From 12de1e21285d2f243ee5fedad013708d4512139c Mon Sep 17 00:00:00 2001
From: Paul Gessinger <paul.gessinger@cern.ch>
Date: Thu, 7 Dec 2023 00:16:25 +0100
Subject: [PATCH] refactor: Factorize GX2F delta param calculation (#2774)

This allows compiling the fairly expensive `colPivHouseholderQr` expression to be compiled in a separate translation unit.
---
 .../TrackFitting/GlobalChiSquareFitter.hpp    | 20 ++++-------
 Core/src/TrackFitting/CMakeLists.txt          |  1 +
 .../TrackFitting/GlobalChiSquareFitter.cpp    | 34 +++++++++++++++++++
 3 files changed, 41 insertions(+), 14 deletions(-)
 create mode 100644 Core/src/TrackFitting/GlobalChiSquareFitter.cpp

diff --git a/Core/include/Acts/TrackFitting/GlobalChiSquareFitter.hpp b/Core/include/Acts/TrackFitting/GlobalChiSquareFitter.hpp
index 6bb62a1d9b4..539c2a87266 100644
--- a/Core/include/Acts/TrackFitting/GlobalChiSquareFitter.hpp
+++ b/Core/include/Acts/TrackFitting/GlobalChiSquareFitter.hpp
@@ -287,6 +287,9 @@ void collector(typename traj_t::TrackStateProxy& trackStateProxy,
   }
 }
 
+BoundVector calculateDeltaParams(bool zeroField, const BoundMatrix& aMatrix,
+                                 const BoundVector& bVector);
+
 /// Global Chi Square fitter (GX2F) implementation.
 ///
 /// @tparam propagator_t Type of the propagation class
@@ -718,20 +721,8 @@ class Gx2Fitter {
       }
 
       // calculate delta params [a] * delta = b
-      deltaParams = BoundVector::Zero();
-      if (gx2fOptions.zeroField) {
-        constexpr std::size_t reducedMatrixSize = 4;
-        deltaParams.topLeftCorner<reducedMatrixSize, 1>() =
-            aMatrix.topLeftCorner<reducedMatrixSize, reducedMatrixSize>()
-                .colPivHouseholderQr()
-                .solve(bVector.topLeftCorner<reducedMatrixSize, 1>());
-      } else {
-        constexpr std::size_t reducedMatrixSize = 5;
-        deltaParams.topLeftCorner<reducedMatrixSize, 1>() =
-            aMatrix.topLeftCorner<reducedMatrixSize, reducedMatrixSize>()
-                .colPivHouseholderQr()
-                .solve(bVector.topLeftCorner<reducedMatrixSize, 1>());
-      }
+      deltaParams =
+          calculateDeltaParams(gx2fOptions.zeroField, aMatrix, bVector);
 
       ACTS_VERBOSE("aMatrix:\n"
                    << aMatrix << "\n"
@@ -822,5 +813,6 @@ class Gx2Fitter {
     return track;
   }
 };
+
 }  // namespace Experimental
 }  // namespace Acts
diff --git a/Core/src/TrackFitting/CMakeLists.txt b/Core/src/TrackFitting/CMakeLists.txt
index 05eccd967ad..c370dc9aa01 100644
--- a/Core/src/TrackFitting/CMakeLists.txt
+++ b/Core/src/TrackFitting/CMakeLists.txt
@@ -9,4 +9,5 @@ target_sources(
     GsfUtils.cpp
     BetheHeitlerApprox.cpp
     GsfMixtureReduction.cpp
+    GlobalChiSquareFitter.cpp
 )
diff --git a/Core/src/TrackFitting/GlobalChiSquareFitter.cpp b/Core/src/TrackFitting/GlobalChiSquareFitter.cpp
new file mode 100644
index 00000000000..c74319982ec
--- /dev/null
+++ b/Core/src/TrackFitting/GlobalChiSquareFitter.cpp
@@ -0,0 +1,34 @@
+// This file is part of the Acts project.
+//
+// Copyright (C) 2023 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 http://mozilla.org/MPL/2.0/.
+
+#include "Acts/TrackFitting/GlobalChiSquareFitter.hpp"
+
+#include "Acts/Definitions/TrackParametrization.hpp"
+
+namespace Acts::Experimental {
+
+BoundVector calculateDeltaParams(bool zeroField, const BoundMatrix& aMatrix,
+                                 const BoundVector& bVector) {
+  BoundVector deltaParams = BoundVector::Zero();
+  if (zeroField) {
+    constexpr std::size_t reducedMatrixSize = 4;
+    deltaParams.topLeftCorner<reducedMatrixSize, 1>() =
+        aMatrix.topLeftCorner<reducedMatrixSize, reducedMatrixSize>()
+            .colPivHouseholderQr()
+            .solve(bVector.topLeftCorner<reducedMatrixSize, 1>());
+  } else {
+    constexpr std::size_t reducedMatrixSize = 5;
+    deltaParams.topLeftCorner<reducedMatrixSize, 1>() =
+        aMatrix.topLeftCorner<reducedMatrixSize, reducedMatrixSize>()
+            .colPivHouseholderQr()
+            .solve(bVector.topLeftCorner<reducedMatrixSize, 1>());
+  }
+
+  return deltaParams;
+}
+}  // namespace Acts::Experimental