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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/Algebra.hpp"
 #include "Acts/Material/IVolumeMaterial.hpp"
 #include "Acts/Material/Interactions.hpp"
 #include "Acts/Material/Material.hpp"
 #include "Acts/Material/MaterialSlab.hpp"
 #include "Acts/Propagator/EigenStepperDefaultExtension.hpp"
 #include "Acts/Utilities/MathHelpers.hpp"
 
 namespace Acts {
 
 /// @brief Evaluator of the k_i's and elements of the transport matrix
 /// D of the RKN4 stepping. This implementation involves energy loss due to
 /// ionisation, bremsstrahlung, pair production and photonuclear interaction
 /// in the propagation and the Jacobian. These effects will only occur if the
 /// propagation is in a TrackingVolume with attached material.
 struct EigenStepperDenseExtension {
   /// Fallback extension
   EigenStepperDefaultExtension defaultExtension;
 
   /// Momentum at a certain point
   double currentMomentum = 0.;
   /// Particles momentum at k1
   double initialMomentum = 0.;
   /// Material that will be passed
   /// TODO : Might not be needed anymore
   Material material = Material::Vacuum();
   /// Derivatives dLambda''dlambda at each sub-step point
   std::array<double, 4> dLdl{};
   /// q/p at each sub-step
   std::array<double, 4> qop{};
   /// Derivatives dPds at each sub-step
   std::array<double, 4> dPds{};
   /// Derivative d(dEds)d(q/p) evaluated at the initial point
   double dgdqopValue = 0.;
   /// Derivative dEds at the initial point
   double g = 0.;
   /// k_i equivalent for the time propagation
   std::array<double, 4> tKi{};
   /// Lambda''_i
   std::array<double, 4> Lambdappi{};
   /// Energy at each sub-step
   std::array<double, 4> energy{};
 
   /// @brief Evaluator of the k_i's of the RKN4. For the case of i = 0 this
   /// step sets up member parameters, too.
   ///
   /// @tparam i Index of the k_i, i = [0, 3]
   /// @tparam stepper_t Type of the stepper
   ///
   /// @param [in] state State of the stepper
   /// @param [in] stepper Stepper of the propagator
   /// @param [in] volumeMaterial Material of the volume
   /// @param [out] knew Next k_i that is evaluated
   /// @param [out] kQoP k_i elements of the momenta
   /// @param [in] bField B-Field at the evaluation position
   /// @param [in] h Step size (= 0. ^ 0.5 * StepSize ^ StepSize)
   /// @param [in] kprev Evaluated k_{i - 1}
   ///
   /// @return Boolean flag if the calculation is valid
   template <int i, typename stepper_t>
   bool k(const typename stepper_t::State& state, const stepper_t& stepper,
          const IVolumeMaterial* volumeMaterial, Vector3& knew,
          const Vector3& bField, std::array<double, 4>& kQoP,
          const double h = 0., const Vector3& kprev = Vector3::Zero())
     requires(i >= 0 && i <= 3)
   {
     if (volumeMaterial == nullptr) {
       return defaultExtension.template k<i>(state, stepper, volumeMaterial,
                                             knew, bField, kQoP, h, kprev);
     }
 
     double q = stepper.charge(state);
     const auto& particleHypothesis = stepper.particleHypothesis(state);
     float mass = particleHypothesis.mass();
 
     // i = 0 is used for setup and evaluation of k
     if constexpr (i == 0) {
       // Set up for energy loss
       Vector3 position = stepper.position(state);
       material = volumeMaterial->material(position.template cast<double>());
       initialMomentum = stepper.absoluteMomentum(state);
       currentMomentum = initialMomentum;
       qop[0] = stepper.qOverP(state);
       initializeEnergyLoss(state, stepper);
       // Evaluate k
       knew = qop[0] * stepper.direction(state).cross(bField);
       // Evaluate k for the time propagation
       Lambdappi[0] = -qop[0] * qop[0] * qop[0] * g * energy[0] / (q * q);
       //~ tKi[0] = std::hypot(1, mass / initialMomentum);
       tKi[0] = fastHypot(1, mass * qop[0]);
       kQoP[0] = Lambdappi[0];
     } else {
       // Update parameters and check for momentum condition
       updateEnergyLoss(mass, h, state, stepper, i);
       if (currentMomentum < state.options.dense.momentumCutOff) {
         return false;
       }
       // Evaluate k
       knew = qop[i] * (stepper.direction(state) + h * kprev).cross(bField);
       // Evaluate k_i for the time propagation
       auto qopNew = qop[0] + h * Lambdappi[i - 1];
       Lambdappi[i] = -qopNew * qopNew * qopNew * g * energy[i] / (q * q);
       tKi[i] = fastHypot(1, mass * qopNew);
       kQoP[i] = Lambdappi[i];
     }
     return true;
   }
 
   /// @brief After a RKN4 step was accepted by the stepper this method has an
   /// additional veto on the quality of the step. The veto lies in evaluation
   /// of the energy loss and the therewith constrained to keep the momentum
   /// after the step in reasonable values.
   ///
   /// @tparam stepper_t Type of the stepper
   ///
   /// @param [in] state State of the stepper
   /// @param [in] stepper Stepper of the propagator
   /// @param [in] volumeMaterial Material of the volume
   /// @param [in] h Step size
   ///
   /// @return Boolean flag if the calculation is valid
   template <typename stepper_t>
   bool finalize(typename stepper_t::State& state, const stepper_t& stepper,
                 const IVolumeMaterial* volumeMaterial, const double h) const {
     if (volumeMaterial == nullptr) {
       return defaultExtension.finalize(state, stepper, volumeMaterial, h);
     }
 
     const auto& particleHypothesis = stepper.particleHypothesis(state);
     float mass = particleHypothesis.mass();
 
     // Evaluate the new momentum
     auto newMomentum =
         stepper.absoluteMomentum(state) +
         (h / 6.) * (dPds[0] + 2. * (dPds[1] + dPds[2]) + dPds[3]);
 
     // Break propagation if momentum becomes below cut-off
     if (newMomentum < state.options.dense.momentumCutOff) {
       return false;
     }
 
     // Add derivative dlambda/ds = Lambda''
     state.derivative(7) = -fastHypot(mass, newMomentum) * g /
                           (newMomentum * newMomentum * newMomentum);
 
     // Update momentum
     state.pars[eFreeQOverP] = stepper.charge(state) / newMomentum;
     // Add derivative dt/ds = 1/(beta * c) = sqrt(m^2 * p^{-2} + c^{-2})
     state.derivative(3) = fastHypot(1, mass / newMomentum);
     // Update time
     state.pars[eFreeTime] +=
         (h / 6.) * (tKi[0] + 2. * (tKi[1] + tKi[2]) + tKi[3]);
 
     return true;
   }
 
   /// @brief After a RKN4 step was accepted by the stepper this method has an
   /// additional veto on the quality of the step. The veto lies in the
   /// evaluation
   /// of the energy loss, the therewith constrained to keep the momentum
   /// after the step in reasonable values and the evaluation of the transport
   /// matrix.
   ///
   /// @tparam stepper_t Type of the stepper
   ///
   /// @param [in] state State of the stepper
   /// @param [in] stepper Stepper of the propagator
   /// @param [in] volumeMaterial Material of the volume
   /// @param [in] h Step size
   /// @param [out] D Transport matrix
   ///
   /// @return Boolean flag if the calculation is valid
   template <typename stepper_t>
   bool finalize(typename stepper_t::State& state, const stepper_t& stepper,
                 const IVolumeMaterial* volumeMaterial, const double h,
                 FreeMatrix& D) const {
     if (volumeMaterial == nullptr) {
       return defaultExtension.finalize(state, stepper, volumeMaterial, h, D);
     }
 
     return finalize(state, stepper, volumeMaterial, h) &&
            transportMatrix(state, stepper, h, D);
   }
 
  private:
   /// @brief Evaluates the transport matrix D for the Jacobian
   ///
   /// @tparam propagator_state_t Type of the state of the propagator
   /// @tparam stepper_t Type of the stepper
   ///
   /// @param [in] state State of the propagator
   /// @param [in] stepper Stepper of the propagator
   /// @param [in] h Step size
   /// @param [out] D Transport matrix
   ///
   /// @return Boolean flag if evaluation is valid
   template <typename stepper_t>
   bool transportMatrix(typename stepper_t::State& state,
                        const stepper_t& stepper, const double h,
                        FreeMatrix& D) const {
     /// The calculations are based on ATL-SOFT-PUB-2009-002. The update of the
     /// Jacobian matrix is requires only the calculation of eq. 17 and 18.
     /// Since the terms of eq. 18 are currently 0, this matrix is not needed
     /// in the calculation. The matrix A from eq. 17 consists out of 3
     /// different parts. The first one is given by the upper left 3x3 matrix
     /// that are calculated by dFdT and dGdT. The second is given by the top 3
     /// lines of the rightmost column. This is calculated by dFdL and dGdL.
     /// The remaining non-zero term is calculated directly. The naming of the
     /// variables is explained in eq. 11 and are directly related to the
     /// initial problem in eq. 7.
     /// The evaluation is based on propagating the parameters T and lambda
     /// (including g(lambda) and E(lambda)) as given in eq. 16 and evaluating
     /// the derivations for matrix A.
     /// @note The translation for u_{n+1} in eq. 7 is in this case a
     /// 3-dimensional vector without a dependency of Lambda or lambda neither in
     /// u_n nor in u_n'. The second and fourth eq. in eq. 14 have the constant
     /// offset matrices h * Id and Id respectively. This involves that the
     /// constant offset does not exist for rectangular matrix dFdu' (due to the
     /// missing Lambda part) and only exists for dGdu' in dlambda/dlambda.
 
     auto& sd = state.stepData;
     auto dir = stepper.direction(state);
     const auto& particleHypothesis = stepper.particleHypothesis(state);
     float mass = particleHypothesis.mass();
 
     D = FreeMatrix::Identity();
     const double half_h = h * 0.5;
 
     // This sets the reference to the sub matrices
     // dFdx is already initialised as (3x3) zero
     auto dFdT = D.block<3, 3>(0, 4);
     auto dFdL = D.block<3, 1>(0, 7);
     // dGdx is already initialised as (3x3) identity
     auto dGdT = D.block<3, 3>(4, 4);
     auto dGdL = D.block<3, 1>(4, 7);
 
     SquareMatrix3 dk1dT = SquareMatrix3::Zero();
     SquareMatrix3 dk2dT = SquareMatrix3::Identity();
     SquareMatrix3 dk3dT = SquareMatrix3::Identity();
     SquareMatrix3 dk4dT = SquareMatrix3::Identity();
 
     Vector3 dk1dL = Vector3::Zero();
     Vector3 dk2dL = Vector3::Zero();
     Vector3 dk3dL = Vector3::Zero();
     Vector3 dk4dL = Vector3::Zero();
 
     /// Propagation of derivatives of dLambda''dlambda at each sub-step
     std::array<double, 4> jdL{};
 
     // Evaluation of the rightmost column without the last term.
     jdL[0] = dLdl[0];
     dk1dL = dir.cross(sd.B_first);
 
     jdL[1] = dLdl[1] * (1. + half_h * jdL[0]);
     dk2dL = (1. + half_h * jdL[0]) * (dir + half_h * sd.k1).cross(sd.B_middle) +
             qop[1] * half_h * dk1dL.cross(sd.B_middle);
 
     jdL[2] = dLdl[2] * (1. + half_h * jdL[1]);
     dk3dL = (1. + half_h * jdL[1]) * (dir + half_h * sd.k2).cross(sd.B_middle) +
             qop[2] * half_h * dk2dL.cross(sd.B_middle);
 
     jdL[3] = dLdl[3] * (1. + h * jdL[2]);
     dk4dL = (1. + h * jdL[2]) * (dir + h * sd.k3).cross(sd.B_last) +
             qop[3] * h * dk3dL.cross(sd.B_last);
 
     dk1dT(0, 1) = sd.B_first.z();
     dk1dT(0, 2) = -sd.B_first.y();
     dk1dT(1, 0) = -sd.B_first.z();
     dk1dT(1, 2) = sd.B_first.x();
     dk1dT(2, 0) = sd.B_first.y();
     dk1dT(2, 1) = -sd.B_first.x();
     dk1dT *= qop[0];
 
     dk2dT += half_h * dk1dT;
     dk2dT = qop[1] * VectorHelpers::cross(dk2dT, sd.B_middle);
 
     dk3dT += half_h * dk2dT;
     dk3dT = qop[2] * VectorHelpers::cross(dk3dT, sd.B_middle);
 
     dk4dT += h * dk3dT;
     dk4dT = qop[3] * VectorHelpers::cross(dk4dT, sd.B_last);
 
     dFdT.setIdentity();
     dFdT += h / 6. * (dk1dT + dk2dT + dk3dT);
     dFdT *= h;
 
     dFdL = h * h / 6. * (dk1dL + dk2dL + dk3dL);
 
     dGdT += h / 6. * (dk1dT + 2. * (dk2dT + dk3dT) + dk4dT);
 
     dGdL = h / 6. * (dk1dL + 2. * (dk2dL + dk3dL) + dk4dL);
 
     // Evaluation of the dLambda''/dlambda term
     D(7, 7) += (h / 6.) * (jdL[0] + 2. * (jdL[1] + jdL[2]) + jdL[3]);
 
     // The following comment lines refer to the application of the time being
     // treated as a position. Since t and qop are treated independently for now,
     // this just serves as entry point for building their relation
     //~ double dtpp1dl = -mass * mass * qop[0] * qop[0] *
     //~ (3. * g + qop[0] * dgdqop(energy[0], .mass,
     //~ absPdg, meanEnergyLoss));
 
     double dtp1dl = qop[0] * mass * mass / fastHypot(1, qop[0] * mass);
     double qopNew = qop[0] + half_h * Lambdappi[0];
 
     //~ double dtpp2dl = -mass * mass * qopNew *
     //~ qopNew *
     //~ (3. * g * (1. + half_h * jdL[0]) +
     //~ qopNew * dgdqop(energy[1], mass, absPdgCode, meanEnergyLoss));
 
     double dtp2dl = qopNew * mass * mass / fastHypot(1, qopNew * mass);
     qopNew = qop[0] + half_h * Lambdappi[1];
 
     //~ double dtpp3dl = -mass * mass * qopNew *
     //~ qopNew *
     //~ (3. * g * (1. + half_h * jdL[1]) +
     //~ qopNew * dgdqop(energy[2], mass, absPdg, meanEnergyLoss));
 
     double dtp3dl = qopNew * mass * mass / fastHypot(1, qopNew * mass);
     qopNew = qop[0] + half_h * Lambdappi[2];
     double dtp4dl = qopNew * mass * mass / fastHypot(1, qopNew * mass);
 
     //~ D(3, 7) = h * mass * mass * qop[0] /
     //~ std::hypot(1., mass * qop[0])
     //~ + h * h / 6. * (dtpp1dl + dtpp2dl + dtpp3dl);
 
     D(3, 7) = (h / 6.) * (dtp1dl + 2. * (dtp2dl + dtp3dl) + dtp4dl);
     return true;
   }
 
   /// @brief Initializer of all parameters related to a RKN4 step with energy
   /// loss of a particle in material
   ///
   /// @tparam stepper_t Type of the stepper
   ///
   /// @param [in] state Deliverer of configurations
   /// @param [in] stepper Stepper of the propagator
   template <typename stepper_t>
   void initializeEnergyLoss(const typename stepper_t::State& state,
                             const stepper_t& stepper) {
     const auto& particleHypothesis = stepper.particleHypothesis(state);
     float mass = particleHypothesis.mass();
     PdgParticle absPdg = particleHypothesis.absolutePdg();
     float absQ = particleHypothesis.absoluteCharge();
 
     energy[0] = fastHypot(initialMomentum, mass);
     // use unit length as thickness to compute the energy loss per unit length
     MaterialSlab slab(material, 1);
     // Use the same energy loss throughout the step.
     if (state.options.dense.meanEnergyLoss) {
       g = -computeEnergyLossMean(slab, absPdg, mass, static_cast<float>(qop[0]),
                                  absQ);
     } else {
       // TODO using the unit path length is not quite right since the most
       //      probably energy loss is not independent from the path length.
       g = -computeEnergyLossMode(slab, absPdg, mass, static_cast<float>(qop[0]),
                                  absQ);
     }
     // Change of the momentum per path length
     // dPds = dPdE * dEds
     dPds[0] = g * energy[0] / initialMomentum;
     if (state.covTransport) {
       // Calculate the change of the energy loss per path length and
       // inverse momentum
       if (state.options.dense.includeGradient) {
         if (state.options.dense.meanEnergyLoss) {
           dgdqopValue = deriveEnergyLossMeanQOverP(
               slab, absPdg, mass, static_cast<float>(qop[0]), absQ);
         } else {
           // TODO path length dependence; see above
           dgdqopValue = deriveEnergyLossModeQOverP(
               slab, absPdg, mass, static_cast<float>(qop[0]), absQ);
         }
       }
       // Calculate term for later error propagation
       dLdl[0] = (-qop[0] * qop[0] * g * energy[0] *
                      (3. - (initialMomentum * initialMomentum) /
                                (energy[0] * energy[0])) -
                  qop[0] * qop[0] * qop[0] * energy[0] * dgdqopValue);
     }
   }
 
   /// @brief Update of the kinematic parameters of the RKN4 sub-steps after
   /// initialization with energy loss of a particle in material
   ///
   /// @tparam stepper_t Type of the stepper
   ///
   /// @param [in] h Stepped distance of the sub-step (1-3)
   /// @param [in] mass Mass of the particle
   /// @param [in] state State of the stepper
   /// @param [in] stepper Stepper of the propagator
   /// @param [in] i Index of the sub-step (1-3)
   template <typename stepper_t>
   void updateEnergyLoss(const double mass, const double h,
                         const typename stepper_t::State& state,
                         const stepper_t& stepper, const int i) {
     // Update parameters related to a changed momentum
     currentMomentum = initialMomentum + h * dPds[i - 1];
     energy[i] = fastHypot(currentMomentum, mass);
     dPds[i] = g * energy[i] / currentMomentum;
     qop[i] = stepper.charge(state) / currentMomentum;
     // Calculate term for later error propagation
     if (state.covTransport) {
       dLdl[i] = (-qop[i] * qop[i] * g * energy[i] *
                      (3. - (currentMomentum * currentMomentum) /
                                (energy[i] * energy[i])) -
                  qop[i] * qop[i] * qop[i] * energy[i] * dgdqopValue);
     }
   }
 };
 
 }  // namespace Acts

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