refactor(gx2f): make function to update parameter (#3840)

Pulling out, as suggested in https://github.com/acts-project/acts/pull/3726/files#r1835623388

Core/include/Acts/TrackFitting/GlobalChiSquareFitter.hpp

@@ -694,6 +694,20 @@ std::size_t countMaterialStates(
   return nMaterialSurfaces;
 }
 
+/// @brief Update parameters (and scattering angles if applicable)
+///
+/// @param params Parameters to be updated
+/// @param deltaParamsExtended Delta parameters for bound parameter and scattering angles
+/// @param nMaterialSurfaces Number of material surfaces in the track
+/// @param scatteringMap Map of geometry identifiers to scattering properties,
+///        containing all scattering angles and covariances
+/// @param geoIdVector Vector of geometry identifiers corresponding to material surfaces
+void updateGx2fParams(
+    BoundTrackParameters& params, const Eigen::VectorXd& deltaParamsExtended,
+    const std::size_t nMaterialSurfaces,
+    std::unordered_map<GeometryIdentifier, ScatteringProperties>& scatteringMap,
+    const std::vector<GeometryIdentifier>& geoIdVector);
+
 /// @brief Calculate and update the covariance of the fitted parameters
 ///
 /// This function calculates the covariance of the fitted parameters using
@@ -704,8 +718,6 @@ std::size_t countMaterialStates(
 ///
 /// @param fullCovariancePredicted The covariance matrix to update
 /// @param extendedSystem All parameters of the current equation system
-///
-/// @return deltaParams The calculated delta parameters.
 void updateGx2fCovarianceParams(BoundMatrix& fullCovariancePredicted,
                                 Gx2fSystem& extendedSystem);
 
@@ -1234,7 +1246,6 @@ class Gx2Fitter {
     using PropagatorOptions = typename propagator_t::template Options<Actors>;
 
     start_parameters_t params = sParameters;
-    BoundVector deltaParams = BoundVector::Zero();
     double chi2sum = 0;
     double oldChi2sum = std::numeric_limits<double>::max();
 
@@ -1272,10 +1283,6 @@ class Gx2Fitter {
     for (nUpdate = 0; nUpdate < gx2fOptions.nUpdateMax; nUpdate++) {
       ACTS_DEBUG("nUpdate = " << nUpdate + 1 << "/" << gx2fOptions.nUpdateMax);
 
-      // update params
-      params.parameters() += deltaParams;
-      ACTS_VERBOSE("Updated parameters: " << params.parameters().transpose());
-
       // set up propagator and co
       Acts::GeometryContext geoCtx = gx2fOptions.geoContext;
       Acts::MagneticFieldContext magCtx = gx2fOptions.magFieldContext;
@@ -1392,14 +1399,10 @@ class Gx2Fitter {
           extendedSystem.aMatrix().colPivHouseholderQr().solve(
               extendedSystem.bVector());
 
-      deltaParams = deltaParamsExtended.topLeftCorner<eBoundSize, 1>().eval();
-
       ACTS_VERBOSE("aMatrix:\n"
                    << extendedSystem.aMatrix() << "\n"
                    << "bVector:\n"
                    << extendedSystem.bVector() << "\n"
-                   << "deltaParams:\n"
-                   << deltaParams << "\n"
                    << "deltaParamsExtended:\n"
                    << deltaParamsExtended << "\n"
                    << "oldChi2sum = " << oldChi2sum << "\n"
@@ -1440,19 +1443,9 @@ class Gx2Fitter {
         break;
       }
 
-      if (multipleScattering) {
-        // update the scattering angles
-        for (std::size_t matSurface = 0; matSurface < nMaterialSurfaces;
-             matSurface++) {
-          const std::size_t deltaPosition = eBoundSize + 2 * matSurface;
-          const GeometryIdentifier geoId = geoIdVector[matSurface];
-          const auto scatteringMapId = scatteringMap.find(geoId);
-          assert(scatteringMapId != scatteringMap.end() &&
-                 "No scattering angles found for material surface.");
-          scatteringMapId->second.scatteringAngles().block<2, 1>(2, 0) +=
-              deltaParamsExtended.block<2, 1>(deltaPosition, 0).eval();
-        }
-      }
+      updateGx2fParams(params, deltaParamsExtended, nMaterialSurfaces,
+                       scatteringMap, geoIdVector);
+      ACTS_VERBOSE("Updated parameters: " << params.parameters().transpose());
 
       oldChi2sum = extendedSystem.chi2();
     }
@@ -1478,7 +1471,6 @@ class Gx2Fitter {
     // step, we will not ignore the boundary checks for measurement surfaces. We
     // want to create trackstates only on surfaces, that we actually hit.
     if (gx2fOptions.nUpdateMax > 0) {
-      ACTS_VERBOSE("Final delta parameters: " << deltaParams.transpose());
       ACTS_VERBOSE("Propagate with the final covariance.");
       // update covariance
       params.covariance() = fullCovariancePredicted;

Core/src/TrackFitting/GlobalChiSquareFitter.cpp

@@ -10,6 +10,30 @@
 
 #include "Acts/Definitions/TrackParametrization.hpp"
 
+void Acts::Experimental::updateGx2fParams(
+    BoundTrackParameters& params, const Eigen::VectorXd& deltaParamsExtended,
+    const std::size_t nMaterialSurfaces,
+    std::unordered_map<GeometryIdentifier, ScatteringProperties>& scatteringMap,
+    const std::vector<GeometryIdentifier>& geoIdVector) {
+  // update params
+  params.parameters() +=
+      deltaParamsExtended.topLeftCorner<eBoundSize, 1>().eval();
+
+  // update the scattering angles.
+  for (std::size_t matSurface = 0; matSurface < nMaterialSurfaces;
+       matSurface++) {
+    const std::size_t deltaPosition = eBoundSize + 2 * matSurface;
+    const GeometryIdentifier geoId = geoIdVector[matSurface];
+    const auto scatteringMapId = scatteringMap.find(geoId);
+    assert(scatteringMapId != scatteringMap.end() &&
+           "No scattering angles found for material surface.");
+    scatteringMapId->second.scatteringAngles().block<2, 1>(2, 0) +=
+        deltaParamsExtended.block<2, 1>(deltaPosition, 0).eval();
+  }
+
+  return;
+}
+
 void Acts::Experimental::updateGx2fCovarianceParams(
     BoundMatrix& fullCovariancePredicted, Gx2fSystem& extendedSystem) {
   // make invertible