关联式容器-基于hash

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5.7 hashtable

1、碰撞问题的解决

线性探测：负载系数在0-1之间（元素个数除以表格大小）。

二次探测：如果位置已被占用，则依序尝试H+1^2, H+2^2, ... , H+i^2，而不是像线性探测那样尝试H+1, H+2 ... , H+i。

开链法：负载系数大于1。

2、哈希表的bucket和node

template <class Value>struct __hashtable_node{__hashtable_node* next;Value val;};

3、哈希表的迭代器

template <class Value, class Key, class HashFcn,          class ExtractKey, class EqualKey, class Alloc>struct __hashtable_iterator {  typedef hashtable<Value, Key, HashFcn, ExtractKey, EqualKey, Alloc>          hashtable;  typedef __hashtable_iterator<Value, Key, HashFcn,                                ExtractKey, EqualKey, Alloc>          iterator;  typedef __hashtable_const_iterator<Value, Key, HashFcn,                                      ExtractKey, EqualKey, Alloc>          const_iterator;  typedef __hashtable_node<Value> node;  typedef forward_iterator_tag iterator_category;  typedef Value value_type;  typedef ptrdiff_t difference_type;  typedef size_t size_type;  typedef Value& reference;  typedef Value* pointer;  node* cur;       //迭代器目前所指的节点  hashtable* ht;   //保持对容器的连结关系（因为可能需要从bucket跳到bucket）  __hashtable_iterator(node* n, hashtable* tab) : cur(n), ht(tab) {}  __hashtable_iterator() {}  reference operator*() const { return cur->val; }#ifndef __SGI_STL_NO_ARROW_OPERATOR  pointer operator->() const { return &(operator*()); }#endif /* __SGI_STL_NO_ARROW_OPERATOR */  iterator& operator++();  iterator operator++(int);  bool operator==(const iterator& it) const { return cur == it.cur; }  bool operator!=(const iterator& it) const { return cur != it.cur; }};

template <class V, class K, class HF, class ExK, class EqK, class A>__hashtable_const_iterator<V, K, HF, ExK, EqK, A>&__hashtable_const_iterator<V, K, HF, ExK, EqK, A>::operator++(){  const node* old = cur;  cur = cur->next;  if (!cur) {    size_type bucket = ht->bkt_num(old->val);  //定位出下一个bucket    while (!cur && ++bucket < ht->buckets.size())      cur = ht->buckets[bucket];  }  return *this;}

4、hashtable的数据结构

template <class Value, class Key, class HashFcn,          class ExtractKey, class EqualKey,          class Alloc>class hashtable {public:  typedef Key key_type;  typedef Value value_type;  typedef HashFcn hasher;    //hash function的函数型别  typedef EqualKey key_equal;    typedef size_t            size_type;  typedef ptrdiff_t         difference_type;  typedef value_type*       pointer;  typedef const value_type* const_pointer;  typedef value_type&       reference;  typedef const value_type& const_reference;  hasher hash_funct() const { return hash; }  key_equal key_eq() const { return equals; }private:  hasher hash;  key_equal equals;      //判断键值相同与否的方法（函数或仿函数）  ExtractKey get_key;    //从节点中取出键值的方法（函数或仿函数）  typedef __hashtable_node<Value> node;  typedef simple_alloc<node, Alloc> node_allocator;  vector<node*,Alloc> buckets;  size_type num_elements;public:  typedef __hashtable_iterator<Value, Key, HashFcn, ExtractKey, EqualKey,                                Alloc>  iterator;  typedef __hashtable_const_iterator<Value, Key, HashFcn, ExtractKey, EqualKey,                                     Alloc>  const_iterator;  friend struct  __hashtable_iterator<Value, Key, HashFcn, ExtractKey, EqualKey, Alloc>;  friend struct  __hashtable_const_iterator<Value, Key, HashFcn, ExtractKey, EqualKey, Alloc>;...};

5、hashtable的构造与内存管理

hashtable(size_type n,          const HashFcn&    hf,          const EqualKey&   eql,          const ExtractKey& ext)  : hash(hf), equals(eql), get_key(ext), num_elements(0){  initialize_buckets(n);}void initialize_buckets(size_type n){  const size_type n_buckets = next_size(n);  //计算出n个元素应当分配多大的哈希表  buckets.reserve(n_buckets);                //buckets是用vector实现的，先预留容量  buckets.insert(buckets.end(), n_buckets, (node*) 0);  num_elements = 0;}

插入分为insert_unique()和insert_equal()。
注意：SGI hashtable的hash仿函数（hash<...>）只能处理short、int、long、char *、const char*、char，不能处理string、float、double。欲处理这些型别，用户必须自行定义它们的hash function。

5.8 hash_set

由于RB-tree有自动排序功能而hashtable没有，所以set的元素有自动排序功能而hash_set没有。除此之外，hash_set的使用方式与set完全相同。

#ifndef __STL_LIMITED_DEFAULT_TEMPLATEStemplate <class Value, class HashFcn = hash<Value>,          class EqualKey = equal_to<Value>,          class Alloc = alloc>#elsetemplate <class Value, class HashFcn, class EqualKey, class Alloc = alloc>#endifclass hash_set{private:  typedef hashtable<Value, Value, HashFcn, identity<Value>,                     EqualKey, Alloc> ht;  ht rep;public:  typedef typename ht::key_type key_type;  typedef typename ht::value_type value_type;  typedef typename ht::hasher hasher;  typedef typename ht::key_equal key_equal;  typedef typename ht::size_type size_type;  typedef typename ht::difference_type difference_type;  typedef typename ht::const_pointer pointer;                 //注意不可以直接修改元素的值  typedef typename ht::const_pointer const_pointer;        typedef typename ht::const_reference reference;  typedef typename ht::const_reference const_reference;  typedef typename ht::const_iterator iterator;  typedef typename ht::const_iterator const_iterator;  hasher hash_funct() const { return rep.hash_funct(); }  key_equal key_eq() const { return rep.key_eq(); }public:  hash_set() : rep(100, hasher(), key_equal()) {}    //缺省使用大小为100的表格，将被hashtable调整为最接近且较大的质数。  explicit hash_set(size_type n) : rep(n, hasher(), key_equal()) {}  hash_set(size_type n, const hasher& hf) : rep(n, hf, key_equal()) {}  hash_set(size_type n, const hasher& hf, const key_equal& eql)    : rep(n, hf, eql) {}  void swap(hash_set& hs) { rep.swap(hs.rep); }  friend bool operator== __STL_NULL_TMPL_ARGS (const hash_set&,                                               const hash_set&);  iterator begin() const { return rep.begin(); }  iterator end() const { return rep.end(); }public:  pair<iterator, bool> insert(const value_type& obj)    {      pair<typename ht::iterator, bool> p = rep.insert_unique(obj);      return pair<iterator, bool>(p.first, p.second);    }  ...};

5.9 hash_map

同样无排序。

#ifndef __STL_LIMITED_DEFAULT_TEMPLATEStemplate <class Key, class T, class HashFcn = hash<Key>,          class EqualKey = equal_to<Key>,          class Alloc = alloc>#elsetemplate <class Key, class T, class HashFcn, class EqualKey,           class Alloc = alloc>#endifclass hash_map{private:  typedef hashtable<pair<const Key, T>, Key, HashFcn,                    select1st<pair<const Key, T> >, EqualKey, Alloc> ht;  ht rep;public:  typedef typename ht::key_type key_type;  typedef T data_type;  typedef T mapped_type;  typedef typename ht::value_type value_type;  typedef typename ht::hasher hasher;  typedef typename ht::key_equal key_equal;  typedef typename ht::size_type size_type;  typedef typename ht::difference_type difference_type;  typedef typename ht::pointer pointer;  typedef typename ht::const_pointer const_pointer;  typedef typename ht::reference reference;  typedef typename ht::const_reference const_reference;  typedef typename ht::iterator iterator;  typedef typename ht::const_iterator const_iterator;  hasher hash_funct() const { return rep.hash_funct(); }  key_equal key_eq() const { return rep.key_eq(); }public:  hash_map() : rep(100, hasher(), key_equal()) {}  explicit hash_map(size_type n) : rep(n, hasher(), key_equal()) {}  hash_map(size_type n, const hasher& hf) : rep(n, hf, key_equal()) {}  hash_map(size_type n, const hasher& hf, const key_equal& eql)    : rep(n, hf, eql) {}  T& operator[](const key_type& key) {    return rep.find_or_insert(value_type(key, T())).second;  }    ...};

5.10 hash_multiset

采用insert_equal()。

5.11 hash_multimap

采用insert_equal()。