aocpp/2022/09.cpp

#include <algorithm>
#include <cstdint>
#include <functional>
#include <iostream>
#include <set>
#include <sstream>
#include <tuple>
#include <doctest.h>

#include <aocpp/Coord.hpp>
#include <aocpp/Startup.hpp>

using namespace aocpp;

namespace {

using DirFn = Coord(Coord);

auto ToDir(char c) -> DirFn* {
  switch (c) {
    case 'L': return Left;
    case 'R': return Right;
    case 'D': return Down;
    case 'U': return Up;
    default: throw std::runtime_error{"bad input"};
  }
}

/// @brief Return the sign of the number
/// @param x number
/// @return -1, 0, or 1 matching the sign of the input
auto Sign(std::int64_t x) -> std::int64_t
{
  return (x>0) - (x<0);
}

class Logic {
  Coord knots[10] {}; // Current location of each knot in the rope
  std::set<Coord> s1{{}}, s9{{}};

public:
  auto answer() const -> std::pair<std::size_t, std::size_t>
  {
    return {s1.size(), s9.size()};
  }

  auto move(DirFn* dirFn) -> void
  {
    knots[0] = dirFn(knots[0]);
    
    std::size_t k;
    for (k = 1; k < 10; ++k) {
      auto const delta {knots[k-1] - knots[k]};
      if (NormInf(delta) <= 1) {
        break;
      }
      knots[k] += {Sign(delta.x), Sign(delta.y)};
    }

    // only update the sets when a knot actually moved
    if (k > 1) {
      s1.insert(knots[1]);
      if (k > 9) {
        s9.insert(knots[9]);
      }
    }
  }
};

auto Solve(std::istream & in) -> std::pair<std::size_t, std::size_t>
{
  Logic logic;
  char d;
  std::size_t n;
  while (in >> d >> n) {
    while (n--) { logic.move(ToDir(d)); }
  }
  return logic.answer();
}

} // namespace

TEST_SUITE("2022-09 examples") {
  TEST_CASE("example") {
    std::istringstream in {
      "R 5\n"
      "U 8\n"
      "L 8\n"
      "D 3\n"
      "R 17\n"
      "D 10\n"
      "L 25\n"
      "U 20\n"};
    auto [p1,p2] = Solve(in);
    CHECK(p1 == 88);
    CHECK(p2 == 36);
  }
}

auto Main(std::istream & in, std::ostream & out) -> void
{
  auto const [p1,p2] = Solve(in);
  out << "Part 1: " << p1 << std::endl;
  out << "Part 2: " << p2 << std::endl;
}