#include <stdio.h>
#include <iostream>
#include <map>
#include <exception>
#include <stdexcept>
 
int main() {
    try {
        FILE* filein;
        FILE* fileout;
        /*char name[256];
        std::cout << "Enter the opening file's path:" << std::endl;
        std::cin >> name;*/
        /*char nameout[256];
        std::cout << "Enter the saving file's path:" << std::endl;
        std::cin >> nameout;*/
        char name[] = "test1.txt";
        char nameout[] = "resout.txt";
        if ((filein = fopen(name, "r")) == NULL)
            throw std::runtime_error("Can't open the file.");
        if ((fileout = fopen(nameout, "w")) == NULL) {
            fclose(filein);
            throw std::runtime_error("Can't create the file.");
        }
        std::multimap<int, int, std::greater <int>> myMap; //создание контейнера
        char str[1024];
        int len;
        int pos;
        std::string s;
        bool counter = 0;
        while (!feof(filein)) {
            if (fgets(str, 1024, filein)) {
                std::cout << str;
                s = str;
                len = s.length();
                pos = ftell(filein);
                if (len >= 11 && len <= 21)
                    counter++;
                myMap.insert(std::make_pair(len, pos - 1)); //заполнение контейнера
            }
        }
        if (counter == 0) {
            std::cout << '\n';
            fclose(filein);
            fclose(fileout);
            throw std::runtime_error("Required strings are missing.");
        }
        std::cout << std::endl;
        std::multimap<int, int>::iterator it; //объявление итератора
        std::cout << "result:" << std::endl;
        std::multimap<int, int>::iterator itlow, itup; //границы контейнера
        itlow = myMap.lower_bound(21);
        itup = myMap.upper_bound(11);               
        for (it = itlow; it != itup; ++it)
        {
            if ((it == myMap.end()) && (it->first >= 11) && (it->first <= 21)) //без доп условия последняя строка не выводится корректно
            {
                fseek(filein, it->second - it->first + 1, SEEK_SET);
                fgets(str, it->first + 1, filein);
            }
            else
            {
                fseek(filein, it->second - it->first, SEEK_SET);
                fgets(str, it->first, filein);
            }
            fputs(str, fileout);
            fputs("\n", fileout);
            //std::cout << it->first << "=" << it->second << std::endl;
            std::cout << str << std::endl;
        }
        fclose(filein);
        fclose(fileout);
    }
    catch (std::exception& exception)
    {   
        std::cerr << "Standard exception: " << exception.what() << std::endl;
    }
}