CurvilinearCovarianceHelper.h

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/*
  Copyright (C) 2002-2026 CERN for the benefit of the ATLAS collaboration
*/
 
#ifndef ACTSTRK_CURVILINEARCOVARIANCEHELPER_H
#define ACTSTRK_CURVILINEARCOVARIANCEHELPER_H
 
#include "Acts/Geometry/GeometryContext.hpp"
#include "Acts/EventData/BoundTrackParameters.hpp"
#include "Acts/Propagator/detail/JacobianEngine.hpp"
 
namespace ActsTrk::detail {
   inline double computeDtDs(const Acts::ParticleHypothesis &hypothesis, double qop) {
      return std::hypot(1.,hypothesis.mass() / hypothesis.extractMomentum(qop));
   }

   inline Acts::FreeToPathMatrix computeFreeToPathDerivatives( const Acts::Vector3 &direction,
                                                               double qop,
                                                               const Acts::Vector3 &bfield,
                                                               const Acts::ParticleHypothesis &particle_hypothesis)
   {
      Acts::FreeToPathMatrix path_length_deriv;
      static_assert(path_length_deriv.cols() == 8);  // ensure that all elements are initialized
      path_length_deriv.segment<3>(Acts::eFreePos0) = direction;
      path_length_deriv(0,Acts::eFreeTime) =  computeDtDs(particle_hypothesis,qop);
      path_length_deriv.segment<3>(Acts::eFreeDir0) = (qop  * direction.cross(bfield)).transpose();
      path_length_deriv(0,Acts::eFreeQOverP) = 0.;
      return path_length_deriv;
   }
 

   inline std::optional<Acts::BoundMatrix> convertActsBoundCovToCurvilinearParam(const Acts::GeometryContext &tgContext,
                                                                                 const Acts::BoundTrackParameters &param,
                                                                                 const Acts::Vector3 &magnFieldVect,
                                                                                 const Acts::ParticleHypothesis &particle_hypothesis)
   {
      if (param.covariance().has_value()) {
         Acts::FreeVector freeParams = Acts::transformBoundToFreeParameters(
            param.referenceSurface(), tgContext, param.parameters());
         Acts::Vector3 position = freeParams.segment<3>(Acts::eFreePos0);
         Acts::Vector3 direction = freeParams.segment<3>(Acts::eFreeDir0);
 
         Acts::BoundToFreeMatrix boundToFreeJacobian = param.referenceSurface().boundToFreeJacobian(tgContext, position, direction);
         Acts::FreeMatrix freeTransportJacobian = Acts::FreeMatrix::Identity();
         Acts::FreeToBoundMatrix freeToBoundJacobian;
         Acts::FreeVector freeToPathDerivatives = computeFreeToPathDerivatives(direction,
                                                                               param.parameters()[Acts::eBoundQOverP],
                                                                               magnFieldVect,
                                                                               particle_hypothesis);
         Acts::BoundMatrix fullTransportJacobian;
         Acts::detail::boundToCurvilinearTransportJacobian(direction,
                                                           boundToFreeJacobian,
                                                           freeTransportJacobian,
                                                           freeToBoundJacobian,
                                                           freeToPathDerivatives,
                                                           fullTransportJacobian);
 
         return fullTransportJacobian * param.covariance().value() * fullTransportJacobian.transpose();
      }
      else {
         return {};
      }
   }
}
 
#endif