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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/EventData/SourceLink.hpp"
 #include "Acts/EventData/TrackParameters.hpp"
 #include "Acts/Seeding/detail/UtilityFunctions.hpp"
 #include "Acts/Utilities/Delegate.hpp"
 #include "Acts/Utilities/GridIterator.hpp"
 
 namespace Acts {
 
 /// @brief Seeding algorigthm that extracts
 /// the IP parameters and sorts the source links
 /// into possible track candidates
 ///
 /// The algorithm to convert the given source links
 /// into seeds -- pairs of IP parameters and the corresponding
 /// source links -- as follows: First the source links
 /// are sorted into a user-defined grid. Then, iteration over the source links
 /// is performed. If a source link is attached to a surface that is
 /// in the reference tracking layer, as defined by the user, the IP parameters
 /// are estimated and the tracking layers are intersected to construct the
 /// core of the "Path". The source links in the subsequent layers are then
 /// added to the seed if they lie within the path width of the core.
 /// Both the list of source links and the IP parameters are stored in the seed
 /// struct.
 ///
 /// @tparam axis_t Type of the axis to bin
 /// the source links
 ///
 /// @note The algorithm is designed to be used in the
 /// context of a telescope-style geometry. The surfaces
 /// are assumed to be planar.
 ///
 /// @note Handling of the rotated surfaces has to happen
 /// in the user-defined delegate functions.
 class PathSeeder {
  public:
   using PathSeed = std::pair<BoundTrackParameters, std::vector<SourceLink>>;
 
   /// @brief Delegate to estimate the IP parameters
   /// and the momentum direction at the reference tracking layer
   ///
   /// @arg Geometry context to use
   /// @arg Pivot source link
   ///
   /// @return Pair of the track parameters at the IP and
   /// the reference tracking layer
   using TrackEstimator =
       Delegate<std::pair<BoundTrackParameters, BoundTrackParameters>(
           const GeometryContext&, const SourceLink&)>;
 
   /// @brief Delegate to find the intersections for the given pivot
   /// source link
   ///
   /// @arg The geometry context to use
   /// @arg Track parameters at the reference tracking layer
   ///
   /// @return Vector of pairs of the geometry identifier
   /// and the local intersection point
   using IntersectionLookup =
       Delegate<std::vector<std::pair<GeometryIdentifier, Vector2>>(
           const GeometryContext&, const BoundTrackParameters&)>;
 
   /// @brief Delegate to provide the path width around
   /// the intersection point to pull the source links
   /// from the grid
   ///
   /// @arg The geometry context to use
   /// @arg The geometry identifier to use if the
   /// path width is varied across different tracking layers
   ///
   /// @return The path width in the bin0 and bin1 direction
   /// defined with respect to the surface normal
   using PathWidthLookup = Delegate<std::pair<double, double>(
       const GeometryContext&, const GeometryIdentifier&)>;
 
   /// @brief The nested configuration struct
   struct Config {
     /// Parameters estimator
     TrackEstimator trackEstimator;
     /// Intersection finder
     IntersectionLookup intersectionFinder;
     /// Path width provider
     PathWidthLookup pathWidthProvider;
     /// Reference layer IDs
     std::vector<GeometryIdentifier> refLayerIds;
   };
 
   /// @brief Constructor
   explicit PathSeeder(const Config& config) : m_cfg(config) {};
 
   /// @brief Destructor
   ~PathSeeder() = default;
 
   /// @brief Extract the IP parameters and
   /// sort the source links into the seeds
   ///
   /// @param gctx The geometry context
   /// @param sourceLinkGridLookup The lookup table for the source links
   /// @param seedCollection The collection of seeds to fill
   template <Acts::detail::SourceLinkGrid grid_t, typename container_t>
   void findSeeds(const GeometryContext& gctx,
                  const std::unordered_map<GeometryIdentifier, grid_t>&
                      sourceLinkGridLookup,
                  container_t& seedCollection) const {
     // Create the seeds
     for (auto& refGeoId : m_cfg.refLayerIds) {
       auto refGrid = sourceLinkGridLookup.at(refGeoId);
 
       for (auto it = refGrid.begin(); it != refGrid.end(); it++) {
         std::vector<SourceLink> pivotSourceLinks = *it;
 
         for (const auto& pivot : pivotSourceLinks) {
           // Get the IP parameters
           auto [ipParameters, refLayerParameters] =
               m_cfg.trackEstimator(gctx, pivot);
 
           // Intersect with the surfaces
           std::vector<std::pair<GeometryIdentifier, Vector2>> intersections =
               m_cfg.intersectionFinder(gctx, refLayerParameters);
 
           // Continue if no intersections
           if (intersections.empty()) {
             continue;
           }
 
           // Iterate over the intersections
           // and get the source links
           // in the subsequent layers
           std::vector<SourceLink> seedSourceLinks;
           for (auto& [geoId, refPoint] : intersections) {
             // Get the path width
             auto [pathWidth0, pathWidth1] =
                 m_cfg.pathWidthProvider(gctx, geoId);
 
             // Get the bounds of the path
             double top0 = refPoint[0] + pathWidth0;
             double bot0 = refPoint[0] - pathWidth0;
             double top1 = refPoint[1] + pathWidth1;
             double bot1 = refPoint[1] - pathWidth1;
 
             // Get the lookup table for the source links
             auto grid = sourceLinkGridLookup.at(geoId);
 
             // Get the range of bins to search for source links
             auto botLeftBin = grid.localBinsFromPosition(Vector2(bot0, bot1));
             auto topRightBin = grid.localBinsFromPosition(Vector2(top0, top1));
 
             // Get the source links from the lookup table
             // by iterating over the bin ranges
             auto currentBin = botLeftBin;
             while (currentBin.at(1) <= topRightBin.at(1)) {
               while (currentBin.at(0) <= topRightBin.at(0)) {
                 auto sourceLinksToAdd = grid.atLocalBins(currentBin);
 
                 seedSourceLinks.insert(seedSourceLinks.end(),
                                        sourceLinksToAdd.begin(),
                                        sourceLinksToAdd.end());
 
                 currentBin.at(0)++;
               }
               currentBin.at(1)++;
               currentBin.at(0) = botLeftBin.at(0);
             }
           }
           PathSeed seed = {ipParameters, seedSourceLinks};
 
           // Add the seed to the collection
           Acts::detail::pushBackOrInsertAtEnd(seedCollection, seed);
         }
       }
     }
   }
 
  private:
   Config m_cfg;
 };
 
 }  // namespace Acts

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