Apollo planning 模块（三）：path decider

lane follow场景为例，包含一个stage，每个stage又包含若干个task。在路径决策方面，依次进行lane_change_decider、path_reuse_decider、path_lane_borrow_decider、path_bounds_decider。在路径优化方面，依次进行piecewise_jerk_path_optimizer、path_assessment_decider、path_decider。

 1.lane_change_decider

2.path_reuse_decider

3.path_lane_borrow_decider

Status PathLaneBorrowDecider::Process(
    Frame* const frame, ReferenceLineInfo* const reference_line_info) {
  // Sanity checks.
  CHECK_NOTNULL(frame);
  CHECK_NOTNULL(reference_line_info);

  // skip path_lane_borrow_decider if reused path
  if (FLAGS_enable_skip_path_tasks && reference_line_info->path_reusable()) {
    // for debug
    AINFO << "skip due to reusing path";
    return Status::OK();
  }

  // By default, don't borrow any lane.
  reference_line_info->set_is_path_lane_borrow(false);
  // Check if lane-borrowing is needed, if so, borrow lane.
  if (Decider::config_.path_lane_borrow_decider_config()
          .allow_lane_borrowing() &&
      IsNecessaryToBorrowLane(*frame, *reference_line_info)) {
    reference_line_info->set_is_path_lane_borrow(true);
  }
  return Status::OK();
}

bool PathLaneBorrowDecider::IsNecessaryToBorrowLane(
    const Frame& frame, const ReferenceLineInfo& reference_line_info) {
  auto* mutable_path_decider_status = injector_->planning_context()
                                          ->mutable_planning_status()
                                          ->mutable_path_decider();
  if (mutable_path_decider_status->is_in_path_lane_borrow_scenario()) {
    // If originally borrowing neighbor lane:
    if (mutable_path_decider_status->able_to_use_self_lane_counter() >= 6) {
      // If have been able to use self-lane for some time, then switch to
      // non-lane-borrowing.
      mutable_path_decider_status->set_is_in_path_lane_borrow_scenario(false);
      mutable_path_decider_status->clear_decided_side_pass_direction();
      AINFO << "Switch from LANE-BORROW path to SELF-LANE path.";
    }
  } else {
    // If originally not borrowing neighbor lane:
    ADEBUG << "Blocking obstacle ID["
           << mutable_path_decider_status->front_static_obstacle_id() << "]";
    // ADC requirements check for lane-borrowing:
    if (!HasSingleReferenceLine(frame)) {
      return false;
    }

    //车速降低到规定车速值以下才允许借道:默认值 5 (单位：m/s或km/h???)
    if (!IsWithinSidePassingSpeedADC(frame)) {
      return false;
    }

    // Obstacle condition check for lane-borrowing:遮挡的障碍远离路口才允许借道，否则可能导致在路口时无法驶入正确车道
    if (!IsBlockingObstacleFarFromIntersection(reference_line_info)) {
      return false;
    }
    
    //是否长期阻挡车道：默认值3秒
    if (!IsLongTermBlockingObstacle()) {
      return false;
    }
    //障碍物与目标点距离是否小于一定值？？？
    if (!IsBlockingObstacleWithinDestination(reference_line_info)) {
      return false;
    }
    //是否为可超越车辆：救护车、警车不可超越
    if (!IsSidePassableObstacle(reference_line_info)) {
      return false;
    }

    // switch to lane-borrowing
    // set side-pass direction
    const auto& path_decider_status =
        injector_->planning_context()->planning_status().path_decider();
    if (path_decider_status.decided_side_pass_direction().empty()) {
      // first time init decided_side_pass_direction
      bool left_borrowable;
      bool right_borrowable;

      //根据当前车道左右车道线的类型，检车左右车道是否可以借道（SOLID_YELLOW或者SOLID_WHITE不可以换道）
      CheckLaneBorrow(reference_line_info, &left_borrowable, &right_borrowable);
      if (!left_borrowable && !right_borrowable) {
        mutable_path_decider_status->set_is_in_path_lane_borrow_scenario(false);
        return false;
      } else {
        mutable_path_decider_status->set_is_in_path_lane_borrow_scenario(true);
        if (left_borrowable) {
          mutable_path_decider_status->add_decided_side_pass_direction(
              PathDeciderStatus::LEFT_BORROW);
        }
        if (right_borrowable) {
          mutable_path_decider_status->add_decided_side_pass_direction(
              PathDeciderStatus::RIGHT_BORROW);
        }
      }
    }

    AINFO << "Switch from SELF-LANE path to LANE-BORROW path.";
  }
  return mutable_path_decider_status->is_in_path_lane_borrow_scenario();
}

4.path_bounds_decider