advent-of-code/2015/puzzle-22-02.cc

#include <algorithm>
#include <cstdlib>
#include <iostream>
#include <limits>
#include <string>
#include <vector>

using namespace std::string_literals;

struct State
{
  State(unsigned long enemy_hp, unsigned long enemy_damage)
      : enemy_hp_(enemy_hp), enemy_damage_(enemy_damage)
  {
  }

  unsigned long my_hp_{50};
  unsigned long my_mana_{500};
  unsigned long my_armour_{0};
  unsigned long enemy_hp_;
  unsigned long enemy_damage_;
  unsigned long shield_counter_{0};
  unsigned long poison_counter_{0};
  unsigned long recharge_counter_{0};
  unsigned long cost_{0};

  bool operator==(State const& rhs) const
  {
    return my_hp_ == rhs.my_hp_ && my_mana_ == rhs.my_mana_ && my_armour_ == rhs.my_armour_ &&
           enemy_hp_ == rhs.enemy_hp_ && enemy_damage_ == rhs.enemy_damage_ &&
           shield_counter_ == rhs.shield_counter_ && poison_counter_ == rhs.poison_counter_ &&
           recharge_counter_ == rhs.recharge_counter_;
  }

  bool apply_effects()
  {
    if (shield_counter_ > 0) {
      if (--shield_counter_ == 0) {
        my_armour_ -= 7;
      }
    }
    if (poison_counter_ > 0) {
      hit_enemy(3);
      --poison_counter_;
    }
    if (recharge_counter_ > 0) {
      my_mana_ += 101;
      --recharge_counter_;
    }
    return enemy_hp_ == 0;
  }

  void hit_enemy(unsigned long damage)
  {
    enemy_hp_ = (damage > enemy_hp_) ? 0 : enemy_hp_ - damage;
  }

  void hit_me()
  {
    auto amt{my_armour_ >= enemy_damage_ ? 1 : enemy_damage_ - my_armour_};
    my_hp_ = (amt > my_hp_) ? 0 : my_hp_ - amt;
  }

  void spend_mana(unsigned long amt)
  {
    assert(my_mana_ >= amt);
    my_mana_ -= amt;
    cost_ += amt;
  }

  void enemy_turn()
  {
    if (!apply_effects()) {
      hit_me();
    }
  }

  [[nodiscard]] State magic_missile() const
  {
    State next{*this};
    next.spend_mana(53);
    next.hit_enemy(4);
    next.enemy_turn();
    return next;
  }

  [[nodiscard]] State drain() const
  {
    State next{*this};
    next.spend_mana(73);
    next.hit_enemy(2);
    next.my_hp_ += 2;
    next.enemy_turn();
    return next;
  }

  [[nodiscard]] State shield() const
  {
    State next{*this};
    assert(shield_counter_ == 0);
    next.spend_mana(113);
    next.my_armour_ += 7;
    next.shield_counter_ = 6;
    next.enemy_turn();
    return next;
  }

  [[nodiscard]] State poison() const
  {
    State next{*this};
    assert(poison_counter_ == 0);
    next.spend_mana(173);
    next.poison_counter_ = 6;
    next.enemy_turn();
    return next;
  }

  [[nodiscard]] State recharge() const
  {
    State next{*this};
    assert(recharge_counter_ == 0);
    next.spend_mana(229);
    next.recharge_counter_ = 5;
    next.enemy_turn();
    return next;
  }
};

unsigned long get_value(std::string const& begin)
{
  std::string line;
  if (!std::getline(std::cin, line)) {
    std::cerr << "Missing line in input\n";
    std::exit(1);
  }
  if (line.substr(0, begin.length()) != begin) {
    std::cerr << "Line doesn't begin with: " << begin << '\n';
    std::exit(1);
  }
  return std::stoul(line.substr(begin.length()));
}

/** \brief  Add \a state to \a states.
 *
 * @param states Vector of states
 * @param state  State to add.
 *
 * If there is already an element in \a states that is the same as \a state we don't add a new
 * state, instead we just update the cost to the minimum.
 */
void add_state(std::vector<State>& states, State const& state)
{
  auto it = std::find(states.begin(), states.end(), state);
  if (it == states.end()) {
    states.push_back(state);
  }
  else if (it->cost_ > state.cost_) {
    it->cost_ = state.cost_;
  }
}

int main()
{
  const std::string hp_begin{"Hit Points: "};
  const std::string d_begin{"Damage: "};

  auto enemy_hp{get_value(hp_begin)};
  auto enemy_damage{get_value(d_begin)};

  /* States - list of states we've generated.  */
  std::vector<State> states;
  states.emplace_back(enemy_hp, enemy_damage);

  /* Cost of the cheapest winning state so far.  */
  auto best_cost{std::numeric_limits<unsigned long>::max()};

  while (!states.empty()) {
    /* Get the lowest cost element in the list of states and process that.  */
    auto it =
      std::min_element(states.begin(), states.end(),
                       [](State const& lhs, State const& rhs) { return lhs.cost_ < rhs.cost_; });
    State candidate{*it};
    states.erase(it);
    if (candidate.cost_ >= best_cost) {
      /* Because we've searched for the minimum element above we know that all future candidates are
       * going to cost more than the current best cost for winning - so just stop here.  */
      break;
    }

    /* Someone has died so this is a winning state - deal with it.  */
    if (candidate.my_hp_ == 0 || candidate.enemy_hp_ == 0) {
      if (candidate.enemy_hp_ == 0 && candidate.cost_ < best_cost) {
        best_cost = candidate.cost_;
      }
      continue;
    }

    /* Part two's addition - we lose health at the start of our turn.  */
    if (--candidate.my_hp_ == 0) {
      continue;
    }

    /* Apply effects at start of our turn.  If someone dies push this state back on the queue.  */
    if (candidate.apply_effects()) {
      states.push_back(candidate);
    }

    /* Handle each of our options (followed by the enemy's turn).  Making sure that when we add the
     * state we don't duplicate ourselves.
     */
    if (candidate.my_mana_ >= 53) {
      add_state(states, candidate.magic_missile());
    }

    if (candidate.my_mana_ >= 73) {
      add_state(states, candidate.drain());
    }

    if (candidate.my_mana_ >= 113 && candidate.shield_counter_ == 0) {
      add_state(states, candidate.shield());
    }

    if (candidate.my_mana_ >= 173 && candidate.poison_counter_ == 0) {
      add_state(states, candidate.poison());
    }

    if (candidate.my_mana_ >= 229 && candidate.recharge_counter_ == 0) {
      add_state(states, candidate.recharge());
    }
  }

  std::cout << "Best cost " << best_cost << "\n";
  return 1;
}