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hashtable
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二叉搜索树具有对数平均时间的表现，它建立在输入数据有足够的随机性的假设
hashtable 有常数平均时间的表现，基于统计，不需依赖输入元素的随机性


hashtalbe 的简单实现：
所有元素都 16-bits 不带正负号的整数，范围 0~65535，配置一个 array，索引号码为 0~65535，初值全部为 0 。
每一个元素值表示索引号对应的元素值出现的次数。
每插入元素 i ，执行 array[i]++; 每删除元素 i ，执行 array[i]--;
搜索元素 i ，只要检查 array[i] 是否为 0 
排序时，只要遍历 array ，输入 array[i] 个 i 
问题：
1.如果元素是字符串(或其它)而非整数，将无法被拿来作为 array 的索引
2.如果元素是 32-bits，需要的索引数是 2^32;
解决：
问题1
整数是由数字组成的，字符串是由字符组成的。
在整数的时候，索引取的是各个数字的十进制组合表示。 如 1234 取索引 1*10^3 + 2*10^2 + 3*10^1 + 4*10^0
在字符串的时候，索引同样也可以取各个字符的ASCII编码组合表示。如 hou 可以取索引 ‘h‘*128^2 + ‘o‘*128^1 + ‘u‘*128^0 
问题2
采用hash function 将元素值映射到大小可接受的索引范围
-->问题：不同元素被映射到相同的位置
-->解决：线性探测、二次探测、开链   
线性、二次指的是碰撞时前进的步伐大小
线性 --> H+1,   H+2,   H+3, ...
二次 --> H+1^2, H+2^2, H+3^2, ...
开链法是指在每一个表元素中维护一个 list ，在那个元素上执行元素的插入、搜寻、删除
SGI STL 的 hash table 采用的是开链法


hashtable 的能容纳的元素个数就是 bucket vector 的大小，当超过这个大小时，hashtable就会调用 resize() 函数重新分配大小，
重建新的 hashtable

#ifndef __SGI_STL_INTERNAL_HASHTABLE_H
#define __SGI_STL_INTERNAL_HASHTABLE_H


// Hashtable class, used to implement the hashed associative containers
// hash_set, hash_map, hash_multiset, and hash_multimap.


#include <stl_algobase.h>
#include <stl_alloc.h>
#include <stl_construct.h>
#include <stl_tempbuf.h>
#include <stl_algo.h>
#include <stl_uninitialized.h>
#include <stl_function.h>
#include <stl_vector.h>
#include <stl_hash_fun.h>


__STL_BEGIN_NAMESPACE


// hashtable 元素所维护的链表节点
template <class Value>
struct __hashtable_node
{
  __hashtable_node* next;
  Value val;
};  


template <class Value, class Key, class HashFcn,
          class ExtractKey, class EqualKey, class Alloc = alloc>
class hashtable;


template <class Value, class Key, class HashFcn,
          class ExtractKey, class EqualKey, class Alloc>
struct __hashtable_iterator;


template <class Value, class Key, class HashFcn,
          class ExtractKey, class EqualKey, class Alloc>
struct __hashtable_const_iterator;


//hashtable 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; }
};


//为什么会有个专门的 __hashtable_const_iterator ，用 const __hashtable_iterator 不行吗？
template <class Value, class Key, class HashFcn,
          class ExtractKey, class EqualKey, class Alloc>
struct __hashtable_const_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 const Value& reference;
  typedef const Value* pointer;


  const node* cur;
  const hashtable* ht;


  __hashtable_const_iterator(const node* n, const hashtable* tab)
    : cur(n), ht(tab) {}
  __hashtable_const_iterator() {}
  __hashtable_const_iterator(const iterator& it) : cur(it.cur), ht(it.ht) {}
  reference operator*() const { return cur->val; }
#ifndef __SGI_STL_NO_ARROW_OPERATOR
  pointer operator->() const { return &(operator*()); }
#endif /* __SGI_STL_NO_ARROW_OPERATOR */
  const_iterator& operator++();
  const_iterator operator++(int);
  bool operator==(const const_iterator& it) const { return cur == it.cur; }
  bool operator!=(const const_iterator& it) const { return cur != it.cur; }
};


//线性探测、二次探测的方法的表大小最好是质数
//开链法的表不需要是质数，不过 SGI STL 仍以质数来设计表的大小
// Note: assumes long is at least 32 bits.
static const int __stl_num_primes = 28;
static const unsigned long __stl_prime_list[__stl_num_primes] =
{
  53,         97,           193,         389,       769,
  1543,       3079,         6151,        12289,     24593,
  49157,      98317,        196613,      393241,    786433,
  1572869,    3145739,      6291469,     12582917,  25165843,
  50331653,   100663319,    201326611,   402653189, 805306457, 
  1610612741, 3221225473ul, 4294967291ul
};


inline unsigned long __stl_next_prime(unsigned long n)
{
  const unsigned long* first = __stl_prime_list;
  const unsigned long* last = __stl_prime_list + __stl_num_primes;
  const unsigned long* pos = lower_bound(first, last, n); //用 lower_bound 来查找与要设计的表的大小最接近的质数
  return pos == last ? *(last - 1) : *pos;
}




template <class Value, class Key, class HashFcn,
          class ExtractKey, class EqualKey,
          class Alloc>
class hashtable {
public:
  typedef Key key_type;     //键值的类型
  typedef Value value_type; //实值的类型
  typedef HashFcn hasher;   //hash function
  typedef EqualKey key_equal;//判断键值是否相同


  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:
  // 三个 function object
  hasher hash;
  key_equal equals;
  ExtractKey get_key;


  typedef __hashtable_node<Value> node;
  typedef simple_alloc<node, Alloc> node_allocator; //每次分配一个 node 大小的空间


  vector<node*,Alloc> buckets; // 以 vector 表示 buckets 数组，每个 bucket 元素维护一个指向链表的 node * 地址
  size_type num_elements; //hashtable 里的元素个数


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>;


public:
  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);
  }


  hashtable(size_type n,
            const HashFcn&    hf,
            const EqualKey&   eql)
    : hash(hf), equals(eql), get_key(ExtractKey()), num_elements(0)
  {
    initialize_buckets(n);
  }


  hashtable(const hashtable& ht)
    : hash(ht.hash), equals(ht.equals), get_key(ht.get_key), num_elements(0)
  {
    copy_from(ht);
  }


  hashtable& operator= (const hashtable& ht)
  {
    if (&ht != this) {
      clear();
      hash = ht.hash;
      equals = ht.equals;
      get_key = ht.get_key;
      copy_from(ht);
    }
    return *this;
  }


  ~hashtable() { clear(); }


  size_type size() const { return num_elements; }
  size_type max_size() const { return size_type(-1); }
  bool empty() const { return size() == 0; }


  void swap(hashtable& ht)
  {
    __STD::swap(hash, ht.hash);
    __STD::swap(equals, ht.equals);
    __STD::swap(get_key, ht.get_key);
    buckets.swap(ht.buckets);
    __STD::swap(num_elements, ht.num_elements);
  }


  iterator begin()
  { 
    for (size_type n = 0; n < buckets.size(); ++n)
      if (buckets[n])
        return iterator(buckets[n], this);
    return end();
  }


  iterator end() { return iterator(0, this); }


  const_iterator begin() const
  {
    for (size_type n = 0; n < buckets.size(); ++n)
      if (buckets[n])
        return const_iterator(buckets[n], this);
    return end();
  }


  const_iterator end() const { return const_iterator(0, this); }


  friend bool
  operator== __STL_NULL_TMPL_ARGS (const hashtable&, const hashtable&);


public:


  size_type bucket_count() const { return buckets.size(); }


  size_type max_bucket_count() const
    { return __stl_prime_list[__stl_num_primes - 1]; } 


  size_type elems_in_bucket(size_type bucket) const
  {
    size_type result = 0;
    for (node* cur = buckets[bucket]; cur; cur = cur->next)
      result += 1;
    return result;
  }


  //插入元素，不许重复
  pair<iterator, bool> insert_unique(const value_type& obj)
  {
    resize(num_elements + 1); //判断是否需要重建表，如需要就扩充
    return insert_unique_noresize(obj);
  }
	
  //插入元素，允许重复
  iterator insert_equal(const value_type& obj)
  {
    resize(num_elements + 1);
    return insert_equal_noresize(obj);
  }


  pair<iterator, bool> insert_unique_noresize(const value_type& obj);
  iterator insert_equal_noresize(const value_type& obj);
 
#ifdef __STL_MEMBER_TEMPLATES
  template <class InputIterator>
  void insert_unique(InputIterator f, InputIterator l)
  {
    insert_unique(f, l, iterator_category(f));
  }


  template <class InputIterator>
  void insert_equal(InputIterator f, InputIterator l)
  {
    insert_equal(f, l, iterator_category(f));
  }


  template <class InputIterator>
  void insert_unique(InputIterator f, InputIterator l,
                     input_iterator_tag)
  {
    for ( ; f != l; ++f)
      insert_unique(*f);
  }


  template <class InputIterator>
  void insert_equal(InputIterator f, InputIterator l,
                    input_iterator_tag)
  {
    for ( ; f != l; ++f)
      insert_equal(*f);
  }


  template <class ForwardIterator>
  void insert_unique(ForwardIterator f, ForwardIterator l,
                     forward_iterator_tag)
  {
    size_type n = 0;
    distance(f, l, n);
    resize(num_elements + n);
    for ( ; n > 0; --n, ++f)
      insert_unique_noresize(*f);
  }


  template <class ForwardIterator>
  void insert_equal(ForwardIterator f, ForwardIterator l,
                    forward_iterator_tag)
  {
    size_type n = 0;
    distance(f, l, n);
    resize(num_elements + n);
    for ( ; n > 0; --n, ++f)
      insert_equal_noresize(*f);
  }


#else /* __STL_MEMBER_TEMPLATES */
  void insert_unique(const value_type* f, const value_type* l)
  {
    size_type n = l - f;
    resize(num_elements + n);
    for ( ; n > 0; --n, ++f)
      insert_unique_noresize(*f);
  }


  void insert_equal(const value_type* f, const value_type* l)
  {
    size_type n = l - f;
    resize(num_elements + n);
    for ( ; n > 0; --n, ++f)
      insert_equal_noresize(*f);
  }


  void insert_unique(const_iterator f, const_iterator l)
  {
    size_type n = 0;
    distance(f, l, n);
    resize(num_elements + n);
    for ( ; n > 0; --n, ++f)
      insert_unique_noresize(*f);
  }


  void insert_equal(const_iterator f, const_iterator l)
  {
    size_type n = 0;
    distance(f, l, n);
    resize(num_elements + n);
    for ( ; n > 0; --n, ++f)
      insert_equal_noresize(*f);
  }
#endif /*__STL_MEMBER_TEMPLATES */


  reference find_or_insert(const value_type& obj);


  iterator find(const key_type& key) 
  {
    //首先得到键值 key 应该落入的 bucket 
    size_type n = bkt_num_key(key);
    node* first;
	//然后遍历这个 bucket 看看能不能找到有相同 key 的元素
    for ( first = buckets[n];
          first && !equals(get_key(first->val), key);
          first = first->next)
      {}
    return iterator(first, this);
  } 


  const_iterator find(const key_type& key) const
  {
    size_type n = bkt_num_key(key);
    const node* first;
    for ( first = buckets[n];
          first && !equals(get_key(first->val), key);
          first = first->next)
      {}
    return const_iterator(first, this);
  } 


  size_type count(const key_type& key) const
  {
    const size_type n = bkt_num_key(key);
    size_type result = 0;


    for (const node* cur = buckets[n]; cur; cur = cur->next)
      if (equals(get_key(cur->val), key))
        ++result;
    return result;
  }


  pair<iterator, iterator> equal_range(const key_type& key);
  pair<const_iterator, const_iterator> equal_range(const key_type& key) const;


  size_type erase(const key_type& key);
  void erase(const iterator& it);
  void erase(iterator first, iterator last);


  void erase(const const_iterator& it);
  void erase(const_iterator first, const_iterator last);


  void resize(size_type num_elements_hint);
  void clear();


private:
  size_type next_size(size_type n) const { return __stl_next_prime(n); }


  void initialize_buckets(size_type n)
  {
    const size_type n_buckets = next_size(n);
    buckets.reserve(n_buckets);
    buckets.insert(buckets.end(), n_buckets, (node*) 0);
    num_elements = 0;
  }


  // 接受键值， 返回 key 位于哪个 bucket
  size_type bkt_num_key(const key_type& key) const
  {
    return bkt_num_key(key, buckets.size());
  }
  
  //接受实值， 返回 obj 位于哪个 bucket
  size_type bkt_num(const value_type& obj) const
  {
	// 调用 get_key 得到 key，调用 bkt_num_key 得到 #bucket
    return bkt_num_key(get_key(obj));
  }
  // 接受键值和 bucket 个数， 返回 key 位于哪个 bucket
  size_type bkt_num_key(const key_type& key, size_t n) const
  {
    return hash(key) % n;
  }
  //接受实值和 bucket 个数， 返回 obj 位于哪个 bucket
  size_type bkt_num(const value_type& obj, size_t n) const
  {
    return bkt_num_key(get_key(obj), n);
  }


  node* new_node(const value_type& obj)
  {
    node* n = node_allocator::allocate(); //分配节点空间
    n->next = 0;
    __STL_TRY {
      construct(&n->val, obj); //构造节点
      return n;
    }
    __STL_UNWIND(node_allocator::deallocate(n));
  }
  
  void delete_node(node* n)
  {
    destroy(&n->val); //析构节点
    node_allocator::deallocate(n); //释放空间
  }


  void erase_bucket(const size_type n, node* first, node* last);
  void erase_bucket(const size_type n, node* last);


  void copy_from(const hashtable& ht);


};


template <class V, class K, class HF, class ExK, class EqK, class A>
__hashtable_iterator<V, K, HF, ExK, EqK, A>&
__hashtable_iterator<V, K, HF, ExK, EqK, A>::operator++()
{
  const node* old = cur;
  cur = cur->next; //如果当前迭代器所指的链表节点有下一个节点，那就将迭代器指向那个节点。否则就要跳到下一个 bucket 了
  if (!cur) {
    size_type bucket = ht->bkt_num(old->val); //定位当前 bucket 的下一个 
    while (!cur && ++bucket < ht->buckets.size()) //找到当前 bucket 之后的第一个不为空的 bucket
      cur = ht->buckets[bucket]; //让 cur 指向 bucket 的头节点
  }
  return *this;
}


//如非必要，还是使用前置 operator++ 吧，后置的编译器要帮它生成和个 int 参数，运行时要产生临时对象
//内部还是调用前置 operator++ 实现的，返回的时候又要调用拷贝构造函数，还要析构掉之前生成的临时对象
template <class V, class K, class HF, class ExK, class EqK, class A>
inline __hashtable_iterator<V, K, HF, ExK, EqK, A>
__hashtable_iterator<V, K, HF, ExK, EqK, A>::operator++(int)
{
  iterator tmp = *this;
  ++*this;
  return tmp;
}


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);
    while (!cur && ++bucket < ht->buckets.size())
      cur = ht->buckets[bucket];
  }
  return *this;
}


template <class V, class K, class HF, class ExK, class EqK, class A>
inline __hashtable_const_iterator<V, K, HF, ExK, EqK, A>
__hashtable_const_iterator<V, K, HF, ExK, EqK, A>::operator++(int)
{
  const_iterator tmp = *this;
  ++*this;
  return tmp;
}


#ifndef __STL_CLASS_PARTIAL_SPECIALIZATION


template <class V, class K, class HF, class ExK, class EqK, class All>
inline forward_iterator_tag
iterator_category(const __hashtable_iterator<V, K, HF, ExK, EqK, All>&)
{
  return forward_iterator_tag();
}


template <class V, class K, class HF, class ExK, class EqK, class All>
inline V* value_type(const __hashtable_iterator<V, K, HF, ExK, EqK, All>&)
{
  return (V*) 0;
}


template <class V, class K, class HF, class ExK, class EqK, class All>
inline hashtable<V, K, HF, ExK, EqK, All>::difference_type*
distance_type(const __hashtable_iterator<V, K, HF, ExK, EqK, All>&)
{
  return (hashtable<V, K, HF, ExK, EqK, All>::difference_type*) 0;
}


template <class V, class K, class HF, class ExK, class EqK, class All>
inline forward_iterator_tag
iterator_category(const __hashtable_const_iterator<V, K, HF, ExK, EqK, All>&)
{
  return forward_iterator_tag();
}


template <class V, class K, class HF, class ExK, class EqK, class All>
inline V* 
value_type(const __hashtable_const_iterator<V, K, HF, ExK, EqK, All>&)
{
  return (V*) 0;
}


template <class V, class K, class HF, class ExK, class EqK, class All>
inline hashtable<V, K, HF, ExK, EqK, All>::difference_type*
distance_type(const __hashtable_const_iterator<V, K, HF, ExK, EqK, All>&)
{
  return (hashtable<V, K, HF, ExK, EqK, All>::difference_type*) 0;
}


#endif /* __STL_CLASS_PARTIAL_SPECIALIZATION */


template <class V, class K, class HF, class Ex, class Eq, class A>
bool operator==(const hashtable<V, K, HF, Ex, Eq, A>& ht1,
                const hashtable<V, K, HF, Ex, Eq, A>& ht2)
{
  typedef typename hashtable<V, K, HF, Ex, Eq, A>::node node;
  if (ht1.buckets.size() != ht2.buckets.size())
    return false;
  for (int n = 0; n < ht1.buckets.size(); ++n) {
    node* cur1 = ht1.buckets[n];
    node* cur2 = ht2.buckets[n];
    for ( ; cur1 && cur2 && cur1->val == cur2->val;
          cur1 = cur1->next, cur2 = cur2->next)
      {}
    if (cur1 || cur2)
      return false;
  }
  return true;
}  


#ifdef __STL_FUNCTION_TMPL_PARTIAL_ORDER


template <class Val, class Key, class HF, class Extract, class EqKey, class A>
inline void swap(hashtable<Val, Key, HF, Extract, EqKey, A>& ht1,
                 hashtable<Val, Key, HF, Extract, EqKey, A>& ht2) {
  ht1.swap(ht2);
}


#endif /* __STL_FUNCTION_TMPL_PARTIAL_ORDER */


//在不需要重建表的情况下插入新节点。键值重复的节点不会插入
template <class V, class K, class HF, class Ex, class Eq, class A>
pair<typename hashtable<V, K, HF, Ex, Eq, A>::iterator, bool> 
hashtable<V, K, HF, Ex, Eq, A>::insert_unique_noresize(const value_type& obj)
{
  const size_type n = bkt_num(obj); //决定 obj 就位于哪个 bucket
  node* first = buckets[n]; //令 first 指向 bucket 对应的串行头部


  for (node* cur = first; cur; cur = cur->next) 
    //发现键值与链表同的某节点键值相同，不插入
    if (equals(get_key(cur->val), get_key(obj)))
      return pair<iterator, bool>(iterator(cur, this), false);
  
  //在链表头插入新节点
  node* tmp = new_node(obj);
  tmp->next = first;
  buckets[n] = tmp;
  ++num_elements;
  return pair<iterator, bool>(iterator(tmp, this), true);
}


//在不需要重建表的情况下插入新节点。插入的节点键值可以重复
template <class V, class K, class HF, class Ex, class Eq, class A>
typename hashtable<V, K, HF, Ex, Eq, A>::iterator 
hashtable<V, K, HF, Ex, Eq, A>::insert_equal_noresize(const value_type& obj)
{
  const size_type n = bkt_num(obj); //决定 obj 就位于哪个 bucket
  node* first = buckets[n]; //令 first 指向 bucket 对应的串行头部


  for (node* cur = first; cur; cur = cur->next) 
    //如果发现与链表中的某键值相同，就马上插入，然后返回
    if (equals(get_key(cur->val), get_key(obj))) {
      node* tmp = new_node(obj);
      tmp->next = cur->next;
      cur->next = tmp;
      ++num_elements;
      return iterator(tmp, this);
    }
  //链表中没有相同键值的节点，将新节点插入在链表
  node* tmp = new_node(obj);
  tmp->next = first;
  buckets[n] = tmp;
  ++num_elements;
  return iterator(tmp, this);
}


template <class V, class K, class HF, class Ex, class Eq, class A>
typename hashtable<V, K, HF, Ex, Eq, A>::reference 
hashtable<V, K, HF, Ex, Eq, A>::find_or_insert(const value_type& obj)
{
  resize(num_elements + 1);


  size_type n = bkt_num(obj);
  node* first = buckets[n];


  for (node* cur = first; cur; cur = cur->next)
    if (equals(get_key(cur->val), get_key(obj)))
      return cur->val;


  node* tmp = new_node(obj);
  tmp->next = first;
  buckets[n] = tmp;
  ++num_elements;
  return tmp->val;
}


template <class V, class K, class HF, class Ex, class Eq, class A>
pair<typename hashtable<V, K, HF, Ex, Eq, A>::iterator,
     typename hashtable<V, K, HF, Ex, Eq, A>::iterator> 
hashtable<V, K, HF, Ex, Eq, A>::equal_range(const key_type& key)
{
  typedef pair<iterator, iterator> pii;
  const size_type n = bkt_num_key(key);


  for (node* first = buckets[n]; first; first = first->next) {
    if (equals(get_key(first->val), key)) {
      for (node* cur = first->next; cur; cur = cur->next)
        if (!equals(get_key(cur->val), key))
          return pii(iterator(first, this), iterator(cur, this));
      for (size_type m = n + 1; m < buckets.size(); ++m)
        if (buckets[m])
          return pii(iterator(first, this),
                     iterator(buckets[m], this));
      return pii(iterator(first, this), end());
    }
  }
  return pii(end(), end());
}


template <class V, class K, class HF, class Ex, class Eq, class A>
pair<typename hashtable<V, K, HF, Ex, Eq, A>::const_iterator, 
     typename hashtable<V, K, HF, Ex, Eq, A>::const_iterator> 
hashtable<V, K, HF, Ex, Eq, A>::equal_range(const key_type& key) const
{
  typedef pair<const_iterator, const_iterator> pii;
  const size_type n = bkt_num_key(key);


  for (const node* first = buckets[n] ; first; first = first->next) {
    if (equals(get_key(first->val), key)) {
      for (const node* cur = first->next; cur; cur = cur->next)
        if (!equals(get_key(cur->val), key))
          return pii(const_iterator(first, this),
                     const_iterator(cur, this));
      for (size_type m = n + 1; m < buckets.size(); ++m)
        if (buckets[m])
          return pii(const_iterator(first, this),
                     const_iterator(buckets[m], this));
      return pii(const_iterator(first, this), end());
    }
  }
  return pii(end(), end());
}


template <class V, class K, class HF, class Ex, class Eq, class A>
typename hashtable<V, K, HF, Ex, Eq, A>::size_type 
hashtable<V, K, HF, Ex, Eq, A>::erase(const key_type& key)
{
  const size_type n = bkt_num_key(key);
  node* first = buckets[n];
  size_type erased = 0;


  if (first) {
    node* cur = first;
    node* next = cur->next;
    while (next) {
      if (equals(get_key(next->val), key)) {
        cur->next = next->next;
        delete_node(next);
        next = cur->next;
        ++erased;
        --num_elements;
      }
      else {
        cur = next;
        next = cur->next;
      }
    }
    if (equals(get_key(first->val), key)) {
      buckets[n] = first->next;
      delete_node(first);
      ++erased;
      --num_elements;
    }
  }
  return erased;
}


template <class V, class K, class HF, class Ex, class Eq, class A>
void hashtable<V, K, HF, Ex, Eq, A>::erase(const iterator& it)
{
  if (node* const p = it.cur) {
    const size_type n = bkt_num(p->val);
    node* cur = buckets[n];


    if (cur == p) {
      buckets[n] = cur->next;
      delete_node(cur);
      --num_elements;
    }
    else {
      node* next = cur->next;
      while (next) {
        if (next == p) {
          cur->next = next->next;
          delete_node(next);
          --num_elements;
          break;
        }
        else {
          cur = next;
          next = cur->next;
        }
      }
    }
  }
}


template <class V, class K, class HF, class Ex, class Eq, class A>
void hashtable<V, K, HF, Ex, Eq, A>::erase(iterator first, iterator last)
{
  size_type f_bucket = first.cur ? bkt_num(first.cur->val) : buckets.size();
  size_type l_bucket = last.cur ? bkt_num(last.cur->val) : buckets.size();


  if (first.cur == last.cur)
    return;
  else if (f_bucket == l_bucket)
    erase_bucket(f_bucket, first.cur, last.cur);
  else {
    erase_bucket(f_bucket, first.cur, 0);
    for (size_type n = f_bucket + 1; n < l_bucket; ++n)
      erase_bucket(n, 0);
    if (l_bucket != buckets.size())
      erase_bucket(l_bucket, last.cur);
  }
}


template <class V, class K, class HF, class Ex, class Eq, class A>
inline void
hashtable<V, K, HF, Ex, Eq, A>::erase(const_iterator first,
                                      const_iterator last)
{
  erase(iterator(const_cast<node*>(first.cur),
                 const_cast<hashtable*>(first.ht)),
        iterator(const_cast<node*>(last.cur),
                 const_cast<hashtable*>(last.ht)));
}


template <class V, class K, class HF, class Ex, class Eq, class A>
inline void
hashtable<V, K, HF, Ex, Eq, A>::erase(const const_iterator& it)
{
  erase(iterator(const_cast<node*>(it.cur),
                 const_cast<hashtable*>(it.ht)));
}


//判断是否需要重建表，如需要就扩充
//为什么要 resize 呢？ 底层的 vector 不是会动态增加大小吗？
// --> ？？因为表太大了， 装载率太大，降低了查找效率
template <class V, class K, class HF, class Ex, class Eq, class A>
void hashtable<V, K, HF, Ex, Eq, A>::resize(size_type num_elements_hint)
{
  const size_type old_n = buckets.size();
  //如果元素个数大于 bucket vector 的大小，就重建表
  if (num_elements_hint > old_n) {
    //新表的大小
    const size_type n = next_size(num_elements_hint);
    if (n > old_n) {
	  //临时的表，用来存放新建立的表，之后会和原来的表 swap。
      vector<node*, A> tmp(n, (node*) 0);
      __STL_TRY {
	    //遍历每一个旧 bucket 
        for (size_type bucket = 0; bucket < old_n; ++bucket) {
          node* first = buckets[bucket];
		  //遍历bucket 里的每一个节点
          while (first) {
            size_type new_bucket = bkt_num(first->val, n); //当前节点应落在新 bucket vector 的哪一个 bucket 里
			// 下面四个操作当前节点链接到新 bucket 下的链表前面
            buckets[bucket] = first->next;
            first->next = tmp[new_bucket];
            tmp[new_bucket] = first;
            first = buckets[bucket];          
          }
        }
        buckets.swap(tmp); //和原来的表交换，由编译器收回 tmp。 --> 为什么要 swap 。把 tmp 作为新的 bucket vector 不好吗？
							--> ??这样重建前指向旧表的迭代器现在不至于指向一个不存在的地址 --> 但指向的内容会发生改变吗？
      }
#         ifdef __STL_USE_EXCEPTIONS
      catch(...) {
        for (size_type bucket = 0; bucket < tmp.size(); ++bucket) {
          while (tmp[bucket]) {
            node* next = tmp[bucket]->next;
            delete_node(tmp[bucket]);
            tmp[bucket] = next;
          }
        }
        throw;
      }
#         endif /* __STL_USE_EXCEPTIONS */
    }
  }
}


template <class V, class K, class HF, class Ex, class Eq, class A>
void                  node* first, node* last)
{
  node* cur = buckets[n];
  if (cur == first)
    erase_bucket(n, last);
  else {
    node* next;
    for (next = cur->next; next != first; cur = next, next = cur->next)
      ;
    while (next) {
      cur->next = ne= cur->next;
      --num_elements;
    }
  }
}


template <class V, class K, class HF, class Ex, class Eq, class A>
void 
hashtable<V, K, HF, Ex, Eq, A>::erase_bucket(const size_type n, node* last)
{
  node* cur = buckets[n];
  while (cur != last) {
    node* next = cur->next;
    delete_node(cur);
    cur = next;
    buckets[n] = cur;
    --num_elements;
  }
}


//整体删除
//hashtable 由两部分组成 bucket vector 和 每一个 bucket 维护的 链表
//释放空间是只需要释放动态生成的空间，即每一个 bucket 维护的链表
//bucket vector 的空间当它离开它的作用域时由编译器收回
template <class V, class K, class HF, class Ex, class Eq, class A>
void hashtable<V, K, HF, Ex, Eq, A>::clear()
{
   //遍历每一个 bucket
  for (size_type i = 0; i < buckets.size(); ++i) {
    node* cur = buckets[i];
	//将 bucket list 中的每一个节点删除掉
    while (cur != 0) {
      node* next = cur->next;
      delete_node(cur);
      cur = next;
    }
    buckets[i] = 0;
  }
  num_elements = 0;
}


 
//如果 ht 就是 this 呢？ 这里怎么没有自我复制检查？   
template <class V, class K, class HF, class Ex, class Eq, class A>
void hashtable<V, K, HF, Ex, Eq, A>::copy_from(const hashtable& ht)
{
  //先清除自己的 bucket vector ，这操作是调用 vector::clear。造成所有元素为 0
  buckets.clear();
  //如果自己的空间大于对方，就什么都不做，否则 reserve() 函数会把 vector 的空间变得和 ht.buckets.size() 一样大
  buckets.reserve(ht.buckets.size());
  //将每个 bucket 初始化为 NULL
  buckets.insert(buckets.end(), ht.buckets.size(), (node*) 0);
  __STL_TRY {
    遍历对方的每一个 bucket
    for(size_type i = 0; i < ht.buckets.size(); ++i){
      //如果 bucket 的链表不空，就复制它的每一个节点
	  if(const node* cur = ht.buckets[i]){
        node* copy = new_node(cur->val);
        buckets[i] = copy;


        for (node* next = cur->next; next; cur = next, next = cur->next) {
          copy->next = new_node(next->val);
          copy = copy->next;
        }
      }
    }
    num_elements = ht.num_elements;
  }
  __STL_UNWIND(clear());
}


__STL_END_NAMESPACE


#endif /* __SGI_STL_INTERNAL_HASHTABLE_H */


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