SplineParameterizer.h

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// Copyright (c) FIRST and other WPILib contributors.
// Open Source Software; you can modify and/or share it under the terms of
// the WPILib BSD license file in the root directory of this project.
 
/*
 * MIT License
 *
 * Copyright (c) 2018 Team 254
 *
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#pragma once
 
#include <stack>
#include <string>
#include <utility>
#include <vector>
 
#include <wpi/SymbolExports.h>
 
#include "frc/spline/Spline.h"
#include "units/angle.h"
#include "units/curvature.h"
#include "units/length.h"
#include "units/math.h"
 
namespace frc {
 
/**
 * Class used to parameterize a spline by its arc length.
 */
class WPILIB_DLLEXPORT SplineParameterizer {
 public:
  using PoseWithCurvature = std::pair<Pose2d, units::curvature_t>;
 
  struct MalformedSplineException : public std::runtime_error {
    explicit MalformedSplineException(const char* what_arg)
        : runtime_error(what_arg) {}
  };
 
  /**
   * Parametrizes the spline. This method breaks up the spline into various
   * arcs until their dx, dy, and dtheta are within specific tolerances.
   *
   * @param spline The spline to parameterize.
   * @param t0 Starting internal spline parameter. It is recommended to leave
   * this as default.
   * @param t1 Ending internal spline parameter. It is recommended to leave this
   * as default.
   *
   * @return A vector of poses and curvatures that represents various points on
   * the spline.
   */
  template <int Dim>
  static std::vector<PoseWithCurvature> Parameterize(const Spline<Dim>& spline,
                                                     double t0 = 0.0,
                                                     double t1 = 1.0) {
    constexpr const char* kMalformedSplineExceptionMsg =
        "Could not parameterize a malformed spline. This means that you "
        "probably had two or more adjacent waypoints that were very close "
        "together with headings in opposing directions.";
    std::vector<PoseWithCurvature> splinePoints;
 
    // The parameterization does not add the initial point. Let's add that.
    if (auto point = spline.GetPoint(t0)) {
      splinePoints.push_back(point.value());
    } else {
      throw MalformedSplineException(kMalformedSplineExceptionMsg);
    }
 
    // We use an "explicit stack" to simulate recursion, instead of a recursive
    // function call This give us greater control, instead of a stack overflow
    std::stack<StackContents> stack;
    stack.emplace(StackContents{t0, t1});
 
    int iterations = 0;
 
    while (!stack.empty()) {
      auto current = stack.top();
      stack.pop();
 
      auto start = spline.GetPoint(current.t0);
      if (!start) {
        throw MalformedSplineException(kMalformedSplineExceptionMsg);
      }
 
      auto end = spline.GetPoint(current.t1);
      if (!end) {
        throw MalformedSplineException(kMalformedSplineExceptionMsg);
      }
 
      const auto twist = start.value().first.Log(end.value().first);
 
      if (units::math::abs(twist.dy) > kMaxDy ||
          units::math::abs(twist.dx) > kMaxDx ||
          units::math::abs(twist.dtheta) > kMaxDtheta) {
        stack.emplace(StackContents{(current.t0 + current.t1) / 2, current.t1});
        stack.emplace(StackContents{current.t0, (current.t0 + current.t1) / 2});
      } else {
        splinePoints.push_back(end.value());
      }
 
      if (iterations++ >= kMaxIterations) {
        throw MalformedSplineException(kMalformedSplineExceptionMsg);
      }
    }
 
    return splinePoints;
  }
 
 private:
  // Constraints for spline parameterization.
  static inline constexpr units::meter_t kMaxDx = 5_in;
  static inline constexpr units::meter_t kMaxDy = 0.05_in;
  static inline constexpr units::radian_t kMaxDtheta = 0.0872_rad;
 
  struct StackContents {
    double t0;
    double t1;
  };
 
  /**
   * A malformed spline does not actually explode the LIFO stack size. Instead,
   * the stack size stays at a relatively small number (e.g. 30) and never
   * decreases. Because of this, we must count iterations. Even long, complex
   * paths don't usually go over 300 iterations, so hitting this maximum should
   * definitely indicate something has gone wrong.
   */
  static constexpr int kMaxIterations = 5000;
 
  friend class CubicHermiteSplineTest;
  friend class QuinticHermiteSplineTest;
};
}  // namespace frc