top_three_elves.cpp

uint64_t top_three(const std::vector<std::string>& data) {
  auto by_elf = data |
	  // group by elf: range{range{string}}
      std::views::lazy_split(std::string{}) |
	  // sum up the calories for each elf: range{uint64_t}	
      std::views::transform([](const auto& elf) -> uint64_t {
          // std::string -> uint64_t
          auto to_unsigned = 
            [](const auto& in) { return std::stoull(in); };
          // make a view of uint64_t: range{string} -> range{uint64_t}
          auto rng = elf | 
            std::views::transform(to_unsigned) | 
            std::views::common; // std::reduce requires a common range
          // reduce into the sum: range{uint64_t} -> uint64_t
          return std::reduce(rng.begin(), rng.end());
      });
  std::vector<uint64_t> top(3);
  // We cannot sort a view, but can partial sort into a vector of three elements
  std::ranges::partial_sort_copy(by_elf, top, std::greater<>{});
  // Sum up the top three
  return std::reduce(top.begin(), top.end());
}