#include <iostream>
   #include <unordered_map>
   #include <string_view>
   #include <fstream>
   #include <sstream>
   using strint_it = std::unordered_map<std::string,int>::iterator;
   std::string_view find_most_useful_string(std::string const & doc)
   {
     // measure all strings in doc in a naive way, finding the one with
   the most repetitions.
     // stop when repetitions * length^2 decreases
     std::unordered_map<std::string_view, int> counts;

     int length = 1;
     std::string_view max_substr;
     for (int length = 1; length < doc.size() / 2; ++ length) {
       int max_count = 0;
       for (int offset = 0; offset < doc.size() - length; ++ offset) {
         std::string_view substr(doc.data() + offset, length);
         if (counts.count(substr)) continue; // already ran thru with this
   one
         int & count = counts[substr]; // reference to count for this
   substr
         for (int offset2 = offset; offset2 < doc.size() - length; ++
   offset2) {
           std::string_view substr2(doc.data() + offset2, length);
           if (substr == substr2) {
             ++ count;
             if (count > max_count) {
               max_count = count;
               max_substr = substr;
             }
           }
         }
       }
       if (max_count > 1)
           std::cout << "len:" << length << " str:" << max_substr << "
   ct:" << max_count << std::endl;
     }
     return max_substr;
   }
   int main()
   {
       std::ifstream stream("useful-string.cpp");
       std::ostringstream sstr;
       sstr << stream.rdbuf();
       find_most_useful_string(sstr.str());
   }