aocpp/day15.cpp

#include <algorithm>
#include <deque>
#include <iostream>
#include <map>
#include <tuple>
#include <utility>
#include <vector>

#include <intcode/intcode.hpp>
#include <intcode/Coord.hpp>
using namespace intcode;

namespace {

using CoordOp = Coord(Coord);
CoordOp* const moves[4] {Up, Down, Left, Right};

auto Interact(Machine & m, ValueType cmd) {
  std::get<Input>(Step(m)).pos = cmd;
  return std::get<Output>(Step(m)).val;
}

auto CounterCommand(ValueType x) -> ValueType {
  return ((x-1)^1)+1;
}

// Depth first search of the maze
auto Explore(Machine m) -> std::map<Coord, ValueType> {
  std::map<Coord, ValueType> world;
  std::vector<ValueType> path {0};
  Coord here {};

  for(;;) {
    while (path.back() < 4) {
      path.back()++;
      auto here_ = moves[path.back() - 1](here);
      auto [it, success] = world.insert({here_,0});
      if (success && 0 != (it->second = Interact(m, path.back()))) {
        here = here_;
        path.push_back(0);
      }
    }

    path.pop_back();
    if (path.empty()) { break; }

    auto rev = CounterCommand(path.back());
    here = moves[rev - 1](here);
    Interact(m, rev);
  }

  return world;
}

// Breadth first search of maze finding shortest path to origin
// and furthest point in the maze from the airsource.
auto Compute12(std::map<Coord, ValueType> world) -> std::pair<int, int> {
  // Find starting coordinate and remove all the walls from the map
  Coord start {};
  for (auto it = world.begin(); it != world.end(); ) {
    switch (it->second) {
      case 2: start = it->first;
      case 0: it = world.erase(it); break;
      default: it++;
    }
  }

  int part1 = -1;
  int part2 = -1;

  for (std::deque<std::pair<int, Coord>> todo {{0,start}};
       !todo.empty();
       todo.pop_front())
  {
    auto && [steps, here] = todo.front();
    if (Coord{0,0} == here) part1 = steps;
    part2 = steps;

    for (auto fn : moves) {
      auto next = fn(here);
      if (world.erase(next)) {
        todo.emplace_back(steps+1, next);
      }
    }
  }
  return {part1, part2};
}

} // namespace

auto main() -> int {
  auto [p1,p2] = Compute12(Explore(Machine{ParseStream(std::cin)}));
  std::cout << "Part 1: " << p1 << std::endl;
  std::cout << "Part 2: " << p2 << std::endl;
}