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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 "Acts/Definitions/Units.hpp"
 #include "Acts/Utilities/Axis.hpp"
 #include "Acts/Utilities/Grid.hpp"
 #include "Acts/Utilities/Logger.hpp"
 #include "Acts/Utilities/Result.hpp"
 
 #include <string>
 #include <vector>
 
 namespace Acts {
 
 /// @class HoughVertexFinder
 ///
 /// @brief Implements the vertex finder based on the spacepoints using Hough transform
 /// For more information, see arXiv:2410.14494
 /// 0. Assumes there is only 1 vertex and that it has a high multiplicity
 /// 1. Estimates what eta range is really necessary
 /// 2. Creates Hough space (z_vtx - cot(theta)) from spacepoints within that eta
 /// range
 /// 3. Subtracts the coincidentally crossed lines in the Hough space
 /// 4. Makes a projection to the Z axis and finds a peak - that is the vertex
 /// position
 /// 5. Repeats 2-4 if necessary
 ///
 template <typename spacepoint_t>
 class HoughVertexFinder {
  public:
   /// Configuration struct
   struct Config {
     /// Ideal amount of spacepoints; |eta| range will be limited to
     /// contain approximately this amount of SPs
     std::uint32_t targetSPs = 10000;
 
     /// Minimum and maximum ranges in |eta|; the |eta| will not be
     /// set outside these bounds even if targetSPs is not reached
     double minAbsEta = 0.3f;
     /// Maximum absolute pseudorapidity for vertex finding
     double maxAbsEta = 4.0f;
 
     /// Minimum number of hits in Hough plane to consider
     /// the cell to contain a track
     std::uint32_t minHits = 4;
 
     /// Number of neighbouring bins in Hough plane
     /// to fill in the cot(theta) direction
     std::uint32_t fillNeighbours = 0;
 
     /// The algorithm dynamically choose |eta| range necessary to
     /// have a reasonable precision, based on the distribution
     /// of the measurements.
     /// The first |eta| range starts at 0., the others start at
     /// the endpoint of the previous range.
     std::vector<double> absEtaRanges{2., 4.};
     /// The amount of measurements in the |eta| range expressed
     /// as a fraction of all measurements within the whole |eta| range.
     /// Measurements are assumed to be distributed uniformly with
     /// the |eta| range.
     std::vector<double> absEtaFractions{0.4f, 0.6f};
 
     /// The algorithm starts peak searching considering wide range
     /// in Z and then iteratively constrain itself to narrower ranges
     /// around previously found peak. At the same time it may adapt
     /// other Hough-image parameters in each step.
     std::vector<double> rangeIterZ{200. * UnitConstants::mm,
                                    30. * UnitConstants::mm,
                                    16. * UnitConstants::mm};
     /// Number of bins along z-axis of the Hough image for each iteration
     std::vector<std::uint32_t> nBinsZIterZ{800, 180, 80};
     /// Number of bins along cot(theta)-axis of the Hough image for
     /// each iteration
     std::vector<std::uint32_t> nBinsCotThetaIterZ{8000, 8000, 8000};
 
     /// If the actual number of measurements is below "targetSPs", then
     /// the number of bins along cot(theta)-axis decreases. Thus, the actual
     /// number of bins can be smaller than stated in "nBinsCotThetaIterZ".
     /// For every magnitude (in natural logarithm) below targetSPs, the number
     /// of bins in cot(theta) will decrease by this factor.
     double binsCotThetaDecrease = 1.35f;
 
     /// Width of the peak when estimating vertex position
     std::uint32_t peakWidth = 3;
 
     /// Default position of the vertex in X, Y, and Z coordinates
     Vector3 defVtxPosition{0. * UnitConstants::mm, 0. * UnitConstants::mm,
                            0. * UnitConstants::mm};
   };
 
   /// Const access to the config
   /// @return Const reference to the configuration object
   const Config& config() const { return m_cfg; }
 
   /// @brief Constructor
   /// @param cfg Configuration object
   /// @param lgr Logging instance
   explicit HoughVertexFinder(
       Config cfg,
       std::unique_ptr<const Logger> lgr = getDefaultLogger("HoughVertexFinder",
                                                            Logging::INFO));
 
   /// Type alias for count values in Hough histogram bins
   using HoughCount_t = std::uint16_t;
   /// Type alias for equidistant axis used in Hough transform
   using HoughAxis = Axis<AxisType::Equidistant, AxisBoundaryType::Open>;
   /// Type alias for 2D histogram used in Hough transform
   using HoughHist = Grid<HoughCount_t, HoughAxis, HoughAxis>;
 
   /// @brief Finds the vertex based on the provided spacepoints
   /// @param spacepoints Vector of the input spacepoints; they do not need to be sorted anyhow
   /// @return Position of the vertex
   Acts::Result<Acts::Vector3> find(
       const std::vector<spacepoint_t>& spacepoints) const;
 
  private:
   /// Configuration instance
   const Config m_cfg;
 
   /// @brief Returns the positions of the peak along Z axis in the pojection of the Hough plane
   /// @param spacepoints Set of all spacepoints within the event
   /// @param vtxOld Previous position of the vertex
   /// @param rangeZ Range in along Z around vtxOld_z to consider when looking for the new vertex
   /// @param numZBins Number of bins along Z axis
   /// @param minCotTheta Minimum theta to consider for the spacepoint
   /// @param maxCotTheta Maximum theta to consider for the spacepoint
   /// @param numCotThetaBins Number of bins along cot(theta) axis
   /// @return Position of the vertex in (X,Y,Z)
   Acts::Result<Acts::Vector3> findHoughVertex(
       const std::vector<spacepoint_t>& spacepoints, const Acts::Vector3& vtxOld,
       double rangeZ, std::uint32_t numZBins, double minCotTheta,
       double maxCotTheta, std::uint32_t numCotThetaBins) const;
 
   /// @brief Finds the peak in the Z axis projection of the Hough space
   /// @param houghZProjection Hough space projection after the cleaning procedure
   /// @param vtxZPositions Bins position in the Hough space projection
   /// @return Position of the peak
   Acts::Result<double> findHoughPeak(
       const std::vector<std::uint32_t>& houghZProjection,
       const std::vector<double>& vtxZPositions) const;
 
   /// Logging instance
   std::unique_ptr<const Logger> m_logger;
 
   /// Private access to logging instance
   const Logger& logger() const { return *m_logger; }
 };
 
 }  // namespace Acts
 
 #include "HoughVertexFinder.ipp"

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