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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/Direction.hpp"
 #include "Acts/Definitions/Units.hpp"
 #include "Acts/EventData/SpacePointContainer.hpp"
 #include "Acts/Utilities/Delegate.hpp"
 #include "Acts/Utilities/detail/ContainerIterator.hpp"
 
 #include <cstdint>
 #include <vector>
 
 namespace Acts {
 
 /// Container for doublets found by the doublet seed finder.
 ///
 /// This implementation uses partial AoS/SoA depending on the access pattern in
 /// the doublet finding process.
 class DoubletsForMiddleSp {
  public:
   /// Type alias for index type used in doublets container
   using Index = std::uint32_t;
   /// Type alias for range of indices in doublets container
   using IndexRange = std::pair<Index, Index>;
   /// Type alias for subset of indices in doublets container
   using IndexSubset = std::span<const Index>;
 
   /// Check if the doublets container is empty
   /// @return True if container has no doublets
   [[nodiscard]] bool empty() const { return m_spacePoints.empty(); }
   /// Get the number of doublets in container
   /// @return Number of doublets stored
   [[nodiscard]] Index size() const {
     return static_cast<Index>(m_spacePoints.size());
   }
 
   /// Clear all stored doublets and associated data
   void clear() {
     m_spacePoints.clear();
     m_cotTheta.clear();
     m_er_iDeltaR.clear();
     m_uv.clear();
     m_xy.clear();
   }
 
   /// Add a new doublet with associated parameters
   /// @param sp space point index for the doublet
   /// @param cotTheta Cotangent of polar angle
   /// @param iDeltaR Inverse delta R parameter
   /// @param er Error in R coordinate
   /// @param u U coordinate parameter
   /// @param v V coordinate parameter
   /// @param x X coordinate
   /// @param y Y coordinate
   void emplace_back(SpacePointIndex sp, float cotTheta, float iDeltaR, float er,
                     float u, float v, float x, float y) {
     m_spacePoints.push_back(sp);
     m_cotTheta.push_back(cotTheta);
     m_er_iDeltaR.push_back({er, iDeltaR});
     m_uv.push_back({u, v});
     m_xy.push_back({x, y});
   }
 
   /// Get reference to space point indices container
   /// @return Const reference to space point indices vector
   const std::vector<SpacePointIndex>& spacePoints() const {
     return m_spacePoints;
   }
   /// Get reference to cotTheta values container
   /// @return Const reference to cotTheta values vector
   const std::vector<float>& cotTheta() const { return m_cotTheta; }
 
   /// Pair of doublet index and cotTheta value.
   struct IndexAndCotTheta {
     /// Doublet index
     Index index{};
     /// Cotangent of theta
     float cotTheta{};
   };
 
   /// Type alias for subset of index and cotTheta pairs
   using IndexAndCotThetaSubset = std::span<const IndexAndCotTheta>;
 
   /// Sort doublets by cotTheta within given range
   /// @param range Index range to sort within
   /// @param indexAndCotTheta Output vector containing sorted index and cotTheta pairs
   void sortByCotTheta(const IndexRange& range,
                       std::vector<IndexAndCotTheta>& indexAndCotTheta) const {
     indexAndCotTheta.clear();
     indexAndCotTheta.reserve(range.second - range.first);
     for (Index i = range.first; i < range.second; ++i) {
       indexAndCotTheta.emplace_back(i, m_cotTheta[i]);
     }
     std::ranges::sort(indexAndCotTheta, {}, [](const IndexAndCotTheta& item) {
       return item.cotTheta;
     });
   }
 
   /// Proxy accessor for a single doublet entry.
   class Proxy {
    public:
     /// Constructor
     /// @param container Pointer to the doublet container
     /// @param index Index of the doublet
     Proxy(const DoubletsForMiddleSp* container, Index index)
         : m_container(container), m_index(index) {}
 
     /// Get the doublet container
     /// @return Reference to the container
     const DoubletsForMiddleSp& container() const { return *m_container; }
     /// Get the doublet index
     /// @return The index
     Index index() const { return m_index; }
 
     /// Get space point index pair
     /// @return The space point index
     SpacePointIndex spacePointIndex() const {
       return m_container->m_spacePoints[m_index];
     }
 
     /// Get cotangent of theta
     /// @return The cotTheta value
     float cotTheta() const { return m_container->m_cotTheta[m_index]; }
     /// Get er value
     /// @return The er value
     float er() const { return m_container->m_er_iDeltaR[m_index][0]; }
     /// Get inverse delta r
     /// @return The inverse delta r value
     float iDeltaR() const { return m_container->m_er_iDeltaR[m_index][1]; }
     /// Get u coordinate
     /// @return The u value
     float u() const { return m_container->m_uv[m_index][0]; }
     /// Get v coordinate
     /// @return The v value
     float v() const { return m_container->m_uv[m_index][1]; }
     /// Get x coordinate
     /// @return The x value
     float x() const { return m_container->m_xy[m_index][0]; }
     /// Get y coordinate
     /// @return The y value
     float y() const { return m_container->m_xy[m_index][1]; }
 
    private:
     const DoubletsForMiddleSp* m_container{};
     Index m_index{};
   };
   /// Same as `Proxy` but also contains `cotTheta`. This is useful after sorting
   /// doublets by `cotTheta` to avoid indirect access.
   class Proxy2 : public Proxy {
    public:
     /// Constructor for Proxy2 with precomputed cotTheta
     /// @param container Pointer to the doublets container
     /// @param indexAndCotTheta Index and cotTheta pair
     Proxy2(const DoubletsForMiddleSp* container,
            IndexAndCotTheta indexAndCotTheta)
         : Proxy(container, indexAndCotTheta.index),
           m_cotTheta(indexAndCotTheta.cotTheta) {}
 
     /// Get precomputed cotTheta value (avoids indirect access)
     /// @return Cotangent of polar angle
     float cotTheta() const { return m_cotTheta; }
 
    private:
     float m_cotTheta{};
   };
 
   /// Access doublet by index
   /// @param index Index of the doublet to access
   /// @return Proxy object for the doublet
   Proxy operator[](Index index) const { return Proxy(this, index); }
   /// Access doublet by index and cotTheta pair
   /// @param indexAndCotTheta Index and cotTheta pair for the doublet
   /// @return Proxy2 object for the doublet with precomputed cotTheta
   Proxy2 operator[](IndexAndCotTheta indexAndCotTheta) const {
     return Proxy2(this, indexAndCotTheta);
   }
 
   /// Type alias for const iterator over doublets in container
   using const_iterator =
       detail::ContainerIterator<DoubletsForMiddleSp, Proxy, Index, true>;
 
   /// Get iterator to beginning of doublets container
   /// @return Const iterator to first doublet
   const_iterator begin() const { return const_iterator(*this, 0); }
   /// Get iterator to end of doublets container
   /// @return Const iterator past the last doublet
   const_iterator end() const { return const_iterator(*this, size()); }
 
   /// Range view over doublets in the container.
   class Range : public detail::ContainerRange<Range, Range, DoubletsForMiddleSp,
                                               Index, true> {
    public:
     /// Base class type alias
     using Base =
         detail::ContainerRange<Range, Range, DoubletsForMiddleSp, Index, true>;
 
     using Base::Base;
   };
 
   /// Get range view of all doublets
   /// @return Range object covering all doublets
   Range range() const noexcept { return Range(*this, {0, size()}); }
   /// Get range view of doublets within specified index range
   /// @param range Index range to create view for
   /// @return Range object covering specified doublets
   Range range(const IndexRange& range) const noexcept {
     return Range(*this, range);
   }
 
   /// Subset view of doublets addressed by indices.
   class Subset
       : public detail::ContainerSubset<Subset, Subset, DoubletsForMiddleSp,
                                        Proxy, std::span<const Index>, true> {
    public:
     /// Base class type alias
     using Base = detail::ContainerSubset<Subset, Subset, DoubletsForMiddleSp,
                                          Proxy, std::span<const Index>, true>;
 
     using Base::Base;
   };
   /// Subset view of doublets addressed by index and cotTheta pairs.
   class Subset2 : public detail::ContainerSubset<
                       Subset2, Subset2, DoubletsForMiddleSp, Proxy2,
                       std::span<const IndexAndCotTheta>, true> {
    public:
     /// Base class type alias
     using Base =
         detail::ContainerSubset<Subset2, Subset2, DoubletsForMiddleSp, Proxy2,
                                 std::span<const IndexAndCotTheta>, true>;
 
     using Base::Base;
   };
 
   /// Create subset view from index subset
   /// @param subset Span of indices to include in subset
   /// @return Subset object for the specified indices
   Subset subset(const IndexSubset& subset) const noexcept {
     return Subset(*this, subset);
   }
   /// Create subset view from index and cotTheta subset
   /// @param subset Span of index and cotTheta pairs to include
   /// @return Subset2 object with precomputed cotTheta values
   Subset2 subset(const IndexAndCotThetaSubset& subset) const noexcept {
     return Subset2(*this, subset);
   }
 
  private:
   std::vector<SpacePointIndex> m_spacePoints;
 
   // parameters required to calculate a circle with linear equation
   std::vector<float> m_cotTheta;
   std::vector<std::array<float, 2>> m_er_iDeltaR;
   std::vector<std::array<float, 2>> m_uv;
   std::vector<std::array<float, 2>> m_xy;
 };
 
 /// Derived quantities for the middle space point in a doublet.
 struct MiddleSpInfo {
   /// minus one over radius of middle SP
   float uIP{};
   /// square of uIP
   float uIP2{};
   /// ratio between middle SP x position and radius
   float cosPhiM{};
   /// ratio between middle SP y position and radius
   float sinPhiM{};
 };
 
 /// Interface and a collection of standard implementations for a doublet seed
 /// finder. Given a starting space point and a collection of candidates, it
 /// finds all doublets that satisfy the selection criteria. For the standard
 /// implementations the criteria are given by interaction point cuts.
 ///
 /// @note The standard implementations rely on virtual function dispatch which
 /// did not turn out to affect the performance after measurement.
 class DoubletSeedFinder {
  public:
   /// Collection of configuration parameters for the doublet seed finder. This
   /// includes doublet cuts, steering switches, and assumptions about the space
   /// points.
   struct Config {
     /// Whether the input space points are sorted by radius
     bool spacePointsSortedByRadius = false;
 
     /// Direction of the doublet candidate space points. Either forward, also
     /// called top doublet, or backward, also called bottom doublet.
     Direction candidateDirection = Direction::Forward();
 
     /// Minimum radial distance between two doublet components
     float deltaRMin = 5 * UnitConstants::mm;
     /// Maximum radial distance between two doublet components
     float deltaRMax = 270 * UnitConstants::mm;
 
     /// Minimal z distance between two doublet components
     float deltaZMin = -std::numeric_limits<float>::infinity();
     /// Maximum z distance between two doublet components
     float deltaZMax = std::numeric_limits<float>::infinity();
 
     /// Maximum value of impact parameter estimation of the seed candidates
     float impactMax = 20 * UnitConstants::mm;
 
     /// Enable cut on the compatibility between interaction point and doublet,
     /// this is an useful approximation to speed up the seeding
     bool interactionPointCut = false;
 
     /// Limiting location of collision region in z-axis used to check if doublet
     /// origin is within reasonable bounds
     float collisionRegionMin = -150 * UnitConstants::mm;
     /// Maximum collision region boundary in z-axis for doublet origin checks
     float collisionRegionMax = +150 * UnitConstants::mm;
 
     /// Maximum allowed cotTheta between two space-points in doublet, used to
     /// check if forward angle is within bounds
     float cotThetaMax = 10.01788;  // equivalent to eta = 3 (pseudorapidity)
 
     /// Minimum transverse momentum (pT) used to check the r-z slope
     /// compatibility of triplets with maximum multiple scattering effect
     /// (produced by the minimum allowed pT particle) + a certain uncertainty
     /// term. Check the documentation for more information
     /// https://acts.readthedocs.io/en/latest/core/reconstruction/pattern_recognition/seeding.html
     float minPt = 400 * UnitConstants::MeV;
     /// Parameter which can loosen the tolerance of the track seed to form a
     /// helix. This is useful for e.g. misaligned seeding.
     float helixCutTolerance = 1;
 
     /// Type alias for delegate to apply experiment specific cuts during doublet
     /// finding
     using ExperimentCuts =
         Delegate<bool(const ConstSpacePointProxy& /*middle*/,
                       const ConstSpacePointProxy& /*other*/, float /*cotTheta*/,
                       bool /*isBottomCandidate*/)>;
 
     /// Delegate to apply experiment specific cuts during doublet finding
     ExperimentCuts experimentCuts;
   };
 
   /// Derived configuration for the doublet seed finder using a magnetic field.
   struct DerivedConfig : public Config {
     /// Constructor for derived configuration with magnetic field
     /// @param config Base configuration to derive from
     /// @param bFieldInZ Magnetic field strength in z-direction
     DerivedConfig(const Config& config, float bFieldInZ);
 
     /// Magnetic field strength in z-direction for helix calculation
     float bFieldInZ = std::numeric_limits<float>::quiet_NaN();
     /// Squared minimum helix diameter derived from magnetic field and minimum
     /// pT
     float minHelixDiameter2 = std::numeric_limits<float>::quiet_NaN();
   };
 
   /// Computes additional quantities from the middle space point which can be
   /// reused during doublet finding.
   /// @param spM Middle space point for doublet computation
   /// @return MiddleSpInfo structure with computed quantities
   static MiddleSpInfo computeMiddleSpInfo(const ConstSpacePointProxy& spM);
 
   /// Creates a new doublet seed finder instance given the configuration.
   /// @param config Configuration for the doublet seed finder
   /// @return Unique pointer to new DoubletSeedFinder instance
   static std::unique_ptr<DoubletSeedFinder> create(const DerivedConfig& config);
 
   virtual ~DoubletSeedFinder() = default;
 
   /// Returns the configuration of the doublet seed finder.
   /// @return Reference to the configuration object
   virtual const DerivedConfig& config() const = 0;
 
   /// Creates compatible dublets by applying a series of cuts that can be
   /// tested with only two SPs.
   ///
   /// @param middleSp Space point candidate to be used as middle SP in a seed
   /// @param middleSpInfo Information about the middle space point
   /// @param candidateSps Subset of space points to be used as candidates for
   ///   middle SP in a seed
   /// @param compatibleDoublets Output container for compatible doublets
   virtual void createDoublets(
       const ConstSpacePointProxy& middleSp, const MiddleSpInfo& middleSpInfo,
       SpacePointContainer::ConstSubset& candidateSps,
       DoubletsForMiddleSp& compatibleDoublets) const = 0;
 
   /// Creates compatible dublets by applying a series of cuts that can be
   /// tested with only two SPs.
   ///
   /// @param middleSp Space point candidate to be used as middle SP in a seed
   /// @param middleSpInfo Information about the middle space point
   /// @param candidateSps Range of space points to be used as candidates for
   ///   middle SP in a seed
   /// @param compatibleDoublets Output container for compatible doublets
   virtual void createDoublets(
       const ConstSpacePointProxy& middleSp, const MiddleSpInfo& middleSpInfo,
       SpacePointContainer::ConstRange& candidateSps,
       DoubletsForMiddleSp& compatibleDoublets) const = 0;
 };
 
 }  // namespace Acts

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