raycaster/scratchpad/a_star.cpp

constexpr const float SCORE_MAX = std::numeric_limits<float>::max()

using AStarPath = std::deque<Point>;

void update_map(Matrix& map, std::deque<Point>& total_path) {
  for(auto &point : total_path) {
    map[point.y][point.x] = 10;
  }
}

AStarPath reconstruct_path(std::unordered_map<Point, Point>& came_from, Point current) {
  std::deque<Point> total_path{current};

  while(came_from.contains(current)) {
    current = came_from[current];
    total_path.push_front(current);
  }

  return total_path;
}

inline float h(Point from, Point to) {
  return std::hypot(float(from.x) - float(to.x),
      float(from.y) - float(to.y));
}

inline float d(Point current, Point neighbor) {
  return std::hypot(float(current.x) - float(neighbor.x),
      float(current.y) - float(neighbor.y));
}

inline Point find_lowest(std::unordered_map<Point, float>& open_set) {
  dbc::check(!open_set.empty(), "open set can't be empty in find_lowest");
  Point result;
  float lowest_score = SCORE_MAX;

  for(auto [point, score] : open_set) {
    if(score < lowest_score) {
      lowest_score = score;
      result = point;
    }
  }

  return result;
}


std::optional<AStarPath> path_to_player(Matrix& map, Point start, Point goal) {
  std::unordered_map<Point, float> open_set;
  std::unordered_map<Point, Point> came_from;
  std::unordered_map<Point, float> g_score;
  g_score[start] = 0;

  open_set[start] = g_score[start] + h(start, goal);

  while(!open_set.empty()) {
    auto current = find_lowest(open_set);

    if(current == goal) {
      return std::make_optional<AStarPath>(reconstruct_path(came_from, current));
    }

    open_set.erase(current);

    for(matrix::compass it{map, current.x, current.y}; it.next();) {
      Point neighbor{it.x, it.y};

      float d_score = d(current, neighbor) + map[it.y][it.x] * SCORE_MAX;
      float tentative_g_score = g_score[current] + d_score;
      float neighbor_g_score = g_score.contains(neighbor) ? g_score[neighbor] : SCORE_MAX;
      if(tentative_g_score < neighbor_g_score) {
        came_from[neighbor] = current;
        g_score[neighbor] = tentative_g_score;
        // open_set gets the fScore
        open_set[neighbor] = tentative_g_score + h(neighbor, goal);
      }
    }
  }

  return std::nullopt;
}