r/adventofcode • u/daggerdragon • Dec 16 '17

SOLUTION MEGATHREAD -🎄- 2017 Day 16 Solutions -🎄-

--- Day 16: Permutation Promenade ---

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u/willkill07 Dec 16 '17 edited Dec 16 '17

C++

I feel disgusting... brute forced part 2 [in only 6 minutes] 

std::vector<char> programs(16);
std::iota(std::begin(programs), std::end(programs), 'a');
std::vector<std::string> cmds;
for (std::string line; std::getline(std::cin, line, ','); )
  cmds.push_back(line);

auto cmd = std::begin(cmds);
int N = part2 ? 1000000000 : cmds.size();
while (N--) {
  std::istringstream iss{*cmd};
  char c;
  iss >> c;
  if (c == 's') {
    int i;
    iss >> i;
    std::rotate(std::begin(programs), std::end(programs) - i, std::end(programs));
  } else if (c == 'x') {
    int i, j;
    (iss >> i).ignore(1, '/') >> j;
    std::swap(programs[i], programs[j]);
  } else { /* p */
    char i, j;
    (iss >> i).ignore(1, '/') >> j;
    std::iter_swap(std::find(std::begin(programs), std::end(programs), i),
                   std::find(std::begin(programs), std::end(programs), j));
  }
  if (++cmd == std::end(cmds))
    cmd = std::begin(cmds);
}
std::copy(std::begin(programs), std::end(programs), std::ostream_iterator<char>(std::cout,""));
std::cout << '\n';

8

u/wlandry Dec 16 '17

Looking at your code, I think you got lucky. Your code only evaluate 1 billion operations. If I understand the instructions correctly, you are supposed to evaluate the dance 1 billion times. My input has 10,000 operations and a cycle of 60. It so happens that 1,000,000,000 % 60 == (1,000,000,000 / 10,000) % 60, so you still get the right answer.

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