dense_hash_map的復雜key的用法

比較了std::map和boost::unordered_map; 后者性能略勝前者。

准備使用dense_hash_map試試。

簡單的例子見http://www.cnblogs.com/dreamcs/articles/1782340.html

官方的例子有

#include <iostream>
#include <google/dense_hash_map>

using google::dense_hash_map;      // namespace where class lives by default
using std::cout;
using std::endl;
using ext::hash;  // or __gnu_cxx::hash, or maybe tr1::hash, depending on your OS

struct eqstr
{
  bool operator()(const char* s1, const char* s2) const
  {
    return (s1 == s2) || (s1 && s2 && strcmp(s1, s2) == 0);
  }
};

int main()
{
  dense_hash_map<const char*, int, hash<const char*>, eqstr> months;
  
  months.set_empty_key(NULL);
  months["january"] = 31;
  months["february"] = 28;
  months["march"] = 31;
  months["april"] = 30;
  months["may"] = 31;
  months["june"] = 30;
  months["july"] = 31;
  months["august"] = 31;
  months["september"] = 30;
  months["october"] = 31;
  months["november"] = 30;
  months["december"] = 31;
  
  cout << "september -> " << months["september"] << endl;
  cout << "april     -> " << months["april"] << endl;
  cout << "june      -> " << months["june"] << endl;
  cout << "november  -> " << months["november"] << endl;
}

　　起初因為很好用，很簡單的例子嘛。但是我的項目里面是一用一對string作為key的，換了一把不好使。

不知道這個dense_hash_map怎么用了，暈菜。

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} Is there an alternative implementation which doesnot use std::pair ?

There is not.  However, sparse_hash_map is a pretty thin wrapper
around sparsehashtable.  You could write your own wrapper around
sparsehashtable with not that much code, and replace pair<> with the
data structure of your choice.  I don't think you'll need to modify
the sparsehashtable code at all.

直到我發現了http://code.google.com/p/google-sparsehash/issues/detail?id=58&can=1，調試了一把，發現可以，於是我猜測單單實現一個比較器不行，必須自己實現一個pair的hash。於是搞定了。

inline unsigned int BKDRHash(const string& str, const string& str2)
{
   unsigned int seed = 131; // 31 131 1313 13131 131313 etc..
   unsigned int hash = 0;

   for(std::size_t i = 0; i < str.size(); i++)
   {
      hash = (hash * seed) + str[i];
   }
   hash = (hash * seed) + '\t';
    for(std::size_t i = 0; i < str2.size(); i++)
   {
      hash = (hash * seed) + str2[i];
   }
   return hash;
}


struct pairKeyHash
{
	size_t operator()(const pair<string,string>& p1) const
	{
		return BKDRHash(p1.first, p2.second);
	}
};

struct pairKeyEq
{
	size_t operator()(const pair<string,string>& p1, const pair<string,string>& p2) const
	{
		return p1 == p2;
	}
};


int main()
{
  dense_hash_map<const string, string,  pairKeyHash, pairKeyEq > test_map;
  test_map.set_empty_key(make_pair("",""));

  test_map[make_pair("1","1")] = "A";

  
  cout << "1 -> " << months[make_pair("1","1")] << endl;
  return 0;
}

搞定了，不過測試起來沒看出這個map有多快，哎，難道是我的hash算法不行！