05.cpp

#include <chrono>
#include <iostream>

#include <openssl/md5.h>

using std::string;

string md5(const string& str) {
  static unsigned char hash[MD5_DIGEST_LENGTH];
  static const char characters[] = "0123456789abcdef";

  MD5(reinterpret_cast<const unsigned char*>(str.c_str()), str.size(), hash);

  // faster method of converting to hexadecimal
  string ret;
  ret.reserve(4 * 2);
  auto buf = std::back_inserter(ret);

  for (int i = 0; i < 4; i++) {
    auto byte = hash[i];
    *buf++ = characters[byte >> 4];
    *buf++ = characters[byte & 0x0F];
  }

  return ret;
}

int main() {
  auto tstart = std::chrono::high_resolution_clock::now();

  string input;
  std::getline(std::cin, input);

  char pt1[9] = {0};
  char pt2[9] = {0};

  int pi = 0;
  int pl = 0;

  for (int i = 0;; i++) {
    string r = md5(input + std::to_string(i));

    if (r.compare(0, 5, "00000") == 0) {
      if (pi < 8) {
        pt1[pi++] = r[5];
      }

      int pi2 = r[5] - '0';
      if (pl < 8 && pi2 >= 0 && pi2 < 8 && pt2[pi2] == 0) {
        pt2[r[5] - '0'] = r[6];
        pl += 1;
      }
    }

    if (pi >= 8 && pl >= 8) {
      break;
    }
  }

  std::cout << "--- Day 5: How About a Nice Game of Chess? ---\n";
  std::cout << "Part 1: " << pt1 << "\n";
  std::cout << "Part 2: " << pt2 << "\n";

  auto tstop = std::chrono::high_resolution_clock::now();
  auto duration =
      std::chrono::duration_cast<std::chrono::microseconds>(tstop - tstart);
  std::cout << "Time: " << duration.count() << " μs"
            << "\n";
  return EXIT_SUCCESS;
}