std::vectorについて

下記構文の
if (result.first != d.start || result.second != d.steps)
のd.stepsはどういう意味ですか？

std::pair<unsigned long long, long> longest_collatz(unsigned long long const limit)
{
   long length = 0;
   unsigned long long number = 0;

   std::vector<int> cache(limit + 1, 0);

   for (unsigned long long i = 2; i <= limit; i++) 
   {
      auto n = i;
      long steps = 0;
      while (n != 1 && n >= i) 
      {
         if ((n % 2) == 0) 
            n = n / 2;
         else 
            n = n * 3 + 1;
         steps++;
      }
      cache[i] = steps + cache[n];

      if (cache[i] > length) 
      {
         length = cache[i];
         number = i;
      }
   }

   return std::make_pair(number, length);
}
int main()
{
    struct test_data
    {
        unsigned long long limit;
        unsigned long long start;
        long steps;
    };

    std::vector<test_data> data
    {
       { 10ULL, 9ULL, 19 },
       { 100ULL, 97ULL, 118 },
       { 1000ULL, 871ULL, 178 },
       { 10000ULL, 6171ULL, 263 },
       { 100000ULL, 77031ULL, 350 },
       { 1000000ULL, 837799ULL, 524 },
       { 10000000ULL, 8400511ULL, 685 },
       { 100000000ULL, 63728127ULL, 949 }
    };

    for (auto const& d : data)
    {
        auto result = longest_collatz(d.limit);

        if (result.first != d.start || result.second != d.steps)
            std::cout << "error on limit " << d.limit << std::endl;
        else
            std::cout
            << "less than      : " << d.limit << std::endl
            << "starting number: " << result.first << std::endl
            << "sequence length: " << result.second << std::endl;
    }
}