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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/Alignment.hpp"
 #include "Acts/Definitions/TrackParametrization.hpp"
 #include "Acts/EventData/MultiTrajectory.hpp"
 #include "Acts/EventData/MultiTrajectoryHelpers.hpp"
 #include "Acts/EventData/TrackParameters.hpp"
 #include "Acts/Geometry/GeometryContext.hpp"
 #include "Acts/Surfaces/Surface.hpp"
 #include "ActsAlignment/Kernel/AlignmentMask.hpp"
 
 #include <unordered_map>
 
 namespace ActsAlignment::detail {
 
 using namespace Acts;
 ///
 ///@brief struct to store info needed for track-based alignment
 ///
 struct TrackAlignmentState {
   // The dimension of measurements
   std::size_t measurementDim = 0;
 
   // The dimension of track parameters
   std::size_t trackParametersDim = 0;
 
   // The contributed alignment degree of freedom
   std::size_t alignmentDof = 0;
 
   // The measurements covariance
   ActsDynamicMatrix measurementCovariance;
 
   // The track parameters covariance
   ActsDynamicMatrix trackParametersCovariance;
 
   // The projection matrix
   ActsDynamicMatrix projectionMatrix;
 
   // The residual
   ActsDynamicVector residual;
 
   // The covariance of residual
   ActsDynamicMatrix residualCovariance;
 
   // The chi2
   double chi2 = 0;
 
   // The derivative of residual w.r.t. alignment parameters
   ActsDynamicMatrix alignmentToResidualDerivative;
 
   // The derivative of chi2 w.r.t. alignment parameters
   ActsDynamicVector alignmentToChi2Derivative;
 
   // The second derivative of chi2 w.r.t. alignment parameters
   ActsDynamicMatrix alignmentToChi2SecondDerivative;
 
   // The alignable surfaces on the track and their indices in both the global
   // alignable surfaces pool and those relevant with this track
   std::unordered_map<const Surface*, std::pair<std::size_t, std::size_t>>
       alignedSurfaces;
 };
 
 /// Reset some columns of the alignment to bound derivative to zero if the
 /// relevant degree of freedom is fixed
 ///
 /// @param alignToBound The alignment to bound parameters derivative
 /// @param mask The alignment mask
 void resetAlignmentDerivative(Acts::AlignmentToBoundMatrix& alignToBound,
                               AlignmentMask mask);
 
 ///
 /// Calculate the first and second derivative of chi2 w.r.t. alignment
 /// parameters for a single track
 ///
 /// Suppose there are n measurements on the track, and m (m<=n) of them are on
 /// alignable surface, then (eAlignmentSize*m) alignment parameters
 /// will be involved for this particular track, i.e. this track will contribute
 /// to at most (eAlignmentSize*m*2) elements of the full chi2
 /// second derivative matrix
 ///
 /// @tparam source_link_t The source link type of the trajectory
 /// @tparam parameters_t The track parameters type
 ///
 /// @param gctx The current geometry context object
 /// @param multiTraj The MultiTrajectory containing the trajectory to be
 /// investigated
 /// @param entryIndex The trajectory entry index
 /// @param globalTrackParamsCov The global track parameters covariance for a
 /// single track and the starting row/column for smoothed states. This contains
 /// all smoothed track states including those non-measurement states. Selection
 /// of certain rows/columns for measurement states is needed.
 /// @param idxedAlignSurfaces The indexed surfaces to be aligned
 ///
 /// @return The track alignment state containing fundamental alignment
 /// ingredients
 template <typename traj_t, typename parameters_t = BoundTrackParameters>
 TrackAlignmentState trackAlignmentState(
     const GeometryContext& gctx, const traj_t& multiTraj,
     const std::size_t& entryIndex,
     const std::pair<ActsDynamicMatrix,
                     std::unordered_map<std::size_t, std::size_t>>&
         globalTrackParamsCov,
     const std::unordered_map<const Surface*, std::size_t>& idxedAlignSurfaces,
     const AlignmentMask& alignMask) {
   using CovMatrix = typename parameters_t::CovarianceMatrix;
 
   // Construct an alignment state
   TrackAlignmentState alignState;
 
   // Remember the index within the trajectory and whether it's alignable
   std::vector<std::pair<std::size_t, bool>> measurementStates;
   measurementStates.reserve(15);
   // Number of smoothed states on the track
   // std::size_t nSmoothedStates = 0; // commented because clang-tidy complains
   // about unused Number of alignable surfaces on the track
   std::size_t nAlignSurfaces = 0;
 
   // Visit the track states on the track
   multiTraj.visitBackwards(entryIndex, [&](const auto& ts) {
     // Remember the number of smoothed states
     if (ts.hasSmoothed()) {
       // nSmoothedStates++; // commented because clang-tidy complains about
       // unused
     } else {
       // @note: this should in principle never happen now. But still keep it as a note
       return true;
     }
 
     // Only measurement states matter (we can't align non-measurement states,
     // no?)
     if (!ts.typeFlags().test(TrackStateFlag::MeasurementFlag)) {
       return true;
     }
     // Check if the reference surface is to be aligned
     bool isAlignable = false;
     const auto surface = &ts.referenceSurface();
     auto it = idxedAlignSurfaces.find(surface);
     if (it != idxedAlignSurfaces.end()) {
       isAlignable = true;
       // Remember the surface and its index
       alignState.alignedSurfaces[surface].first = it->second;
       nAlignSurfaces++;
     }
     // Remember the index of the state within the trajectory and whether it's
     // alignable
     measurementStates.push_back({ts.index(), isAlignable});
     // Add up measurement dimension
     alignState.measurementDim += ts.calibratedSize();
     return true;
   });
 
   // Return now if the track contains no alignable surfaces
   if (nAlignSurfaces == 0) {
     return alignState;
   }
 
   // The alignment degree of freedom
   alignState.alignmentDof = eAlignmentSize * nAlignSurfaces;
   // Dimension of global track parameters (from only measurement states)
   alignState.trackParametersDim = eBoundSize * measurementStates.size();
 
   // Initialize the alignment matrices with components from the measurement
   // states
   // The measurement covariance
   alignState.measurementCovariance = ActsDynamicMatrix::Zero(
       alignState.measurementDim, alignState.measurementDim);
   // The bound parameters to measurement projection matrix
   alignState.projectionMatrix = ActsDynamicMatrix::Zero(
       alignState.measurementDim, alignState.trackParametersDim);
   // The derivative of residual w.r.t. alignment parameters
   alignState.alignmentToResidualDerivative = ActsDynamicMatrix::Zero(
       alignState.measurementDim, alignState.alignmentDof);
   // The track parameters covariance
   alignState.trackParametersCovariance = ActsDynamicMatrix::Zero(
       alignState.trackParametersDim, alignState.trackParametersDim);
   // The residual
   alignState.residual = ActsDynamicVector::Zero(alignState.measurementDim);
 
   // Unpack global track parameters covariance and the starting row/column for
   // all smoothed states.
   // Note that the dimension of provided global track parameters covariance
   // should be same as eBoundSize * nSmoothedStates
   const auto& [sourceTrackParamsCov, stateRowIndices] = globalTrackParamsCov;
 
   // Loop over the measurement states to fill those alignment matrices
   // This is done in reverse order
   std::size_t iMeasurement = alignState.measurementDim;
   std::size_t iParams = alignState.trackParametersDim;
   std::size_t iSurface = nAlignSurfaces;
   for (const auto& [rowStateIndex, isAlignable] : measurementStates) {
     const auto& state = multiTraj.getTrackState(rowStateIndex);
     const std::size_t measdim = state.calibratedSize();
     // Update index of current measurement and parameter
     iMeasurement -= measdim;
     iParams -= eBoundSize;
     // (a) Get and fill the measurement covariance matrix
     const ActsDynamicMatrix measCovariance =
         state.effectiveCalibratedCovariance();
     alignState.measurementCovariance.block(iMeasurement, iMeasurement, measdim,
                                            measdim) = measCovariance;
 
     // (b) Get and fill the bound parameters to measurement projection matrix
     const ActsDynamicMatrix H =
         state.projectorSubspaceHelper().fullProjector().topLeftCorner(
             measdim, eBoundSize);
     alignState.projectionMatrix.block(iMeasurement, iParams, measdim,
                                       eBoundSize) = H;
     // (c) Get and fill the residual
     alignState.residual.segment(iMeasurement, measdim) =
         state.effectiveCalibrated() - H * state.smoothed();
 
     // (d) Get the derivative of alignment parameters w.r.t. measurement
     // or residual
     if (isAlignable) {
       iSurface -= 1;
       const auto surface = &state.referenceSurface();
       alignState.alignedSurfaces.at(surface).second = iSurface;
       // The free parameters transformed from the smoothed parameters
       const FreeVector freeParams =
           Acts::MultiTrajectoryHelpers::freeSmoothed(gctx, state);
       // The position
       const Vector3 position = freeParams.segment<3>(eFreePos0);
       // The direction
       const Vector3 direction = freeParams.segment<3>(eFreeDir0);
       // The derivative of free parameters w.r.t. path length. @note Here, we
       // assume a linear track model, i.e. neglecting the change of track
       // direction. Otherwise, we need to know the magnetic field at the free
       // parameters
       FreeVector pathDerivative = FreeVector::Zero();
       pathDerivative.head<3>() = direction;
       // Get the derivative of bound parameters w.r.t. alignment parameters
       AlignmentToBoundMatrix alignToBound = surface->alignmentToBoundDerivative(
           gctx, position, direction, pathDerivative);
       // Set the degree of freedom per surface.
       // @Todo: don't allocate memory for fixed degree of freedom and consider surface/layer/volume wise align mask (instead of using global mask as now)
       resetAlignmentDerivative(alignToBound, alignMask);
 
       // Residual is calculated as the (measurement - parameters), thus we need
       // a minus sign below
       alignState.alignmentToResidualDerivative.block(
           iMeasurement, iSurface * eAlignmentSize, measdim, eAlignmentSize) =
           -H * (alignToBound);
     }
 
     // (e) Extract and fill the track parameters covariance matrix for only
     // measurement states
     // @Todo: add helper function to select rows/columns of a matrix
     for (unsigned int iColState = 0; iColState < measurementStates.size();
          iColState++) {
       std::size_t colStateIndex = measurementStates.at(iColState).first;
       // Retrieve the block from the source covariance matrix
       CovMatrix correlation =
           sourceTrackParamsCov.block<eBoundSize, eBoundSize>(
               stateRowIndices.at(rowStateIndex),
               stateRowIndices.at(colStateIndex));
       // Fill the block of the target covariance matrix
       std::size_t iCol =
           alignState.trackParametersDim - (iColState + 1) * eBoundSize;
       alignState.trackParametersCovariance.block<eBoundSize, eBoundSize>(
           iParams, iCol) = correlation;
     }
   }
 
   // Calculate the chi2 and chi2 derivatives based on the alignment matrixs
   alignState.chi2 = alignState.residual.transpose() *
                     alignState.measurementCovariance.inverse() *
                     alignState.residual;
   alignState.alignmentToChi2Derivative =
       ActsDynamicVector::Zero(alignState.alignmentDof);
   alignState.alignmentToChi2SecondDerivative =
       ActsDynamicMatrix::Zero(alignState.alignmentDof, alignState.alignmentDof);
   // The covariance of residual
   alignState.residualCovariance = ActsDynamicMatrix::Zero(
       alignState.measurementDim, alignState.measurementDim);
   alignState.residualCovariance = alignState.measurementCovariance -
                                   alignState.projectionMatrix *
                                       alignState.trackParametersCovariance *
                                       alignState.projectionMatrix.transpose();
 
   alignState.alignmentToChi2Derivative =
       2 * alignState.alignmentToResidualDerivative.transpose() *
       alignState.measurementCovariance.inverse() *
       alignState.residualCovariance *
       alignState.measurementCovariance.inverse() * alignState.residual;
   alignState.alignmentToChi2SecondDerivative =
       2 * alignState.alignmentToResidualDerivative.transpose() *
       alignState.measurementCovariance.inverse() *
       alignState.residualCovariance *
       alignState.measurementCovariance.inverse() *
       alignState.alignmentToResidualDerivative;
 
   return alignState;
 }
 
 }  // namespace ActsAlignment::detail

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