0;virtual void* Lookup(char const* key) const = 0;virtual unsigned numEntries() const = 0;	// Returns 0 if not foundBoolean 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 noneprotected: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
};

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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。

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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;

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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。

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🎄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; // forwardpublic: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 happyclass Iterator: public HashTable::Iterator {// 迭代器，用于迭代传入的哈希表 2024-07-22 23:43:05public:Iterator(BasicHashTable const& table);private: // implementation of inherited pure virtual functionsvoid* next(char const*& key); // returns 0 if noneprivate:BasicHashTable const& fTable;unsigned fNextIndex; // index of next bucket to be enumerated after thisTableEntry* 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 2024-07-24virtual void* Add(char const* key, void* value);// Returns the old value if different, otherwise 0virtual Boolean Remove(char const* key);virtual void* Lookup(char const* key) const;// Returns 0 if not foundvirtual 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 noneBoolean 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 tablevoid 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 increasesunsigned hashIndexFromKey(char const* key) const;// used to implement many of the routines aboveunsigned randomIndex(uintptr_t i) const {return (unsigned)(((i*1103515245) >> fDownShift) & fMask);}private:TableEntry** fBuckets; // pointer to bucket arrayTableEntry* fStaticBuckets[SMALL_HASH_TABLE_SIZE];// used for small tablesunsigned 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;
};

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✨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 keyoldValue = 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。

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✨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 entrydeleteEntry(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 DEBUGfprintf(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;}
}

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✨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 entryreturn entry->value;
}

通过key找到键值对entry，然后将value返回。

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✨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;
}

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🎄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：

// 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;
}

🎄五、总结

👉本文介绍了Live555的哈希表实现，最后给出了使用例子，对于想了解哈希表实现或Live555源码的同学有一定的帮助。

如果文章有帮助的话，点赞👍、收藏⭐，支持一波，谢谢 😁😁😁