有什么方法可以更快地读取文件吗？

Question

我正在尝试创建一个程序来查看字符串的所有排列，然后打印出所有有效的单词。我可以获得所有排列，但检查一个单词是否在字典文本文件中大约需要 3 秒。当我尝试 7 个字母时，花了 47:19。有什么方法可以更快地从文件中读取？
任何帮助将不胜感激。

#include <iostream>
#include <fstream>
#include <string>
#include <vector>




bool in(std::string arr, char element)
{
    for (int i = 0; i < arr.size(); i++)
    {
        if (arr[i] == element)
        {
            return true;
        }
    }

    return false;
}


bool inDictionary(std::string str)
{
    std::ifstream dictionary;
    dictionary.open("words.txt");
    if (dictionary.is_open())
    {
        std::string word;
        while (std::getline(dictionary, word))
        {
            if (word == str)
            {
                return true;
            }
        }
    }
    return false;
}




std::string remainingCharacters(std::string orginal, std::string newString)
{
    std::string characters = "";
    for (int i = 0; i < orginal.size(); i++)
    {
        if (!in(newString, orginal[i]))
        {
            characters += orginal[i];
        }
    }
    





    return characters;
}


void combinations(std::string cur, std::string original, std::vector<std::string>& permutations)
{

    for (int i = 0; i < remainingCharacters(original, cur).size(); i++)
    {
        
        permutations.push_back(cur + remainingCharacters(original, cur)[i]);
        combinations(cur + remainingCharacters(original, cur)[i], original, permutations);
    }
}

int main()
{

    std::vector<std::string> permutations;
    combinations("", "wdsrock",  permutations);
    for (int i = 0; i < permutations.size(); i++)
    {
        if (inDictionary(permutations[i]))
        {
            std::cout << permutations[i] << ", ";
        }
    }
;
}
   

Answer 1

有一些方法可以减少读取操作的数量和测试的排列数量：

• 将字典存储在内存中
• 将单词字母与所有可能的排列（字谜）相关联。
• iterate over combination 而不是排列

所以

std::unordered_map<std::string, std::vector<std::string>> read_dictionary()
{
    std::ifstream dictionary;
    dictionary.open("words.txt");
    if (!dictionary.is_open()) { throw std::runtime_error("No dictionary"); }
    std::unordered_map<std::string, std::vector<std::string>> res;
    std::string word;
    while (std::getline(dictionary, word))
    {
        auto anagram = word;
        std::sort(anagram.begin(), anagram.end());

        res[anagram].push_back(word);
    }
    return res;
}

template <typename Iterator>
bool next_combination(const Iterator first, Iterator k, const Iterator last)
{
    /* Credits: Thomas Draper */
    if ((first == last) || (first == k) || (last == k))
        return false;
     Iterator itr1 = first;
     Iterator itr2 = last;
     ++itr1;
     if (last == itr1)
         return false;
     itr1 = last;
     --itr1;
     itr1 = k;
     --itr2;
     while (first != itr1)
     {
         if (*--itr1 < *itr2)
         {
             Iterator j = k;
             while (!(*itr1 < *j)) ++j;
             std::iter_swap(itr1,j);
             ++itr1;
             ++j;
             itr2 = k;
             std::rotate(itr1,j,last);
             while (last != j)
             {
                 ++j;
                 ++itr2;
             }
             std::rotate(k,itr2,last);
             return true;
        }
    }
    std::rotate(first,k,last);
    return false;
}

int main()
{
    const auto dictionary = read_dictionary();
    std::string letters = "dsrock";
    std::sort(letters.begin(), letters.end());

    for (std::size_t i = 1; i != letters.length() + 1; ++i) {
        do {
            auto it = dictionary.find(letters.substr(0, i));
            if (it != dictionary.end()) {
                for (const auto& word : it->second) {
                    std::cout << word << std::endl;
                }
            }
        } while (next_combination(letters.begin(), letters.begin() + i, letters.end()));
    }
}

Demo

Answer 2

1. 最好的方法是将文件映射到内存并读取它。 Boost 库提供 API 来读取内存映射文件

#include <iostream>
#include <boost/iostreams/device/mapped_file.hpp>
using namespace std;
int main()
{
boost::iostreams::mapped_file_params arg;
arg.path = "yourfile.txt";
arg.new_file_size = pow(1024, 2); // 1 MB
boost::iostreams::mapped_file::mapmode::readonly;
boost::iostreams::mapped_file mf;
mf.open(arg);
char* bytes = (char*) mf.const_data();
cout << bytes << endl;
mf.close();
return 0;
}

1. 如果您不想使用内存映射文件，另一种方法是读取数据块。读取数据块比单独读取字符更快

ifstream is(filename,readmode);
if (is) {
char* buffer = new char[length+1];
is.read(buffer, length);
buffer[length] = '\0';
is.close();