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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/.
 
 #include "Acts/Material/Interactions.hpp"
 
 #include "Acts/Definitions/PdgParticle.hpp"
 #include "Acts/Definitions/Units.hpp"
 #include "Acts/Material/Material.hpp"
 #include "Acts/Utilities/MathHelpers.hpp"
 
 #include <cassert>
 #include <cmath>
 
 using namespace Acts::UnitLiterals;
 
 namespace {
 
 // values from RPP2018 table 33.1
 // electron mass
 constexpr float Me = 0.5109989461_MeV;
 // Bethe formular prefactor. 1/mol unit is just a factor 1 here.
 constexpr float K = 30.7075_MeV * 1_mm2;
 // Energy scale for plasma energy.
 constexpr float PlasmaEnergyScale = 28.816_eV;
 
 /// Additional derived relativistic quantities.
 struct RelativisticQuantities {
   float q2OverBeta2 = 0.0f;
   float beta2 = 0.0f;
   float betaGamma = 0.0f;
   float gamma = 0.0f;
 
   RelativisticQuantities(float mass, float qOverP, float absQ) {
     assert((0 < mass) && "Mass must be positive");
     assert((qOverP != 0) && "q/p must be non-zero");
     assert((absQ > 0) && "Absolute charge must be non-zero and positive");
     // beta²/q² = (p/E)²/q² = p²/(q²m² + q²p²) = 1/(q² + (m²(q/p)²)
     // q²/beta² = q² + m²(q/p)²
     q2OverBeta2 = absQ * absQ + (mass * qOverP) * (mass * qOverP);
     assert((q2OverBeta2 >= 0) && "Negative q2OverBeta2");
     // 1/p = q/(qp) = (q/p)/q
     const float mOverP = mass * std::abs(qOverP / absQ);
     const float pOverM = 1.0f / mOverP;
     // beta² = p²/E² = p²/(m² + p²) = 1/(1 + (m/p)²)
     beta2 = 1.0f / (1.0f + mOverP * mOverP);
     assert((beta2 >= 0) && "Negative beta2");
     // beta*gamma = (p/sqrt(m² + p²))*(sqrt(m² + p²)/m) = p/m
     betaGamma = pOverM;
     assert((betaGamma >= 0) && "Negative betaGamma");
     // gamma = sqrt(m² + p²)/m = sqrt(1 + (p/m)²)
     gamma = Acts::fastHypot(1.0f, pOverM);
   }
 };
 
 /// Compute q/p derivative of beta².
 inline float deriveBeta2(float qOverP, const RelativisticQuantities& rq) {
   return -2 / (qOverP * rq.gamma * rq.gamma);
 }
 
 /// Compute the 2 * mass * (beta * gamma)² mass term.
 inline float computeMassTerm(float mass, const RelativisticQuantities& rq) {
   return 2 * mass * rq.betaGamma * rq.betaGamma;
 }
 
 /// Compute mass term logarithmic derivative w/ respect to q/p.
 inline float logDeriveMassTerm(float qOverP) {
   // only need to compute d((beta*gamma)²)/(beta*gamma)²; rest cancels.
   return -2 / qOverP;
 }
 
 /// Compute the maximum energy transfer in a single collision.
 ///
 /// Uses RPP2018 eq. 33.4.
 inline float computeWMax(float mass, const RelativisticQuantities& rq) {
   const float mfrac = Me / mass;
   const float nominator = 2 * Me * rq.betaGamma * rq.betaGamma;
   const float denonimator = 1.0f + 2 * rq.gamma * mfrac + mfrac * mfrac;
   return nominator / denonimator;
 }
 
 /// Compute WMax logarithmic derivative w/ respect to q/p.
 inline float logDeriveWMax(float mass, float qOverP,
                            const RelativisticQuantities& rq) {
   // this is (q/p) * (beta/q).
   // both quantities have the same sign and the product must always be
   // positive. we can thus reuse the known (unsigned) quantity (q/beta)².
   const float a = std::abs(qOverP / std::sqrt(rq.q2OverBeta2));
   // (m² + me²) / me = me (1 + (m/me)²)
   const float b = Me * (1.0f + (mass / Me) * (mass / Me));
   return -2 * (a * b - 2 + rq.beta2) / (qOverP * (a * b + 2));
 }
 
 /// Compute epsilon energy pre-factor for RPP2018 eq. 33.11.
 ///
 /// Defined as
 ///
 ///     (K/2) * (Z/A)*rho * x * (q²/beta²)
 ///
 /// where (Z/A)*rho is the electron density in the material and x is the
 /// traversed length (thickness) of the material.
 inline float computeEpsilon(float molarElectronDensity, float thickness,
                             const RelativisticQuantities& rq) {
   return 0.5f * K * molarElectronDensity * thickness * rq.q2OverBeta2;
 }
 
 /// Compute epsilon logarithmic derivative w/ respect to q/p.
 inline float logDeriveEpsilon(float qOverP, const RelativisticQuantities& rq) {
   // only need to compute d(q²/beta²)/(q²/beta²); everything else cancels.
   return 2 / (qOverP * rq.gamma * rq.gamma);
 }
 
 /// Compute the density correction factor delta/2.
 inline float computeDeltaHalf(float meanExcitationEnergy,
                               float molarElectronDensity,
                               const RelativisticQuantities& rq) {
   // Uses RPP2018 eq. 33.6 which is only valid for high energies.
   // only relevant for very high ernergies; use arbitrary cutoff
   if (rq.betaGamma < 10.0f) {
     return 0.0f;
   }
   // pre-factor according to RPP2019 table 33.1
   const float plasmaEnergy =
       PlasmaEnergyScale *
       std::sqrt(molarElectronDensity / static_cast<float>(1 / 1_cm3));
   return std::log(rq.betaGamma) +
          std::log(plasmaEnergy / meanExcitationEnergy) - 0.5f;
 }
 
 /// Compute derivative w/ respect to q/p for the density correction.
 inline float deriveDeltaHalf(float qOverP, const RelativisticQuantities& rq) {
   // original equation is of the form
   //     log(beta*gamma) + log(eplasma/I) - 1/2
   // which the resulting derivative as
   //     d(beta*gamma)/(beta*gamma)
   return (rq.betaGamma < 10.0f) ? 0.0f : (-1.0f / qOverP);
 }
 
 /// Convert Landau full-width-half-maximum to an equivalent Gaussian sigma,
 ///
 /// Full-width-half-maximum for a Gaussian is given as
 ///
 ///     fwhm = 2 * sqrt(2 * log(2)) * sigma
 /// -> sigma = fwhm / (2 * sqrt(2 * log(2)))
 ///
 inline float convertLandauFwhmToGaussianSigma(float fwhm) {
   // return fwhm / (2 * std::sqrt(2 * std::log(2.0f)));
   return fwhm * 0.42466090014400953f;
 }
 
 namespace detail {
 
 inline float computeEnergyLossLandauFwhm(const Acts::MaterialSlab& slab,
                                          const RelativisticQuantities& rq) {
   // return early in case of vacuum or zero thickness
   if (slab.isVacuum()) {
     return 0.0f;
   }
 
   const float Ne = slab.material().molarElectronDensity();
   const float thickness = slab.thickness();
   // the Landau-Vavilov fwhm is 4*eps (see RPP2018 fig. 33.7)
   return 4 * computeEpsilon(Ne, thickness, rq);
 }
 
 }  // namespace detail
 
 }  // namespace
 
 float Acts::computeEnergyLossBethe(const MaterialSlab& slab, float m,
                                    float qOverP, float absQ) {
   // return early in case of vacuum or zero thickness
   if (slab.isVacuum()) {
     return 0.0f;
   }
 
   const RelativisticQuantities rq{m, qOverP, absQ};
   const float I = slab.material().meanExcitationEnergy();
   const float Ne = slab.material().molarElectronDensity();
   const float thickness = slab.thickness();
   const float eps = computeEpsilon(Ne, thickness, rq);
   // TODO calculation of dhalf is not always necessary
   const float dhalf = computeDeltaHalf(I, Ne, rq);
   const float u = computeMassTerm(Me, rq);
   const float wmax = computeWMax(m, rq);
   // uses RPP2018 eq. 33.5 scaled from mass stopping power to linear stopping
   // power and multiplied with the material thickness to get a total energy loss
   // instead of an energy loss per length.
   // the required modification only change the prefactor which becomes
   // identical to the prefactor epsilon for the most probable value.
   const float running =
       std::log(u / I) + std::log(wmax / I) - 2.0f * rq.beta2 - 2.0f * dhalf;
   return eps * running;
 }
 
 float Acts::deriveEnergyLossBetheQOverP(const MaterialSlab& slab, float m,
                                         float qOverP, float absQ) {
   // return early in case of vacuum or zero thickness
   if (slab.isVacuum()) {
     return 0.0f;
   }
 
   const RelativisticQuantities rq{m, qOverP, absQ};
   const float I = slab.material().meanExcitationEnergy();
   const float Ne = slab.material().molarElectronDensity();
   const float thickness = slab.thickness();
   const float eps = computeEpsilon(Ne, thickness, rq);
   const float dhalf = computeDeltaHalf(I, Ne, rq);
   const float u = computeMassTerm(Me, rq);
   const float wmax = computeWMax(m, rq);
   // original equation is of the form
   //
   //     eps * (log(u/I) + log(wmax/I) - 2 * beta² - delta)
   //     = eps * (log(u) + log(wmax) - 2 * log(I) - 2 * beta² - delta)
   //
   // with the resulting derivative as
   //
   //     d(eps) * (log(u/I) + log(wmax/I) - 2 * beta² - delta)
   //     + eps * (d(u)/(u) + d(wmax)/(wmax) - 2 * d(beta²) - d(delta))
   //
   // where we can use d(eps) = eps * (d(eps)/eps) for further simplification.
   const float logDerEps = logDeriveEpsilon(qOverP, rq);
   const float derDHalf = deriveDeltaHalf(qOverP, rq);
   const float logDerU = logDeriveMassTerm(qOverP);
   const float logDerWmax = logDeriveWMax(m, qOverP, rq);
   const float derBeta2 = deriveBeta2(qOverP, rq);
   const float rel = logDerEps * (std::log(u / I) + std::log(wmax / I) -
                                  2.0f * rq.beta2 - 2.0f * dhalf) +
                     logDerU + logDerWmax - 2.0f * derBeta2 - 2.0f * derDHalf;
   return eps * rel;
 }
 
 float Acts::computeEnergyLossLandau(const MaterialSlab& slab, float m,
                                     float qOverP, float absQ) {
   // return early in case of vacuum or zero thickness
   if (slab.isVacuum()) {
     return 0.0f;
   }
 
   const RelativisticQuantities rq{m, qOverP, absQ};
   const float I = slab.material().meanExcitationEnergy();
   const float Ne = slab.material().molarElectronDensity();
   const float thickness = slab.thickness();
   const float eps = computeEpsilon(Ne, thickness, rq);
   const float dhalf = computeDeltaHalf(I, Ne, rq);
   const float u = computeMassTerm(Me, rq);
   // uses RPP2018 eq. 33.12
   const float running =
       std::log(u / I) + std::log(eps / I) + 0.2f - rq.beta2 - 2 * dhalf;
   return eps * running;
 }
 
 float Acts::deriveEnergyLossLandauQOverP(const MaterialSlab& slab, float m,
                                          float qOverP, float absQ) {
   // return early in case of vacuum or zero thickness
   if (slab.isVacuum()) {
     return 0.0f;
   }
 
   const RelativisticQuantities rq{m, qOverP, absQ};
   const float I = slab.material().meanExcitationEnergy();
   const float Ne = slab.material().molarElectronDensity();
   const float thickness = slab.thickness();
   const float eps = computeEpsilon(Ne, thickness, rq);
   const float dhalf = computeDeltaHalf(I, Ne, rq);
   const float t = computeMassTerm(Me, rq);
   // original equation is of the form
   //
   //     eps * (log(t/I) - log(eps/I) - 0.2 - beta² - delta)
   //     = eps * (log(t) - log(eps) - 2*log(I) - 0.2 - beta² - delta)
   //
   // with the resulting derivative as
   //
   //     d(eps) * (log(t/I) - log(eps/I) - 0.2 - beta² - delta)
   //     + eps * (d(t)/t + d(eps)/eps - d(beta²) - 2*d(delta/2))
   //
   // where we can use d(eps) = eps * (d(eps)/eps) for further simplification.
   const float logDerEps = logDeriveEpsilon(qOverP, rq);
   const float derDHalf = deriveDeltaHalf(qOverP, rq);
   const float logDerT = logDeriveMassTerm(qOverP);
   const float derBeta2 = deriveBeta2(qOverP, rq);
   const float rel = logDerEps * (std::log(t / I) + std::log(eps / I) - 0.2f -
                                  rq.beta2 - 2 * dhalf) +
                     logDerT + logDerEps - derBeta2 - 2 * derDHalf;
   return eps * rel;
 }
 
 float Acts::computeEnergyLossLandauSigma(const MaterialSlab& slab, float m,
                                          float qOverP, float absQ) {
   // return early in case of vacuum or zero thickness
   if (slab.isVacuum()) {
     return 0.0f;
   }
 
   const RelativisticQuantities rq{m, qOverP, absQ};
   const float Ne = slab.material().molarElectronDensity();
   const float thickness = slab.thickness();
   // the Landau-Vavilov fwhm is 4*eps (see RPP2018 fig. 33.7)
   const float fwhm = 4 * computeEpsilon(Ne, thickness, rq);
   return convertLandauFwhmToGaussianSigma(fwhm);
 }
 
 float Acts::computeEnergyLossLandauFwhm(const MaterialSlab& slab, float m,
                                         float qOverP, float absQ) {
   const RelativisticQuantities rq{m, qOverP, absQ};
   return detail::computeEnergyLossLandauFwhm(slab, rq);
 }
 
 float Acts::computeEnergyLossLandauSigmaQOverP(const MaterialSlab& slab,
                                                float m, float qOverP,
                                                float absQ) {
   const RelativisticQuantities rq{m, qOverP, absQ};
   const float fwhm = detail::computeEnergyLossLandauFwhm(slab, rq);
   const float sigmaE = convertLandauFwhmToGaussianSigma(fwhm);
   //  var(q/p) = (d(q/p)/dE)² * var(E)
   // d(q/p)/dE = d/dE (q/sqrt(E²-m²))
   //           = q * -(1/2) * 1/p³ * 2E
   //           = -q/p² E/p = -(q/p)² * 1/(q*beta) = -(q/p)² * (q/beta) / q²
   //  var(q/p) = (q/p)^4 * (q/beta)² * (1/q)^4 * var(E)
   //           = (1/p)^4 * (q/beta)² * var(E)
   // do not need to care about the sign since it is only used squared
   const float pInv = qOverP / absQ;
   const float qOverBeta = std::sqrt(rq.q2OverBeta2);
   return qOverBeta * pInv * pInv * sigmaE;
 }
 
 namespace {
 
 /// Compute mean energy loss from bremsstrahlung per radiation length.
 inline float computeBremsstrahlungLossMean(float mass, float energy) {
   return energy * (Me / mass) * (Me / mass);
 }
 
 /// Derivative of the bremsstrahlung loss per rad length with respect to energy.
 inline float deriveBremsstrahlungLossMeanE(float mass) {
   return (Me / mass) * (Me / mass);
 }
 
 /// Expansion coefficients for the muon radiative loss as a function of energy
 ///
 /// Taken from ATL-SOFT-PUB-2008-003 eq. 7,8 where the expansion is expressed
 /// with terms E^n/X0 with fixed units [E] = MeV and [X0] = mm. The evaluated
 /// expansion has units MeV/mm. In this implementation, the X0 dependence is
 /// factored out and the coefficients must be scaled to the native units such
 /// that the evaluated expansion with terms E^n has dimension energy in
 /// native units.
 constexpr float MuonHighLowThreshold = 1_TeV;
 // [low0 / X0] = MeV / mm -> [low0] = MeV
 constexpr double MuonLow0 = -0.5345_MeV;
 // [low1 * E / X0] = MeV / mm -> [low1] = 1
 constexpr double MuonLow1 = 6.803e-5;
 // [low2 * E^2 / X0] = MeV / mm -> [low2] = 1/MeV
 constexpr double MuonLow2 = 2.278e-11 / 1_MeV;
 // [low3 * E^3 / X0] = MeV / mm -> [low3] = 1/MeV^2
 constexpr double MuonLow3 = -9.899e-18 / (1_MeV * 1_MeV);
 // units are the same as low0
 constexpr double MuonHigh0 = -2.986_MeV;
 // units are the same as low1
 constexpr double MuonHigh1 = 9.253e-5;
 
 /// Compute additional radiation energy loss for muons per radiation length.
 inline float computeMuonDirectPairPhotoNuclearLossMean(double energy) {
   if (energy < MuonHighLowThreshold) {
     return MuonLow0 +
            (MuonLow1 + (MuonLow2 + MuonLow3 * energy) * energy) * energy;
   } else {
     return MuonHigh0 + MuonHigh1 * energy;
   }
 }
 
 /// Derivative of the additional rad loss per rad length with respect to energy.
 inline float deriveMuonDirectPairPhotoNuclearLossMeanE(double energy) {
   if (energy < MuonHighLowThreshold) {
     return MuonLow1 + (2 * MuonLow2 + 3 * MuonLow3 * energy) * energy;
   } else {
     return MuonHigh1;
   }
 }
 
 }  // namespace
 
 float Acts::computeEnergyLossRadiative(const MaterialSlab& slab,
                                        PdgParticle absPdg, float m,
                                        float qOverP, float absQ) {
   assert((absPdg == Acts::makeAbsolutePdgParticle(absPdg)) &&
          "pdg is not absolute");
 
   // return early in case of vacuum or zero thickness
   if (slab.isVacuum()) {
     return 0.0f;
   }
 
   // relative radiation length
   const float x = slab.thicknessInX0();
   // particle momentum and energy
   // do not need to care about the sign since it is only used squared
   const float momentum = absQ / qOverP;
   const float energy = fastHypot(m, momentum);
 
   float dEdx = computeBremsstrahlungLossMean(m, energy);
 
   // muon- or muon+
   // TODO magic number 8_GeV
   if ((absPdg == PdgParticle::eMuon) && (energy > 8_GeV)) {
     dEdx += computeMuonDirectPairPhotoNuclearLossMean(energy);
   }
   // scale from energy loss per unit radiation length to total energy
   return dEdx * x;
 }
 
 float Acts::deriveEnergyLossRadiativeQOverP(const MaterialSlab& slab,
                                             PdgParticle absPdg, float m,
                                             float qOverP, float absQ) {
   assert((absPdg == Acts::makeAbsolutePdgParticle(absPdg)) &&
          "pdg is not absolute");
 
   // return early in case of vacuum or zero thickness
   if (slab.isVacuum()) {
     return 0.0f;
   }
 
   // relative radiation length
   const float x = slab.thicknessInX0();
   // particle momentum and energy
   // do not need to care about the sign since it is only used squared
   const float momentum = absQ / qOverP;
   const float energy = fastHypot(m, momentum);
 
   // compute derivative w/ respect to energy.
   float derE = deriveBremsstrahlungLossMeanE(m);
 
   // muon- or muon+
   // TODO magic number 8_GeV
   if ((absPdg == PdgParticle::eMuon) && (8_GeV < energy)) {
     derE += deriveMuonDirectPairPhotoNuclearLossMeanE(energy);
   }
   // compute derivative w/ respect to q/p by using the chain rule
   //     df(E)/d(q/p) = df(E)/dE dE/d(q/p)
   // with
   //     E = sqrt(m² + p²) = sqrt(m² + q²/(q/p)²)
   // and the resulting derivative
   //     dE/d(q/p) = -q² / ((q/p)³ * E)
   const float derQOverP = -(absQ * absQ) / (qOverP * qOverP * qOverP * energy);
   return derE * derQOverP * x;
 }
 
 float Acts::computeEnergyLossMean(const MaterialSlab& slab, PdgParticle absPdg,
                                   float m, float qOverP, float absQ) {
   return computeEnergyLossBethe(slab, m, qOverP, absQ) +
          computeEnergyLossRadiative(slab, absPdg, m, qOverP, absQ);
 }
 
 float Acts::deriveEnergyLossMeanQOverP(const MaterialSlab& slab,
                                        PdgParticle absPdg, float m,
                                        float qOverP, float absQ) {
   return deriveEnergyLossBetheQOverP(slab, m, qOverP, absQ) +
          deriveEnergyLossRadiativeQOverP(slab, absPdg, m, qOverP, absQ);
 }
 
 float Acts::computeEnergyLossMode(const MaterialSlab& slab, PdgParticle absPdg,
                                   float m, float qOverP, float absQ) {
   // see ATL-SOFT-PUB-2008-003 section 3 for the relative fractions
   // TODO this is inconsistent with the text of the note
   return 0.9f * computeEnergyLossBethe(slab, m, qOverP, absQ) +
          0.15f * computeEnergyLossRadiative(slab, absPdg, m, qOverP, absQ);
 }
 
 float Acts::deriveEnergyLossModeQOverP(const MaterialSlab& slab,
                                        PdgParticle absPdg, float m,
                                        float qOverP, float absQ) {
   // see ATL-SOFT-PUB-2008-003 section 3 for the relative fractions
   // TODO this is inconsistent with the text of the note
   return 0.9f * deriveEnergyLossBetheQOverP(slab, m, qOverP, absQ) +
          0.15f * deriveEnergyLossRadiativeQOverP(slab, absPdg, m, qOverP, absQ);
 }
 
 namespace {
 
 /// Multiple scattering theta0 for minimum ionizing particles.
 inline float theta0Highland(float xOverX0, float momentumInv,
                             float q2OverBeta2) {
   // RPP2018 eq. 33.15 (treats beta and q² consistently)
   const float t = std::sqrt(xOverX0 * q2OverBeta2);
   // log((x/X0) * (q²/beta²)) = log((sqrt(x/X0) * (q/beta))²)
   //                          = 2 * log(sqrt(x/X0) * (q/beta))
   return 13.6_MeV * momentumInv * t * (1.0f + 0.038f * 2 * std::log(t));
 }
 
 /// Multiple scattering theta0 for electrons.
 inline float theta0RossiGreisen(float xOverX0, float momentumInv,
                                 float q2OverBeta2) {
   // TODO add source paper/ resource
   const float t = std::sqrt(xOverX0 * q2OverBeta2);
   return 17.5_MeV * momentumInv * t *
          (1.0f + 0.125f * std::log10(10.0f * xOverX0));
 }
 
 }  // namespace
 
 float Acts::computeMultipleScatteringTheta0(const MaterialSlab& slab,
                                             PdgParticle absPdg, float m,
                                             float qOverP, float absQ) {
   assert((absPdg == Acts::makeAbsolutePdgParticle(absPdg)) &&
          "pdg is not absolute");
 
   // return early in case of vacuum or zero thickness
   if (slab.isVacuum()) {
     return 0.0f;
   }
 
   // relative radiation length
   const float xOverX0 = slab.thicknessInX0();
   // 1/p = q/(pq) = (q/p)/q
   const float momentumInv = std::abs(qOverP / absQ);
   // q²/beta²; a smart compiler should be able to remove the unused computations
   const float q2OverBeta2 = RelativisticQuantities(m, qOverP, absQ).q2OverBeta2;
 
   // electron or positron
   if (absPdg == PdgParticle::eElectron) {
     return theta0RossiGreisen(xOverX0, momentumInv, q2OverBeta2);
   } else {
     return theta0Highland(xOverX0, momentumInv, q2OverBeta2);
   }
 }
 
 float Acts::approximateHighlandScattering(float xOverX0) {
   // similar to `theta0Highland` but without momentum and charge
 
   const float q2OverBeta2 = 1;  // q^2=1, beta^2~1
   // RPP2018 eq. 33.15 (treats beta and q² consistently)
   const float t = std::sqrt(xOverX0 * q2OverBeta2);
   // log((x/X0) * (q²/beta²)) = log((sqrt(x/X0) * (q/beta))²)
   //                          = 2 * log(sqrt(x/X0) * (q/beta))
   return 13.6 * UnitConstants::MeV * t * (1.0f + 0.038f * 2 * std::log(t));
 }

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