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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/detail/TrackParametersUtils.hpp"
 #include "Acts/Geometry/GeometryContext.hpp"
 
 #include <map>
 #include <memory>
 #include <mutex>
 #include <stdexcept>
 #include <utility>
 
 namespace Acts {
 
 /// @brief Class to accumulate and average track lookup tables
 ///
 /// @tparam Grid type for track parameters accumulation
 ///
 /// This class is used to accumulate track parameters in
 /// reference layer grids and average them to create a lookup
 /// table for track parameter estimation in seeding
 ///
 /// @note Geometry context is left to be handled by the user
 /// outside of accumulation
 template <detail::TrackParamsGrid grid_t>
 class TrackParamsLookupAccumulator {
  public:
   /// Type alias for track parameter lookup grid
   using LookupGrid = grid_t;
   /// Type alias for track parameters type
   using TrackParameters = typename std::pointer_traits<
       typename grid_t::value_type::first_type>::element_type;
 
   /// @brief Constructor
   /// @param grid Grid to use for track parameter lookup accumulation
   explicit TrackParamsLookupAccumulator(grid_t grid)
       : m_grid(std::move(grid)) {}
 
   /// @brief Add track parameters to the accumulator
   ///
   /// @param ipTrackParameters Track parameters at the IP
   /// @param refTrackParameters Track parameters at the reference layer
   /// @param position Local position of the track hit on the reference layer
   void addTrack(const TrackParameters& ipTrackParameters,
                 const TrackParameters& refTrackParameters,
                 const Vector2& position) {
     std::lock_guard<std::mutex> lock(m_gridMutex);
 
     auto bin = m_grid.localBinsFromPosition(position);
 
     if (m_countGrid[bin] == 0) {
       m_grid.atLocalBins(bin).first =
           std::make_shared<TrackParameters>(ipTrackParameters);
       m_grid.atLocalBins(bin).second =
           std::make_shared<TrackParameters>(refTrackParameters);
 
       m_countGrid.at(bin)++;
       return;
     }
 
     *m_grid.atLocalBins(bin).first =
         addTrackParameters(*m_grid.atLocalBins(bin).first, ipTrackParameters);
     *m_grid.atLocalBins(bin).second =
         addTrackParameters(*m_grid.atLocalBins(bin).second, refTrackParameters);
     m_countGrid.at(bin)++;
   }
 
   /// @brief Finalize the lookup table
   ///
   /// @return Grid with the bin track parameters averaged
   LookupGrid finalizeLookup() {
     auto meanTrack = [&](const TrackParameters& track, std::size_t count) {
       if constexpr (detail::isGenericBoundTrackParams<TrackParameters>) {
         Acts::GeometryContext gctx;
 
         auto res = TrackParameters::create(
             gctx, track.referenceSurface().getSharedPtr(),
             track.fourPosition(gctx) / count, track.momentum().normalized(),
             count * track.charge() / track.momentum().norm(),
             track.covariance(), track.particleHypothesis());
 
         if (!res.ok()) {
           throw std::invalid_argument("Bound track grid finalization failed");
         }
         return res.value();
       } else {
         return TrackParameters(track.fourPosition() / count,
                                track.momentum().normalized(),
                                count * track.charge() / track.momentum().norm(),
                                track.covariance(), track.particleHypothesis());
       }
     };
 
     for (auto [bin, count] : m_countGrid) {
       if (count == 0) {
         continue;
       }
       *m_grid.atLocalBins(bin).first =
           meanTrack(*m_grid.atLocalBins(bin).first, count);
       *m_grid.atLocalBins(bin).second =
           meanTrack(*m_grid.atLocalBins(bin).second, count);
     }
 
     return m_grid;
   }
 
  private:
   /// @brief Add two track parameters
   ///
   /// @param a First track parameter in the sum
   /// @param b Second track parameter in the sum
   ///
   /// @return Sum of track parameters a + b
   ///
   /// @note Covariances of the track parameters
   /// are not added and instead assumed to be
   /// generated by the same random process for
   /// both a and b, making its averaging redundant
   TrackParameters addTrackParameters(const TrackParameters& a,
                                      const TrackParameters& b) {
     if (a.particleHypothesis() != b.particleHypothesis()) {
       throw std::invalid_argument(
           "Cannot accumulate track parameters with different particle "
           "hypotheses");
     }
     if (a.charge() != b.charge()) {
       throw std::invalid_argument(
           "Cannot accumulate track parameters with different charges");
     }
     if constexpr (detail::isGenericBoundTrackParams<TrackParameters>) {
       if (a.referenceSurface() != b.referenceSurface()) {
         throw std::invalid_argument(
             "Cannot accumulate bound track parameters with different reference "
             "surfaces");
       }
     }
 
     Acts::Vector3 momentum = a.momentum() + b.momentum();
 
     // Assume track parameters being i.i.d.
     if constexpr (detail::isGenericBoundTrackParams<TrackParameters>) {
       Acts::GeometryContext gctx;
 
       Acts::Vector4 fourPosition = a.fourPosition(gctx) + b.fourPosition(gctx);
 
       auto res = TrackParameters::create(
           gctx, a.referenceSurface().getSharedPtr(), fourPosition,
           momentum.normalized(), a.charge() / momentum.norm(), a.covariance(),
           a.particleHypothesis());
 
       if (!res.ok()) {
         throw std::runtime_error("Invalid bound track parameters");
       }
       return res.value();
     } else {
       Acts::Vector4 fourPosition = a.fourPosition() + b.fourPosition();
       return TrackParameters(fourPosition, momentum.normalized(),
                              a.charge() / momentum.norm(), a.covariance(),
                              a.particleHypothesis());
     }
   }
 
   /// Grids to accumulate IP and reference
   /// layer track parameters
   LookupGrid m_grid;
 
   /// Mutex for protecting grid access
   std::mutex m_gridMutex;
 
   /// Map to keep the accumulation count
   /// in the occupied grid bins
   std::map<std::array<std::size_t, LookupGrid::DIM>, std::size_t> m_countGrid;
 };
 
 }  // namespace Acts

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