//********************************************************
// The following code example is taken from the book
//  C++17 - The Complete Guide
//  by Nicolai M. Josuttis (www.josuttis.com)
//  http://www.cppstd17.com
//
// The code is licensed under a
//  Creative Commons Attribution 4.0 International License
//  http://creativecommons.org/licenses/by/4.0/
//********************************************************


#include <vector>
#include <iostream>
#include <numeric>     // for transform_reduce()
#include <execution>   // for the execution policy
#include <filesystem>  // filesystem library
#include <cstdlib>     // for EXIT_FAILURE

int main(int argc, char* argv[])
{
  // root directory is passed as command line argument:
  if (argc < 2) {
    std::cout << "Usage: " << argv[0] << " <path> \n";
    return EXIT_FAILURE;
  }
  std::filesystem::path root{argv[1]};

  // init list of all file paths in passed file tree:
  std::vector<std::filesystem::path> paths;
  try {
    std::filesystem::recursive_directory_iterator dirpos{root};
    std::copy(begin(dirpos), end(dirpos),
              std::back_inserter(paths));
  }
  catch (const std::exception& e) {
    std::cerr << "EXCEPTION: " << e.what() << std::endl;
    return EXIT_FAILURE;
  }

  // accumulate size of all regular files:
  auto sz = std::transform_reduce(
                   std::execution::par,                 // parallel execution
                   paths.cbegin(), paths.cend(),        // range
                   std::uintmax_t{0},                   // initial value
                   std::plus<>(),                       // accumulate ...
                   [](const std::filesystem::path& p) { //  file size if regular file
                     return is_regular_file(p) ? file_size(p)
                                               : std::uintmax_t{0};
                   }); 
  std::cout << "size of all " << paths.size()
            << " regular files: " << sz << '\n';
}