text: #include <math.h>
  #include <uWS/uWS.h>
  #include <iostream>
  #include <string>
  #include "json.hpp"
  #include "particle_filter.h"
  
  // for convenience
  using nlohmann::json;
  using std::string;
  using std::vector;
  
  // Checks if the SocketIO event has JSON data.
  // If there is data the JSON object in string format will be returned,
  // else the empty string "" will be returned.
  string hasData(string s) {
    auto found_null = s.find("null");
    auto b1 = s.find_first_of("[");
    auto b2 = s.find_first_of("]");
    if (found_null != string::npos) {
      return "";
    } else if (b1 != string::npos && b2 != string::npos) {
      return s.substr(b1, b2 - b1 + 1);
    }
    return "";
  }
  
  int main() {
    uWS::Hub h;
  
    // Set up parameters here
    double delta_t = 0.1;  // Time elapsed between measurements [sec]
    double sensor_range = 50;  // Sensor range [m]
  
    // GPS measurement uncertainty [x [m], y [m], theta [rad]]
    double sigma_pos [3] = {0.3, 0.3, 0.01};
    // Landmark measurement uncertainty [x [m], y [m]]
    double sigma_landmark [2] = {0.3, 0.3};
  
    // Read map data
    Map map;
    if (!read_map_data("../data/map_data.txt", map)) {
      std::cout << "Error: Could not open map file" << std::endl;
      return -1;
    }
  
    // Create particle filter
    ParticleFilter pf;
  
    h.onMessage([&pf,&map,&delta_t,&sensor_range,&sigma_pos,&sigma_landmark]
                (uWS::WebSocket<uWS::SERVER> ws, char *data, size_t length, 
                 uWS::OpCode opCode) {
      // "42" at the start of the message means there's a websocket message event.
      // The 4 signifies a websocket message
      // The 2 signifies a websocket event
      if (length && length > 2 && data[0] == '4' && data[1] == '2') {
        auto s = hasData(string(data));
  
        if (s != "") {
          auto j = json::parse(s);
  
          string event = j[0].get<string>();
          
          if (event == "telemetry") {
            // j[1] is the data JSON object
            if (!pf.initialized()) {
              // Sense noisy position data from the simulator
              double sense_x = std::stod(j[1]["sense_x"].get<string>());
              double sense_y = std::stod(j[1]["sense_y"].get<string>());
              double sense_theta = std::stod(j[1]["sense_theta"].get<string>());
  
              pf.init(sense_x, sense_y, sense_theta, sigma_pos);
            } else {
              // Predict the vehicle's next state from previous 
              //   (noiseless control) data.
              double previous_velocity = std::stod(j[1]["previous_velocity"].get<string>());
              double previous_yawrate = std::stod(j[1]["previous_yawrate"].get<string>());
  
              pf.prediction(delta_t, sigma_pos, previous_velocity, previous_yawrate);
            }
  
            // receive noisy observation data from the simulator
            // sense_observations in JSON format 
            //   [{obs_x,obs_y},{obs_x,obs_y},...{obs_x,obs_y}] 
            vector<LandmarkObs> noisy_observations;
            string sense_observations_x = j[1]["sense_observations_x"];
            string sense_observations_y = j[1]["sense_observations_y"];
  
            vector<float> x_sense;
            std::istringstream iss_x(sense_observations_x);
  
            std::copy(std::istream_iterator<float>(iss_x),
            std::istream_iterator<float>(),
            std::back_inserter(x_sense));
  
            vector<float> y_sense;
            std::istringstream iss_y(sense_observations_y);
  
            std::copy(std::istream_iterator<float>(iss_y),
            std::istream_iterator<float>(),
            std::back_inserter(y_sense));
  
            for (int i = 0; i < x_sense.size(); ++i) {
              LandmarkObs obs;
              obs.x = x_sense[i];
              obs.y = y_sense[i];
              noisy_observations.push_back(obs);
            }
  
            // Update the weights and resample
            pf.updateWeights(sensor_range, sigma_landmark, noisy_observations, map);
            pf.resample();
  
            // Calculate and output the average weighted error of the particle 
            //   filter over all time steps so far.
            vector<Particle> particles = pf.m_particles;
            int num_particles = particles.size();
            double highest_weight = -1.0;
            Particle best_particle;
            double weight_sum = 0.0;
            for (int i = 0; i < num_particles; ++i) {
              if (particles[i].weight > highest_weight) {
                highest_weight = particles[i].weight;
                best_particle = particles[i];
              }
  
              weight_sum += particles[i].weight;
            }
  
            std::cout << "highest w " << highest_weight << std::endl;
            std::cout << "average w " << weight_sum/num_particles << std::endl;
  
            json msgJson;
            msgJson["best_particle_x"] = best_particle.x;
            msgJson["best_particle_y"] = best_particle.y;
            msgJson["best_particle_theta"] = best_particle.theta;
  
            // Optional message data used for debugging particle's sensing 
            //   and associations
            msgJson["best_particle_associations"] = pf.getAssociations(best_particle);
            msgJson["best_particle_sense_x"] = pf.getSenseCoord(best_particle, "X");
            msgJson["best_particle_sense_y"] = pf.getSenseCoord(best_particle, "Y");
  
            auto msg = "42[\"best_particle\"," + msgJson.dump() + "]";
            // std::cout << msg << std::endl;
            ws.send(msg.data(), msg.length(), uWS::OpCode::TEXT);
          }  // end "telemetry" if
        } else {
          string msg = "42[\"manual\",{}]";
          ws.send(msg.data(), msg.length(), uWS::OpCode::TEXT);
        }
      }  // end websocket message if
    }); // end h.onMessage
  
    h.onConnection([&h](uWS::WebSocket<uWS::SERVER> ws, uWS::HttpRequest req) {
      std::cout << "Connected!!!" << std::endl;
    });
  
    h.onDisconnection([&h](uWS::WebSocket<uWS::SERVER> ws, int code, 
                           char *message, size_t length) {
      ws.close();
      std::cout << "Disconnected" << std::endl;
    });
  
    int port = 4567;
    if (h.listen(port)) {
      std::cout << "Listening to port " << port << std::endl;
    } else {
      std::cerr << "Failed to listen to port" << std::endl;
      return -1;
    }
    
    h.run();
  }