侃侃哈希表

 

 

说到哈希表，相信初通数据结构的人士应该耳熟能详，其相关的结构细节虽然并不繁复，但就快速查找数据而言，该结构优异的性能表现绝对可算一枝独秀，平均情况下O(1)的时间复杂度更是令人心旷神怡 :)，这不，在近几天编写的一个简短程序中，我自己便遇到了需要使用哈希表的情况，由于自己惯于使用MinGW，其中的STL（SGI版本）刚好提供了一个优雅的哈希表的模板实现，名曰hashtable，并在此基础之上进一步构建起了hash_map、hash_multimap、hash_set以及hash_multiset，正好与标准模板库中的map与set容器一一对应，此番作为的确大快人心，可惜的是，作为SGI单独的扩展模块，哈希表现今仍然不在C++标准之列，这不能不令人扼腕叹息，所以即便我在MinGW中将hashtable用的生龙活虎，但只要稍稍转变一下编程环境，譬如转至MS的VS，那么等待我的大抵也就是一大堆的未定义错误，而上述的什么hashtable则更是踪迹全无……虽然有心人士早已提供了很多第三方库(如STLPort)用以解围，但这般编程境况仍然给我带来了些许不和谐之感，总觉着不是太合乎标准正道( 在此严重期待C++新一代标准的早日降临 :) )，没办法，最后想想还是决定走一走重造车轮的荆棘路，自己来实现一个简单的hashtable，当然，追求如STL库中那般的通用性并不是我的编程初衷，相反，简单够用倒是我的编写原则，既然如此，那么事不宜迟，就让我马上动手吧 :)。

 

既然需要编写一个ADT，那么就先让我做一个最简单的哈希表设计，首先哈希函数，以及哈希键值函数，感觉应该以模板参数提供，以此来增加灵活性，具体的当以仿函数(函数对象)的形式实现，而原程序中则应该提供针对部分常用类型的仿函数实现，以方便使用。

 

然后的便是冲突的处理，对于哈希值相同的元素，我本想采用简单的一次线性探测方式，但经过后来的几番实践，发现线性探测的实现方式会引发很多问题，其中对于探测失败的处理尤为恼人，建立公共溢出区或是将原哈希表增长等处理感觉都不是很清晰，所以最终还是改成了链地址法（拉链法），顺便说一句，SGI版本中的哈希实现也是用了这种方法 :)

 

最后就是模块应该提供的外部接口了，首先自然是插入和删除操作，接着便是查找，除了这些必要的功能之外，我想在不甚影响程序整体结构以及效率的情况下仍可以适当添加，不过本着KISS原则，这些就以后再说吧 :)

 

好了，废话到此结束，对于编程这个行当，有时候代码胜于千言，所以接下来便贴出我编写一点代码，总体来说颇为拙劣，希望大家不吝嘲笑，多多海涵 :)

 

 

/* Name: heHashTable.h Copyright: No Copyright Author: Hugo Yu Date: 16-11-09 16:20 Description: Provide A Very Simple Hash Table :) ( Because Of The C++ Standard ... :( ) */ #ifndef HE_HASHTABLE_H #define HE_HASHTABLE_H #include <vector> using std::vector; #include <algorithm> using std::lower_bound; #include <string> using std::string; //prime array static const unsigned long PrimeTable[] = { 53ul, 97ul, 193ul, 389ul, 769ul, 1543ul, 3079ul, 6151ul, 12289ul, 24593ul, 49157ul, 98317ul, 196613ul, 393241ul, 786433ul, 1572869ul, 3145739ul, 6291469ul, 12582917ul, 25165843ul, 50331653ul, 100663319ul, 201326611ul, 402653189ul, 805306457ul, 1610612741ul, 3221225473ul, 4294967291ul }; //the size of prime array static const unsigned long PrimeTableSize = sizeof( PrimeTable ); //get the first prime that is bigger than size unsigned long GetUpperPrime( unsigned long size ) { const unsigned long* first = PrimeTable; const unsigned long* last = PrimeTable + PrimeTableSize; const unsigned long* pos = std::lower_bound( first, last, size ); if( pos != last ) return *pos; else return PrimeTable[PrimeTableSize-1]; } template<typename T> class DefaultHashKey { public: unsigned long operator() ( const T& t ) const { return (unsigned long)t; } }; template<> class DefaultHashKey<std::string>//ELFhash algorithm { public: unsigned long operator() ( const std::string& str ) const { const char* key = str.c_str(); unsigned long h = 0; while( *key ) { h = (h<<4) + *key++; unsigned long g = h & 0xF0000000L; if( g ) h ^= g>>24; h &= ~g; } return h; } }; class DefaultHashFunc { public: unsigned long operator() ( unsigned long hashKey, unsigned long tableSize ) const { return hashKey % tableSize; } }; template<typename T, typename HashKey = DefaultHashKey<T>, typename HashFunc = DefaultHashFunc> class heHashTable { private: template<typename T1> struct ValueList//the value list { T1 value; ValueList* pNext; ValueList():value(T1()),pNext(NULL) {}; ValueList( const T1& val, ValueList* p = NULL ):value(val),pNext(p) {}; private: ValueList( const ValueList& vl ); ValueList& operator = ( const ValueList& vl ); }; public: typedef ValueList<T>* Iterator;//just define simple iterator typedef const ValueList<T>* ConstIterator; public: heHashTable( unsigned long size ); ~heHashTable(); Iterator Insert( const T& val );//return the element's iterator that is inserted Iterator Remove( Iterator it ); Iterator Remove( const T& val );//return the following element's iterator that is removed ConstIterator Find( const T& val ) const;//find the element by iterator Iterator Find( const T& val ); bool IsIn( const T& val ) const;//just for the convience unsigned long Elements() const { return m_num_elements; }//get the elements count private: unsigned long getPos( const T& val ) const;//get the insert position by HashKey and HashFunc vector< ValueList<T>* > m_buckets; unsigned long m_num_elements; HashKey m_hashKey; HashFunc m_hashFunc; }; #endif #ifndef HE_HASHTABLE_CPP #define HE_HASHTABLE_CPP #include "heHashTable.h" //should use memory pool... template<typename T,typename HashKey,typename HashFunc> heHashTable<T,HashKey,HashFunc>::heHashTable<T,HashKey,HashFunc>( unsigned long size ) :m_buckets( GetUpperPrime( size ), NULL ),m_num_elements(0) { } template<typename T,typename HashKey,typename HashFunc> heHashTable<T,HashKey,HashFunc>::~heHashTable<T,HashKey,HashFunc>() { typename std::vector< ValueList<T>* >::iterator it = m_buckets.begin(); for( ; it != m_buckets.end(); ++it ) { if( *it != NULL ) { ValueList<T>* pNxt = (*it)->pNext; cout<<(*it)->value<<endl; delete (*it); while( pNxt ) { pNxt = this->Remove( pNxt ); } } } } template<typename T,typename HashKey,typename HashFunc> typename heHashTable<T,HashKey,HashFunc>::Iterator heHashTable<T,HashKey,HashFunc>::Insert( const T& val ) { unsigned long pos = getPos( val ); if( m_buckets[pos] == NULL ) { m_buckets[pos] = new ValueList<T>( val ); } else { ValueList<T>* pNxt = m_buckets[pos]; m_buckets[pos] = new ValueList<T>( val, pNxt ); } ++m_num_elements; return m_buckets[pos]; } template<typename T,typename HashKey,typename HashFunc> typename heHashTable<T,HashKey,HashFunc>::Iterator heHashTable<T,HashKey,HashFunc>::Remove( Iterator it ) { if( it != NULL ) { ValueList<T>* pNxt = it->pNext; delete it; --m_num_elements; return pNxt; } return NULL; } template<typename T,typename HashKey,typename HashFunc> typename heHashTable<T,HashKey,HashFunc>::Iterator heHashTable<T,HashKey,HashFunc>::Remove( const T& val ) { unsigned long pos = getPos( val ); if( m_buckets[pos]->value == val ) { ValueList<T>* tmp = m_buckets[pos]->pNext; delete m_buckets[pos]; m_buckets[pos] = tmp; --m_num_elements; return m_buckets[pos]; } else { ValueList<T>* pPre = m_buckets[pos]; ValueList<T>* pNxt = m_buckets[pos]->pNext; while( pNxt ) { if( pNxt->value == val ) { pPre->pNext = pNxt->pNext; delete pNxt; --m_num_elements; return pPre->pNext; } pPre = pNxt; pNxt = pNxt->pNext; } } return NULL; } template<typename T,typename HashKey,typename HashFunc> typename heHashTable<T,HashKey,HashFunc>::ConstIterator heHashTable<T,HashKey,HashFunc>::Find( const T& val ) const { unsigned long pos = getPos( val ); if( m_buckets[pos] == NULL || m_buckets[pos]->value == val )//val need to support operator '==‘ return m_buckets[pos]; else { ValueList<T>* pNxt = m_buckets[pos]->pNext; while( pNxt ) { if( pNxt->value == val ) return pNxt; pNxt = pNxt->pNext; } } return NULL; } template<typename T,typename HashKey,typename HashFunc> typename heHashTable<T,HashKey,HashFunc>::Iterator heHashTable<T,HashKey,HashFunc>::Find( const T& val ) { unsigned long pos = getPos( val ); if( m_buckets[pos]->value == val )//val need to support operator '==‘ return m_buckets[pos]; else { ValueList<T>* pNxt = m_buckets[pos]->pNext; while( pNxt ) { if( pNxt->value == val ) return pNxt; pNxt = pNxt->pNext; } } return NULL; } template<typename T,typename HashKey,typename HashFunc> inline bool heHashTable<T,HashKey,HashFunc>::IsIn( const T& val ) const { return this->Find( val ) == NULL ? false : true; } template<typename T,typename HashKey,typename HashFunc> inline unsigned long heHashTable<T,HashKey,HashFunc>::getPos( const T& val ) const { return m_hashFunc( m_hashKey( val ), m_buckets.size() ) ; } #endif