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 // This file is part of the ACTS project.
 //
 // Copyright (C) 2016 CERN for the benefit of the ACTS project
 //
 // This Source Code Form is subject to the terms of the Mozilla Public
 // License, v. 2.0. If a copy of the MPL was not distributed with this
 // file, You can obtain one at https://mozilla.org/MPL/2.0/.
 
 #pragma once
 
 #include "Acts/Definitions/Algebra.hpp"
 
 #include <type_traits>
 #include <variant>
 
 namespace Acts {
 
 /// @brief Variant-like type to capture different types of boundary tolerances
 ///
 /// Since our track hypothesis comes with uncertainties, we sometimes need to
 /// check if the track is not just within the boundary of the surface but also
 /// within a certain tolerance. This class captures different parameterizations
 /// of such tolerances. The surface class will then use these tolerances to
 /// check if a ray is within the boundary+tolerance of the surface.
 ///
 /// Different types of boundary tolerances implemented:
 /// - Infinite: Infinite tolerance i.e. no boundary check will be performed.
 /// - None: No tolerance i.e. exact boundary check will be performed.
 /// - AbsoluteEuclidean: Absolute tolerance in Euclidean distance.
 ///   The tolerance is defined as a single absolute value for the Euclidean
 ///   distance. The Euclidean distance can be calculated via the local bound
 ///   Jacobian and the bound coordinate residual. If the distance is within
 ///   the tolerance, the boundary check is considered as passed.
 /// - Chi2Bound: Chi2 tolerance in bound coordinates.
 ///   The tolerance is defined as a maximum chi2 value and a weight matrix,
 ///   which is the inverse of the bound covariance matrix. The chi2 value is
 ///   calculated from the bound coordinates residual and the weight matrix.
 ///   If the chi2 value is below the maximum chi2 value, the boundary check
 ///   is considered as passed.
 /// - Chi2Cartesian: Chi2 tolerance in Cartesian coordinates.
 ///   Similar to Chi2Bound, but the chi2 value is calculated in Cartesian
 ///   coordinates.
 ///
 /// The bound coordinates residual is defined as the difference between the
 /// point checked and the closest point on the boundary. The Jacobian is the
 /// derivative of the bound coordinates with respect to the Cartesian
 /// coordinates.
 ///
 class BoundaryTolerance {
  public:
   struct InfiniteParams {};
 
   struct NoneParams {};
 
   struct AbsoluteEuclideanParams {
     double tolerance{};
   };
 
   struct Chi2BoundParams {
     double maxChi2{};
     std::array<double, 4> weight{};
 
     Eigen::Map<SquareMatrix2> weightMatrix() {
       return Eigen::Map<SquareMatrix2>(weight.data());
     }
 
     Eigen::Map<const SquareMatrix2> weightMatrix() const {
       return Eigen::Map<const SquareMatrix2>(weight.data());
     }
   };
 
   struct Chi2CartesianParams {
     double maxChi2{};
     std::array<double, 4> weight{};
 
     Eigen::Map<SquareMatrix2> weightMatrix() {
       return Eigen::Map<SquareMatrix2>(weight.data());
     }
 
     Eigen::Map<const SquareMatrix2> weightMatrix() const {
       return Eigen::Map<const SquareMatrix2>(weight.data());
     }
   };
 
   static_assert(std::is_trivially_copyable_v<Chi2BoundParams>);
 
  private:
   /// Underlying variant type
   using Variant =
       std::variant<InfiniteParams, NoneParams, AbsoluteEuclideanParams,
                    Chi2BoundParams, Chi2CartesianParams>;
   static_assert(std::is_trivially_copyable_v<Variant>);
 
   /// Construct from variant
   constexpr explicit BoundaryTolerance(Variant variant) : m_variant{variant} {}
 
  public:
   /// Infinite tolerance i.e. no boundary check
   constexpr static auto Infinite() noexcept {
     return BoundaryTolerance{InfiniteParams{}};
   }
 
   /// No tolerance i.e. exact boundary check
   constexpr static auto None() noexcept {
     return BoundaryTolerance{NoneParams{}};
   }
 
   /// Absolute tolerance in Euclidean distance
   constexpr static auto AbsoluteEuclidean(double tolerance) noexcept {
     return BoundaryTolerance{AbsoluteEuclideanParams{tolerance}};
   }
 
   /// Chi2 tolerance in bound coordinates
   static auto Chi2Bound(const SquareMatrix2& weight, double maxChi2) noexcept {
     Chi2BoundParams tolerance{maxChi2, {}};
     tolerance.weightMatrix() = weight;
     return BoundaryTolerance{tolerance};
   }
 
   /// Chi2 tolerance in Cartesian coordinates
   static auto Chi2Cartesian(const SquareMatrix2& weight,
                             double maxChi2) noexcept {
     Chi2CartesianParams tolerance{maxChi2, {}};
     tolerance.weightMatrix() = weight;
     return BoundaryTolerance{tolerance};
   }
 
   BoundaryTolerance(const BoundaryTolerance& other) noexcept = default;
   BoundaryTolerance& operator=(const BoundaryTolerance& other) noexcept =
       default;
   BoundaryTolerance(BoundaryTolerance&& other) noexcept = default;
   BoundaryTolerance& operator=(BoundaryTolerance&& other) noexcept = default;
 
   enum class ToleranceMode {
     Extend,  //< Extend the boundary
     None,    //< No tolerance
     Shrink   //< Shrink the boundary
   };
 
   /// Check if the tolerance is infinite.
   constexpr bool isInfinite() const { return holdsVariant<InfiniteParams>(); }
   /// Check if the is no tolerance.
   constexpr bool isNone() const { return holdsVariant<NoneParams>(); }
   /// Check if the tolerance is absolute with Euclidean distance.
   constexpr bool hasAbsoluteEuclidean() const {
     return holdsVariant<AbsoluteEuclideanParams>();
   }
   /// Check if the tolerance is chi2 with bound coordinates.
   constexpr bool hasChi2Bound() const {
     return holdsVariant<Chi2BoundParams>();
   }
   /// Check if the tolerance is chi2 with Cartesian coordinates.
   constexpr bool hasChi2Cartesian() const {
     return holdsVariant<Chi2CartesianParams>();
   }
 
   /// Get the tolerance mode.
   ToleranceMode toleranceMode() const;
 
   /// Get the tolerance as absolute Euclidean.
   constexpr const AbsoluteEuclideanParams& asAbsoluteEuclidean() const {
     return getVariant<AbsoluteEuclideanParams>();
   }
   /// Get the tolerance as chi2 bound.
   constexpr const Chi2BoundParams& asChi2Bound() const {
     return getVariant<Chi2BoundParams>();
   }
   /// Get the tolerance as chi2 Cartesian.
   constexpr const Chi2CartesianParams& asChi2Cartesian() const {
     return getVariant<Chi2CartesianParams>();
   }
 
   /// Check if the bound position delta is tolerated.
   bool isTolerated(const Vector2& boundDelta,
                    const SquareMatrix2& boundToCartesian) const;
 
  private:
   Variant m_variant;
 
   /// Check if the boundary check is of a specific type.
   template <typename T>
   constexpr bool holdsVariant() const {
     return std::holds_alternative<T>(m_variant);
   }
 
   /// Get the specific underlying type.
   template <typename T>
   constexpr const T& getVariant() const {
     return std::get<T>(m_variant);
   }
 
   template <typename T>
   constexpr const T* getVariantPtr() const {
     return holdsVariant<T>() ? &getVariant<T>() : nullptr;
   }
 };
 
 static_assert(std::is_trivially_copyable_v<BoundaryTolerance>);
 static_assert(std::is_trivially_move_constructible_v<BoundaryTolerance>);
 
 }  // namespace Acts

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