node_lattice.cpp

// Copyright (c) 2021, Samsung Research America
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License. Reserved.
 
#include <math.h>
#include <chrono>
#include <vector>
#include <memory>
#include <algorithm>
#include <queue>
#include <limits>
#include <string>
#include <fstream>
#include <cmath>
 
#include "ompl/base/ScopedState.h"
#include "ompl/base/spaces/DubinsStateSpace.h"
#include "ompl/base/spaces/ReedsSheppStateSpace.h"
#include "ompl/base/spaces/SE2StateSpace.h"
 
#include "nav2_smac_planner/node_lattice.hpp"
 
using namespace std::chrono;  // NOLINT
 
namespace nav2_smac_planner
{
 
// defining static member for all instance to share
LatticeMotionTable NodeLattice::motion_table;
float NodeLattice::size_lookup = 25;
LookupTable NodeLattice::dist_heuristic_lookup_table;
 
// Each of these tables are the projected motion models through
// time and space applied to the search on the current node in
// continuous map-coordinates (e.g. not meters but partial map cells)
// Currently, these are set to project *at minimum* into a neighboring
// cell. Though this could be later modified to project a certain
// amount of time or particular distance forward.
void LatticeMotionTable::initMotionModel(
  unsigned int & size_x_in,
  SearchInfo & search_info)
{
  size_x = size_x_in;
  change_penalty = search_info.change_penalty;
  non_straight_penalty = search_info.non_straight_penalty;
  cost_penalty = search_info.cost_penalty;
  reverse_penalty = search_info.reverse_penalty;
  travel_distance_reward = 1.0f - search_info.retrospective_penalty;
  allow_reverse_expansion = search_info.allow_reverse_expansion;
  rotation_penalty = search_info.rotation_penalty;
  min_turning_radius = search_info.minimum_turning_radius;
 
  if (current_lattice_filepath == search_info.lattice_filepath) {
    return;
  }
  current_lattice_filepath = search_info.lattice_filepath;
 
  // Get the metadata about this minimum control set
  lattice_metadata = getLatticeMetadata(current_lattice_filepath);
  std::ifstream latticeFile(current_lattice_filepath);
  if (!latticeFile.is_open()) {
    throw std::runtime_error("Could not open lattice file");
  }
  nlohmann::json json;
  latticeFile >> json;
  num_angle_quantization = lattice_metadata.number_of_headings;
 
  if (!state_space) {
    if (lattice_metadata.motion_model == "omni") {
      // Holonomic robots: straight-line analytic expansion
      state_space = std::make_shared<ompl::base::SE2StateSpace>();
      motion_model = MotionModel::OMNI;
    } else if (!allow_reverse_expansion) {
      state_space = std::make_shared<ompl::base::DubinsStateSpace>(
        lattice_metadata.min_turning_radius);
      motion_model = MotionModel::DUBIN;
    } else {
      state_space = std::make_shared<ompl::base::ReedsSheppStateSpace>(
        lattice_metadata.min_turning_radius);
      motion_model = MotionModel::REEDS_SHEPP;
    }
  }
 
  // Populate the motion primitives at each heading angle
  float prev_start_angle = 0.0;
  std::vector<MotionPrimitive> primitives;
  nlohmann::json json_primitives = json["primitives"];
  for (unsigned int i = 0; i < json_primitives.size(); ++i) {
    MotionPrimitive new_primitive;
    fromJsonToMotionPrimitive(json_primitives[i], new_primitive);
 
    if (prev_start_angle != new_primitive.start_angle) {
      motion_primitives.push_back(primitives);
      primitives.clear();
      prev_start_angle = new_primitive.start_angle;
    }
    primitives.push_back(new_primitive);
  }
  motion_primitives.push_back(primitives);
 
  // Populate useful precomputed values to be leveraged
  trig_values.reserve(lattice_metadata.number_of_headings);
  for (unsigned int i = 0; i < lattice_metadata.heading_angles.size(); ++i) {
    trig_values.emplace_back(
      cos(lattice_metadata.heading_angles[i]),
      sin(lattice_metadata.heading_angles[i]));
  }
}
 
MotionPrimitivePtrs LatticeMotionTable::getMotionPrimitives(
  const NodeLattice * node,
  unsigned int & direction_change_index)
{
  MotionPrimitives & prims_at_heading = motion_primitives[node->pose.theta];
  MotionPrimitivePtrs primitive_projection_list;
  for (unsigned int i = 0; i != prims_at_heading.size(); i++) {
    primitive_projection_list.push_back(&prims_at_heading[i]);
  }
 
  // direction change index
  direction_change_index = static_cast<unsigned int>(primitive_projection_list.size());
 
  if (allow_reverse_expansion) {
    // Find normalized heading bin of the reverse expansion
    double reserve_heading = node->pose.theta - (num_angle_quantization / 2);
    if (reserve_heading < 0) {
      reserve_heading += num_angle_quantization;
    }
    if (reserve_heading > num_angle_quantization) {
      reserve_heading -= num_angle_quantization;
    }
 
    MotionPrimitives & prims_at_reverse_heading = motion_primitives[reserve_heading];
    for (unsigned int i = 0; i != prims_at_reverse_heading.size(); i++) {
      primitive_projection_list.push_back(&prims_at_reverse_heading[i]);
    }
  }
 
  return primitive_projection_list;
}
 
LatticeMetadata LatticeMotionTable::getLatticeMetadata(const std::string & lattice_filepath)
{
  std::ifstream lattice_file(lattice_filepath);
  if (!lattice_file.is_open()) {
    throw std::runtime_error("Could not open lattice file!");
  }
 
  nlohmann::json j;
  lattice_file >> j;
  LatticeMetadata metadata;
  fromJsonToMetaData(j["lattice_metadata"], metadata);
  return metadata;
}
 
unsigned int LatticeMotionTable::getClosestAngularBin(const double & theta)
{
  float min_dist = std::numeric_limits<float>::max();
  unsigned int closest_idx = 0;
  float dist = 0.0;
  for (unsigned int i = 0; i != lattice_metadata.heading_angles.size(); i++) {
    dist = fabs(angles::shortest_angular_distance(theta, lattice_metadata.heading_angles[i]));
    if (dist < min_dist) {
      min_dist = dist;
      closest_idx = i;
    }
  }
  return closest_idx;
}
 
float & LatticeMotionTable::getAngleFromBin(const unsigned int & bin_idx)
{
  return lattice_metadata.heading_angles[bin_idx];
}
 
double LatticeMotionTable::getAngle(const double & theta)
{
  return getClosestAngularBin(theta);
}
 
NodeLattice::NodeLattice(const uint64_t index)
: parent(nullptr),
  pose(0.0f, 0.0f, 0.0f),
  _cell_cost(std::numeric_limits<float>::quiet_NaN()),
  _accumulated_cost(std::numeric_limits<float>::max()),
  _index(index),
  _was_visited(false),
  _motion_primitive(nullptr),
  _backwards(false),
  _is_node_valid(false)
{
}
 
NodeLattice::~NodeLattice()
{
  parent = nullptr;
}
 
void NodeLattice::reset()
{
  parent = nullptr;
  _cell_cost = std::numeric_limits<float>::quiet_NaN();
  _accumulated_cost = std::numeric_limits<float>::max();
  _was_visited = false;
  pose.x = 0.0f;
  pose.y = 0.0f;
  pose.theta = 0.0f;
  _motion_primitive = nullptr;
  _backwards = false;
  _is_node_valid = false;
}
 
bool NodeLattice::isNodeValid(
  const bool & traverse_unknown,
  GridCollisionChecker * collision_checker,
  MotionPrimitive * motion_primitive,
  bool is_backwards)
{
  // Already found, we can return the result
  if (!std::isnan(_cell_cost)) {
    return _is_node_valid;
  }
 
  // Check primitive end pose
  // Convert grid quantization of primitives to radians, then collision checker quantization
  static const double bin_size = 2.0 * M_PI / collision_checker->getPrecomputedAngles().size();
  const double angle = std::fmod(motion_table.getAngleFromBin(this->pose.theta),
      2.0 * M_PI) / bin_size;
  if (collision_checker->inCollision(
      this->pose.x, this->pose.y, angle /*bin in collision checker*/, traverse_unknown))
  {
    _is_node_valid = false;
    _cell_cost = collision_checker->getCost();
    return false;
  }
 
  // Set the cost of a node to the highest cost across the primitive
  float max_cell_cost = collision_checker->getCost();
 
  // If valid motion primitives are set, check intermediary poses > 1 cell apart
  if (motion_primitive) {
    const float & grid_resolution = motion_table.lattice_metadata.grid_resolution;
    const float & resolution_diag_sq = 2.0 * grid_resolution * grid_resolution;
    MotionPose last_pose(1e9, 1e9, 1e9, TurnDirection::UNKNOWN);
    MotionPose pose_dist(0.0, 0.0, 0.0, TurnDirection::UNKNOWN);
 
    // Back out the initial node starting point to move motion primitive relative to
    MotionPose initial_pose, prim_pose;
    initial_pose._x = this->pose.x - (motion_primitive->poses.back()._x / grid_resolution);
    initial_pose._y = this->pose.y - (motion_primitive->poses.back()._y / grid_resolution);
    initial_pose._theta = motion_table.getAngleFromBin(motion_primitive->start_angle);
 
    for (auto it = motion_primitive->poses.begin(); it != motion_primitive->poses.end(); ++it) {
      // poses are in metric coordinates from (0, 0), not grid space yet
      pose_dist = *it - last_pose;
      // Avoid square roots by (hypot(x, y) > res) == (x*x+y*y > diag*diag)
      if (pose_dist._x * pose_dist._x + pose_dist._y * pose_dist._y > resolution_diag_sq) {
        last_pose = *it;
        // Convert primitive pose into grid space if it should be checked
        prim_pose._x = initial_pose._x + (it->_x / grid_resolution);
        prim_pose._y = initial_pose._y + (it->_y / grid_resolution);
        // If reversing, invert the angle because the robot is backing into the primitive
        // not driving forward with it
        if (is_backwards) {
          prim_pose._theta = std::fmod(it->_theta + M_PI, 2.0 * M_PI);
        } else {
          prim_pose._theta = std::fmod(it->_theta, 2.0 * M_PI);
        }
        if (collision_checker->inCollision(
            prim_pose._x,
            prim_pose._y,
            prim_pose._theta / bin_size /*bin in collision checker*/,
            traverse_unknown))
        {
          _is_node_valid = false;
          _cell_cost = std::max(max_cell_cost, collision_checker->getCost());
          return false;
        }
        max_cell_cost = std::max(max_cell_cost, collision_checker->getCost());
      }
    }
  }
 
  _cell_cost = max_cell_cost;
  _is_node_valid = true;
  return _is_node_valid;
}
 
float NodeLattice::getTraversalCost(const NodePtr & child)
{
  const float normalized_cost = child->getCost() / 252.0;
  if (std::isnan(normalized_cost)) {
    throw std::runtime_error(
            "Node attempted to get traversal "
            "cost without a known collision cost!");
  }
 
  // this is the first node
  MotionPrimitive * prim = this->getMotionPrimitive();
  MotionPrimitive * transition_prim = child->getMotionPrimitive();
  const float prim_length =
    transition_prim->trajectory_length / motion_table.lattice_metadata.grid_resolution;
  if (prim == nullptr) {
    return prim_length;
  }
 
  // Pure rotation in place 1 angular bin in either direction
  if (transition_prim->trajectory_length < 1e-4) {
    return motion_table.rotation_penalty * (1.0 + motion_table.cost_penalty * normalized_cost);
  }
 
  float travel_cost = 0.0;
  float travel_cost_raw = prim_length *
    (motion_table.travel_distance_reward + motion_table.cost_penalty * normalized_cost);
 
  if (transition_prim->arc_length < 0.001) {
    // New motion is a straight motion, no additional costs to be applied
    travel_cost = travel_cost_raw;
  } else {
    if (prim->left_turn == transition_prim->left_turn) {
      // Turning motion but keeps in same general direction: encourages to commit to actions
      travel_cost = travel_cost_raw * motion_table.non_straight_penalty;
    } else {
      // Turning motion and velocity directions: penalizes wiggling.
      travel_cost = travel_cost_raw *
        (motion_table.non_straight_penalty + motion_table.change_penalty);
    }
  }
 
  // If backwards flag is set, this primitive is moving in reverse
  if (child->isBackward()) {
    // reverse direction
    travel_cost *= motion_table.reverse_penalty;
  }
 
  return travel_cost;
}
 
float NodeLattice::getHeuristicCost(
  const Coordinates & node_coords,
  const Coordinates & goal_coords)
{
  // get obstacle heuristic value
  const float obstacle_heuristic = getObstacleHeuristic(
    node_coords, goal_coords, motion_table.cost_penalty);
  const float distance_heuristic =
    getDistanceHeuristic(node_coords, goal_coords, obstacle_heuristic);
  return std::max(obstacle_heuristic, distance_heuristic);
}
 
void NodeLattice::initMotionModel(
  const MotionModel & motion_model,
  unsigned int & size_x,
  unsigned int & /*size_y*/,
  unsigned int & /*num_angle_quantization*/,
  SearchInfo & search_info)
{
  if (motion_model != MotionModel::STATE_LATTICE) {
    throw std::runtime_error(
            "Invalid motion model for Lattice node. Please select"
            " STATE_LATTICE and provide a valid lattice file.");
  }
 
  motion_table.initMotionModel(size_x, search_info);
}
 
float NodeLattice::getDistanceHeuristic(
  const Coordinates & node_coords,
  const Coordinates & goal_coords,
  const float & obstacle_heuristic)
{
  // rotate and translate node_coords such that goal_coords relative is (0,0,0)
  // Due to the rounding involved in exact cell increments for caching,
  // this is not an exact replica of a live heuristic, but has bounded error.
  // (Usually less than 1 cell length)
 
  // This angle is negative since we are de-rotating the current node
  // by the goal angle; cos(-th) = cos(th) & sin(-th) = -sin(th)
  const TrigValues & trig_vals = motion_table.trig_values[goal_coords.theta];
  const float cos_th = trig_vals.first;
  const float sin_th = -trig_vals.second;
  const float dx = node_coords.x - goal_coords.x;
  const float dy = node_coords.y - goal_coords.y;
 
  double dtheta_bin = node_coords.theta - goal_coords.theta;
  if (dtheta_bin < 0) {
    dtheta_bin += motion_table.num_angle_quantization;
  }
  if (dtheta_bin > motion_table.num_angle_quantization) {
    dtheta_bin -= motion_table.num_angle_quantization;
  }
 
  Coordinates node_coords_relative(
    round(dx * cos_th - dy * sin_th),
    round(dx * sin_th + dy * cos_th),
    round(dtheta_bin));
 
  // Check if the relative node coordinate is within the localized window around the goal
  // to apply the distance heuristic. Since the lookup table is contains only the positive
  // X axis, we mirror the Y and theta values across the X axis to find the heuristic values.
  float motion_heuristic = 0.0;
  const int floored_size = floor(size_lookup / 2.0);
  const int ceiling_size = ceil(size_lookup / 2.0);
  const float mirrored_relative_y = abs(node_coords_relative.y);
  if (abs(node_coords_relative.x) < floored_size && mirrored_relative_y < floored_size) {
    // Need to mirror angle if Y coordinate was mirrored
    int theta_pos;
    if (node_coords_relative.y < 0.0) {
      theta_pos = motion_table.num_angle_quantization - node_coords_relative.theta;
    } else {
      theta_pos = node_coords_relative.theta;
    }
    const int x_pos = node_coords_relative.x + floored_size;
    const int y_pos = static_cast<int>(mirrored_relative_y);
    const int index =
      x_pos * ceiling_size * motion_table.num_angle_quantization +
      y_pos * motion_table.num_angle_quantization +
      theta_pos;
    motion_heuristic = dist_heuristic_lookup_table[index];
  } else if (obstacle_heuristic == 0.0) {
    static ompl::base::ScopedState<> from(motion_table.state_space), to(motion_table.state_space);
    to[0] = goal_coords.x;
    to[1] = goal_coords.y;
    to[2] = motion_table.getAngleFromBin(goal_coords.theta);
    from[0] = node_coords.x;
    from[1] = node_coords.y;
    from[2] = motion_table.getAngleFromBin(node_coords.theta);
    motion_heuristic = motion_table.state_space->distance(from(), to());
  }
 
  return motion_heuristic;
}
 
void NodeLattice::precomputeDistanceHeuristic(
  const float & lookup_table_dim,
  const MotionModel & /*motion_model*/,
  const unsigned int & dim_3_size,
  const SearchInfo & search_info)
{
  motion_table.lattice_metadata =
    LatticeMotionTable::getLatticeMetadata(search_info.lattice_filepath);
 
  // Select state space based on motion model from lattice file
  if (motion_table.lattice_metadata.motion_model == "omni") {
    // Holonomic robots: Euclidean distance heuristic
    motion_table.state_space = std::make_shared<ompl::base::SE2StateSpace>();
    motion_table.motion_model = MotionModel::OMNI;
  } else if (!search_info.allow_reverse_expansion) {
    motion_table.state_space = std::make_shared<ompl::base::DubinsStateSpace>(
      search_info.minimum_turning_radius);
    motion_table.motion_model = MotionModel::DUBIN;
  } else {
    motion_table.state_space = std::make_shared<ompl::base::ReedsSheppStateSpace>(
      search_info.minimum_turning_radius);
    motion_table.motion_model = MotionModel::REEDS_SHEPP;
  }
 
  ompl::base::ScopedState<> from(motion_table.state_space), to(motion_table.state_space);
  to[0] = 0.0;
  to[1] = 0.0;
  to[2] = 0.0;
  size_lookup = lookup_table_dim;
  float motion_heuristic = 0.0;
  unsigned int index = 0;
  int dim_3_size_int = static_cast<int>(dim_3_size);
 
  // Create a lookup table of Dubin/Reeds-Shepp distances in a window around the goal
  // to help drive the search towards admissible approaches. Deu to symmetries in the
  // Heuristic space, we need to only store 2 of the 4 quadrants and simply mirror
  // around the X axis any relative node lookup. This reduces memory overhead and increases
  // the size of a window a platform can store in memory.
  dist_heuristic_lookup_table.resize(size_lookup * ceil(size_lookup / 2.0) * dim_3_size_int);
  for (float x = ceil(-size_lookup / 2.0); x <= floor(size_lookup / 2.0); x += 1.0) {
    for (float y = 0.0; y <= floor(size_lookup / 2.0); y += 1.0) {
      for (int heading = 0; heading != dim_3_size_int; heading++) {
        from[0] = x;
        from[1] = y;
        from[2] = motion_table.getAngleFromBin(heading);
        motion_heuristic = motion_table.state_space->distance(from(), to());
        dist_heuristic_lookup_table[index] = motion_heuristic;
        index++;
      }
    }
  }
}
 
void NodeLattice::getNeighbors(
  std::function<bool(const uint64_t &,
  nav2_smac_planner::NodeLattice * &)> & NeighborGetter,
  GridCollisionChecker * collision_checker,
  const bool & traverse_unknown,
  NodeVector & neighbors)
{
  uint64_t index = 0;
  bool backwards = false;
  NodePtr neighbor = nullptr;
  Coordinates initial_node_coords, motion_projection;
  unsigned int direction_change_index = 0;
  MotionPrimitivePtrs motion_primitives = motion_table.getMotionPrimitives(
    this,
    direction_change_index);
  const float & grid_resolution = motion_table.lattice_metadata.grid_resolution;
 
  for (unsigned int i = 0; i != motion_primitives.size(); i++) {
    const MotionPose & end_pose = motion_primitives[i]->poses.back();
    motion_projection.x = this->pose.x + (end_pose._x / grid_resolution);
    motion_projection.y = this->pose.y + (end_pose._y / grid_resolution);
    motion_projection.theta = motion_primitives[i]->end_angle /*this is the ending angular bin*/;
 
    // if i >= idx, then we're in a reversing primitive. In that situation,
    // the orientation of the robot is mirrored from what it would otherwise
    // appear to be from the motion primitives file. We want to take this into
    // account in case the robot base footprint is asymmetric.
    backwards = false;
    if (i >= direction_change_index) {
      backwards = true;
      float opposite_heading_theta =
        motion_projection.theta - (motion_table.num_angle_quantization / 2);
      if (opposite_heading_theta < 0) {
        opposite_heading_theta += motion_table.num_angle_quantization;
      }
      if (opposite_heading_theta > motion_table.num_angle_quantization) {
        opposite_heading_theta -= motion_table.num_angle_quantization;
      }
      motion_projection.theta = opposite_heading_theta;
    }
 
    index = NodeLattice::getIndex(
      static_cast<unsigned int>(motion_projection.x),
      static_cast<unsigned int>(motion_projection.y),
      static_cast<unsigned int>(motion_projection.theta));
 
    if (NeighborGetter(index, neighbor) && !neighbor->wasVisited()) {
      // Cache the initial pose in case it was visited but valid
      // don't want to disrupt continuous coordinate expansion
      initial_node_coords = neighbor->pose;
 
      neighbor->setPose(
        Coordinates(
          motion_projection.x,
          motion_projection.y,
          motion_projection.theta));
 
      // Using a special isNodeValid API here, giving the motion primitive to use to
      // validity check the transition of the current node to the new node over
      if (neighbor->isNodeValid(
          traverse_unknown, collision_checker, motion_primitives[i], backwards))
      {
        neighbor->setMotionPrimitive(motion_primitives[i]);
        // Marking if this search was obtained in the reverse direction
        neighbor->backwards(backwards);
        neighbors.push_back(neighbor);
      } else {
        neighbor->setPose(initial_node_coords);
      }
    }
  }
}
 
bool NodeLattice::backtracePath(CoordinateVector & path)
{
  if (!this->parent) {
    return false;
  }
 
  NodePtr current_node = this;
 
  while (current_node->parent) {
    addNodeToPath(current_node, path);
    current_node = current_node->parent;
  }
 
  // add start to path
  addNodeToPath(current_node, path);
 
  return true;
}
 
void NodeLattice::addNodeToPath(
  NodeLattice::NodePtr current_node,
  NodeLattice::CoordinateVector & path)
{
  Coordinates initial_pose, prim_pose;
  MotionPrimitive * prim = nullptr;
  const float & grid_resolution = NodeLattice::motion_table.lattice_metadata.grid_resolution;
  prim = current_node->getMotionPrimitive();
  // if motion primitive is valid, then was searched (rather than analytically expanded),
  // include dense path of subpoints making up the primitive at grid resolution
  if (prim) {
    initial_pose.x = current_node->pose.x - (prim->poses.back()._x / grid_resolution);
    initial_pose.y = current_node->pose.y - (prim->poses.back()._y / grid_resolution);
    initial_pose.theta = NodeLattice::motion_table.getAngleFromBin(prim->start_angle);
 
    for (auto it = prim->poses.crbegin(); it != prim->poses.crend(); ++it) {
      // Convert primitive pose into grid space if it should be checked
      prim_pose.x = initial_pose.x + (it->_x / grid_resolution);
      prim_pose.y = initial_pose.y + (it->_y / grid_resolution);
      // If reversing, invert the angle because the robot is backing into the primitive
      // not driving forward with it
      if (current_node->isBackward()) {
        prim_pose.theta = std::fmod(it->_theta + M_PI, 2.0 * M_PI);
      } else {
        prim_pose.theta = it->_theta;
      }
      path.push_back(prim_pose);
    }
  } else {
    // For analytic expansion nodes where there is no valid motion primitive
    path.push_back(current_node->pose);
    path.back().theta = NodeLattice::motion_table.getAngleFromBin(path.back().theta);
  }
}
 
}  // namespace nav2_smac_planner