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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/.
 
 /// This file implements the tools for a hough transform.
 
 #pragma once
 
 #include "Acts/Utilities/Grid.hpp"
 
 #include <array>
 #include <span>
 #include <unordered_set>
 
 namespace Acts::HoughTransformUtils {
 
 /// this type is responsible for encoding the parameters of our hough space
 using CoordType = double;
 
 // this type is used to encode hit counts.
 // Floating point to allow hit weights to be applied
 using YieldType = float;
 
 /// @brief this function represents a mapping of a coordinate point in detector space to a line in
 /// hough space. Given the value of the first hough coordinate, it shall return
 /// the corresponding second coordinate according to the line parametrisation.
 /// Should be implemented by the user.
 /// @tparam PointType: The class representing a point in detector space (can differ between implementations)
 template <class PointType>
 using LineParametrisation =
     std::function<CoordType(CoordType, const PointType&)>;
 
 /// @brief struct to define the ranges of the hough histogram.
 /// Used to move between parameter and bin index coordinates.
 /// Disconnected from the hough plane binning to be able to re-use
 /// a plane with a given binning for several parameter ranges
 struct HoughAxisRanges {
   CoordType xMin = 0.0f;  // minimum value of the first coordinate
   CoordType xMax = 0.0f;  // maximum value of the first coordinate
   CoordType yMin = 0.0f;  // minimum value of the second coordinate
   CoordType yMax = 0.0f;  // maximum value of the second coordinate
 };
 
 /// convenience functions to link bin indices to axis coordinates
 
 /// @brief find the bin index corresponding to a certain abscissa
 /// of the coordinate axis, based on the axis limits and binning.
 /// @param min: Start of axis range
 /// @param max: End of axis range
 /// @param nSteps: Number of bins in axis
 /// @param val: value to find the corresponding bin for
 /// @return the bin number.
 /// No special logic to prevent over-/underflow, checking these is
 /// left to the caller
 inline int binIndex(double min, double max, unsigned nSteps, double val) {
   return static_cast<int>((val - min) / (max - min) * nSteps);
 }
 // Returns the lower bound of the bin specified by step
 /// @param min: Start of axis range
 /// @param max: End of axis range
 /// @param nSteps: Number of bins in axis
 /// @param binIndex: The index of the bin
 /// @return the parameter value at the lower bin edge.
 /// No special logic to prevent over-/underflow, checking these is
 /// left to the caller
 inline double lowerBinEdge(double min, double max, unsigned nSteps,
                            std::size_t binIndex) {
   return min + (max - min) * binIndex / nSteps;
 }
 // Returns the lower bound of the bin specified by step
 /// @param min: Start of axis range
 /// @param max: End of axis range
 /// @param nSteps: Number of bins in axis
 /// @param binIndex: The index of the bin
 /// @return the parameter value at the bin center.
 /// No special logic to prevent over-/underflow, checking these is
 /// left to the caller
 inline double binCenter(double min, double max, unsigned nSteps,
                         std::size_t binIndex) {
   return min + (max - min) * 0.5 * (2 * binIndex + 1) / nSteps;
 }
 
 /// @brief data class for the information to store for each
 /// cell of the hough histogram.
 /// @tparam identifier_t: Type of the identifier to associate to the hits
 ///                       Should be sortable. Used to uniquely identify each
 ///                       hit and to eventually return the list of hits per cell
 template <class identifier_t>
 class HoughCell {
  public:
   /// @brief construct the cell as empty
   HoughCell() = default;
   /// @brief add an entry to this cell
   /// @param identifier: Identifier of the hit (used to distinguish hits from another)
   /// @param layer: Layer of the hit (used when counting layers)
   /// @param weight: Optional weight to assign to the hit
   void fill(const identifier_t& identifier, unsigned int layer,
             YieldType weight = 1.);
   /// @brief access the number of layers with hits compatible with this cell
   YieldType nLayers() const { return m_nLayers; }
   /// @brief access the number of unique hits compatible with this cell
   YieldType nHits() const { return m_nHits; }
   /// @brief access the span of layers compatible with this cell
   std::span<const unsigned, std::dynamic_extent> getLayers() const;
   // /// @brief access the pan of unique hits compatible with this cell
   std::span<const identifier_t, std::dynamic_extent> getHits() const;
 
   /// @brief reset this cell, removing any existing content.
   void reset();
 
  private:
   /// (weighted) number of layers with hits on this cell
   YieldType m_nLayers{0};
   /// (weighted) number of unique hits on this cell
   YieldType m_nHits{0};
 
   /// index for the hits -- keeps track of vector's size
   std::size_t m_iHit{0};
   /// index for the layers -- keeps track of vector's size
   std::size_t m_iLayer{0};
 
   /// a batch to resize the vector of the hits or the layers
   std::size_t m_assignBatch{20};
 
   /// vector of layers with hits on this cell
   std::vector<unsigned> m_layers{std::vector<unsigned>(m_assignBatch)};
 
   /// vector of hits on this cell
   std::vector<identifier_t> m_hits{std::vector<identifier_t>(m_assignBatch)};
 };
 
 /// @brief Configuration - number of bins in each axis.
 /// The Hough plane is agnostic of how the bins map to
 /// coordinates, allowing to re-use a plane for several
 /// (sub) detectors of different dimensions if the bin number
 /// remains applicable
 struct HoughPlaneConfig {
   std::size_t nBinsX = 0;  // number of bins in the first coordinate
   std::size_t nBinsY = 0;  // number of bins in the second coordinate
 };
 
 /// @brief Representation of the hough plane - the histogram used
 /// for the hough transform with methods to fill and evaluate
 /// the histogram. Templated to a class used as identifier for the hits
 template <class identifier_t>
 class HoughPlane {
  public:
   /// @brief hough histogram representation as a 2D-indexable vector of hough cells
   using Axis =
       Acts::Axis<Acts::AxisType::Equidistant, Acts::AxisBoundaryType::Bound>;
   using HoughHist = Grid<HoughCell<identifier_t>, Axis, Axis>;
   using Index = typename HoughHist::index_t;
 
   /// @brief instantiate the (empty) hough plane
   /// @param cfg: configuration
   explicit HoughPlane(const HoughPlaneConfig& cfg);
 
   /// fill and reset methods to modify the grid content
 
   /// @brief add one measurement to the hough plane
   /// @tparam PointType: Type of the objects to use when adding measurements (e.g. experiment EDM object)
   /// @param measurement: The measurement to add
   /// @param axisRanges: Ranges of the hough axes, used to map the bin numbers to parameter values
   /// @param linePar: The function y(x) parametrising the hough space line for a given measurement
   /// @param widthPar: The function dy(x) parametrising the width of the y(x) curve
   ///                   for a given measurement
   /// @param identifier: The unique identifier for the given hit
   /// @param layer: A layer index for this hit
   /// @param weight: An optional weight to assign to this hit
   template <class PointType>
   void fill(const PointType& measurement, const HoughAxisRanges& axisRanges,
             LineParametrisation<PointType> linePar,
             LineParametrisation<PointType> widthPar,
             const identifier_t& identifier, unsigned layer = 0,
             YieldType weight = 1.0f);
   /// @brief resets the contents of the grid. Can be used to avoid reallocating the histogram
   /// when switching regions / (sub)detectors
   void reset();
 
   //// user-facing accessors
 
   /// @brief get the layers with hits in one cell of the histogram
   /// @param xBin: bin index in the first coordinate
   /// @param yBin: bin index in the second coordinate
   /// @return the set of layer indices that have hits for this cell
   std::unordered_set<unsigned> layers(std::size_t xBin,
                                       std::size_t yBin) const {
     return m_houghHist.atLocalBins({xBin, yBin}).layers();
   }
 
   /// @brief get the (weighted) number of layers  with hits in one cell of the histogram
   /// @param xBin: bin index in the first coordinate
   /// @param yBin: bin index in the second coordinate
   /// @return the (weighed) number of layers that have hits for this cell
   YieldType nLayers(std::size_t xBin, std::size_t yBin) const {
     return m_houghHist.atLocalBins({xBin, yBin}).nLayers();
   }
 
   /// @brief get the identifiers of all hits in one cell of the histogram
   /// @param xBin: bin index in the first coordinate
   /// @param yBin: bin index in the second coordinate
   /// @return the set of identifiers of the hits for this cell
   std::unordered_set<identifier_t> hitIds(std::size_t xBin,
                                           std::size_t yBin) const {
     const auto hits_span = m_houghHist.atLocalBins({xBin, yBin}).getHits();
 
     return std::unordered_set<identifier_t>(hits_span.begin(), hits_span.end());
   }
   /// @brief access the (weighted) number of hits in one cell of the histogram from bin's coordinates
   /// @param xBin: bin index in the first coordinate
   /// @param yBin: bin index in the second coordinate
   /// @return the (weighted) number of hits for this cell
   YieldType nHits(std::size_t xBin, std::size_t yBin) const {
     return m_houghHist.atLocalBins({xBin, yBin}).nHits();
   }
 
   /// @brief access the (weighted) number of hits in one cell of the histogram from globalBin index
   /// @param globalBin: global bin index
   /// @return the (weighted) number of hits for this cell
   YieldType nHits(std::size_t globalBin) const {
     return m_houghHist.at(globalBin).nHits();
   }
 
   /// @brief get the number of bins on the first coordinate
   std::size_t nBinsX() const { return m_cfg.nBinsX; }
   /// @brief get the number of bins on the second coordinate
   std::size_t nBinsY() const { return m_cfg.nBinsY; }
 
   /// @brief get the maximum number of (weighted) hits seen in a single
   /// cell across the entire histrogram.
   YieldType maxHits() const { return m_maxHits; }
 
   /// @brief get the list of cells with non-zero content.
   /// Useful for peak-finders in sparse data
   /// to avoid looping over all cells
   const std::unordered_set<std::size_t>& getNonEmptyBins() const {
     return m_touchedBins;
   }
 
   /// @brief get the coordinates of the bin given the global bin index
   Index axisBins(std::size_t globalBin) const {
     return m_houghHist.localBinsFromGlobalBin(globalBin);
   }
 
   /// @brief get the globalBin index given the coordinates of the bin
   std::size_t globalBin(Index indexBin) const {
     return m_houghHist.globalBinFromLocalBins(indexBin);
   }
 
   /// @brief get the bin indices of the cell containing the largest number
   /// of (weighted) hits across the entire histogram
   std::pair<std::size_t, std::size_t> locMaxHits() const {
     return m_maxLocHits;
   }
 
   /// @brief get the maximum number of (weighted) layers with hits  seen
   /// in a single cell across the entire histrogram.
   YieldType maxLayers() const { return m_maxLayers; }
 
   /// @brief get the bin indices of the cell containing the largest number
   /// of (weighted) layers with hits across the entire histogram
   std::pair<std::size_t, std::size_t> locMaxLayers() const {
     return m_maxLocLayers;
   }
 
  private:
   /// @brief Helper method to fill a bin of the hough histogram.
   /// Updates the internal helper data structures (maximum tracker etc).
   /// @param binX: bin number along x
   /// @param binY: bin number along y
   /// @param identifier: hit identifier
   /// @param layer: layer index
   /// @param w: optional hit weight
   void fillBin(std::size_t binX, std::size_t binY,
                const identifier_t& identifier, unsigned layer, double w = 1.0f);
 
   YieldType m_maxHits = 0.0f;    // track the maximum number of hits seen
   YieldType m_maxLayers = 0.0f;  // track the maximum number of layers seen
 
   /// track the location of the maximum in hits
   std::pair<std::size_t, std::size_t> m_maxLocHits = {0, 0};
   /// track the location of the maximum in layers
   std::pair<std::size_t, std::size_t> m_maxLocLayers = {0, 0};
 
   std::size_t m_assignBatch{20};
 
   /// track the bins with non-trivial content
   std::unordered_set<std::size_t> m_touchedBins{};
 
   std::size_t m_iBin = 0;
 
   HoughPlaneConfig m_cfg;  // the configuration object
   HoughHist m_houghHist;   // the histogram data object
 };
 
 /// example peak finders.
 namespace PeakFinders {
 /// configuration for the LayerGuidedCombinatoric peak finder
 struct LayerGuidedCombinatoricConfig {
   YieldType threshold = 3.0f;  // min number of layers to obtain a maximum
   int localMaxWindowSize = 0;  // Only create candidates from a local maximum
 };
 
 /// @brief Peak finder inspired by ATLAS ITk event filter developments.
 /// Builds peaks based on layer counts and allows for subsequent resolution
 /// of the combinatorics by building multiple candidates per peak if needed.
 /// In flat regions, peak positions are moved towards smaller values of the
 /// second and first coordinate.
 /// @tparam identifier_t: The identifier type to use. Should match the one used in the hough plane.
 template <class identifier_t>
 class LayerGuidedCombinatoric {
  public:
   /// @brief data class representing the found maxima.
   /// Here, we just have a list of cluster identifiers
   struct Maximum {
     std::unordered_set<identifier_t> hitIdentifiers =
         {};  // identifiers of contributing hits
   };
   /// @brief constructor
   /// @param cfg: Configuration object
   explicit LayerGuidedCombinatoric(const LayerGuidedCombinatoricConfig& cfg);
 
   /// @brief main peak finder method.
   /// @param plane: Filled hough plane to search
   /// @return vector of found maxima
   std::vector<Maximum> findPeaks(const HoughPlane<identifier_t>& plane) const;
 
  private:
   /// @brief check if a given bin is a local maximum.
   /// @param plane: The filled hough plane
   /// @param xBin: x bin index
   /// @param yBin: y bin index
   /// @return true if a maximum, false otherwise
   bool passThreshold(const HoughPlane<identifier_t>& plane, std::size_t xBin,
                      std::size_t yBin) const;  // did we pass extensions?
 
   LayerGuidedCombinatoricConfig m_cfg;  // configuration data object
 };
 /// @brief Configuration for the IslandsAroundMax peak finder
 struct IslandsAroundMaxConfig {
   YieldType threshold =
       3.0f;  // min number of weigted hits required in a maximum
   YieldType fractionCutoff =
       0;  // Fraction of the global maximum at which to cut off maxima
   std::pair<CoordType, CoordType> minSpacingBetweenPeaks = {
       0.0f, 0.0f};  // minimum distance of a new peak from existing peaks in
                     // parameter space
 };
 /// @brief Peak finder inspired by ATLAS muon reconstruction.
 /// Looks for regions above a given fraction of the global maximum
 /// hit count and connects them into islands comprising adjacent bins
 /// above the threshold. Peak positions are averaged across cells in the island,
 /// weighted by hit counts
 /// @tparam identifier_t: The identifier type to use. Should match the one used in the hough plane.
 template <class identifier_t>
 class IslandsAroundMax {
  public:
   /// @brief data struct representing a local maximum.
   /// Comes with a position estimate and a list of hits within the island
   struct Maximum {
     CoordType x = 0;   // x value of the maximum
     CoordType y = 0;   // y value of the maximum
     CoordType wx = 0;  // x width of the maximum
     CoordType wy = 0;  // y width of the maximum
     std::unordered_set<identifier_t> hitIdentifiers =
         {};  // identifiers of contributing hits
   };
   /// @brief constructor.
   /// @param cfg: configuration object
   explicit IslandsAroundMax(const IslandsAroundMaxConfig& cfg);
 
   /// @brief main peak finder method.
   /// @param plane: The filled hough plane to search
   /// @param ranges: The axis ranges used for mapping between parameter space and bins.
   /// @return List of the found maxima
   std::vector<Maximum> findPeaks(const HoughPlane<identifier_t>& plane,
                                  const HoughAxisRanges& ranges);
 
  private:
   /// @brief method to incrementally grow an island by adding adjacent cells
   /// Performs a breadth-first search for neighbours above threshold and adds
   /// them to candidate. Stops when no suitable neighbours are left.
   /// @param houghPlane: The current hough Plane we are looking for maxima
   /// @param inMaximum: List of cells found in the island. Incrementally populated by calls to the method
   /// @param toExplore: List of the global Bin indices of neighbour cell candidates left to explore. Method will not do anything once this is empty
   /// @param threshold: the threshold to apply to check if a cell should be added to an island
   /// @param yieldMap: A map of the hit content of above-threshold cells. Used cells will be set to empty content to avoid re-use by subsequent calls
   void extendMaximum(const HoughPlane<identifier_t>& houghPlane,
                      std::vector<std::array<std::size_t, 2>>& inMaximum,
                      std::vector<std::size_t>& toExplore, YieldType threshold,
                      std::unordered_map<std::size_t, YieldType>& yieldMap);
 
   IslandsAroundMaxConfig m_cfg;  // configuration data object
 
   /// @brief array of steps to consider when exploring neighbouring cells.
   const std::array<std::pair<int, int>, 8> m_stepDirections{
       std::make_pair(-1, -1), std::make_pair(0, -1), std::make_pair(1, -1),
       std::make_pair(-1, 0),  std::make_pair(1, 0),  std::make_pair(-1, 1),
       std::make_pair(0, 1),   std::make_pair(1, 1)};
 };
 }  // namespace PeakFinders
 }  // namespace Acts::HoughTransformUtils
 
 #include "HoughTransformUtils.ipp"

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