Resolve #345 "Add loki mtdocachi2 functor to thor"

Closes #345 (closed). Requires !2656 (closed)

Phys/FunctorCore/include/Functors/Composite.h

 /*****************************************************************************\
-* (c) Copyright 2019-20 CERN for the benefit of the LHCb Collaboration        *
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+ * (c) Copyright 2019-20 CERN for the benefit of the LHCb Collaboration        *
+ *                                                                             *
+ * This software is distributed under the terms of the GNU General Public      *
+ * Licence version 3 (GPL Version 3), copied verbatim in the file "COPYING".   *
+ *                                                                             *
+ * In applying this licence, CERN does not waive the privileges and immunities *
+ * granted to it by virtue of its status as an Intergovernmental Organization  *
+ * or submit itself to any jurisdiction.                                       *
 \*****************************************************************************/
 #pragma once
 #include "Event/FlavourTag.h"
@@ -20,6 +20,10 @@
 #include "SelKernel/Utilities.h"
 #include "SelKernel/VertexRelation.h"
 
+#include "Event/Particle.h"
+#include "Event/Particle_v2.h"
+#include "Functors/TrackLike.h"
+
 /** @file  Composite.h
  *  @brief Definitions of functors for composite-particle-like objects.
  */
@@ -47,6 +51,43 @@ namespace Functors::detail {
 } // namespace Functors::detail
 
 namespace Functors::Composite {
+
+  /**MTDOCACHI2**/
+  template <int N>
+  struct MotherTrajectoryDistanceOfClosestApproachChi2 : public Function {
+    static_assert( N >= 1, "Indices start from 1 for LoKi compatibility." );
+    MotherTrajectoryDistanceOfClosestApproachChi2( std::integral_constant<int, N> = {} ) {}
+
+    void bind( TopLevelInfo& top_level ) { m_dist_calc.emplace( top_level.algorithm() ); }
+
+    template <typename VContainer, typename Particle>
+    auto operator()( VContainer const& vertices, Particle const& composite ) const {
+      auto const bestPV = Sel::getBestPV( composite, vertices );
+
+      using Sel::Utils::decayProducts;
+      auto const& children = decayProducts( composite );
+      auto const& pN       = children[N - 1];
+
+      auto const& pN_state = Sel::stateVectorFromComposite( pN );
+
+      using Sel::Utils::posCovMatrix;
+      using Sel::Utils::referencePoint;
+      using Sel::Utils::threeMomCovMatrix;
+      using Sel::Utils::threeMomentum;
+      using Sel::Utils::threeMomPosCovMatrix;
+
+      auto const& composite_state = Sel::stateVectorComputations(
+          endVertexPos( bestPV ), threeMomentum( composite ), threeMomCovMatrix( composite ), posCovMatrix( bestPV ),
+          threeMomPosCovMatrix( composite ) );
+
+      const auto& dist_calc = *m_dist_calc;
+      return dist_calc.stateDOCAChi2( composite_state, pN_state );
+    }
+
+  private:
+    std::optional<Functors::detail::DefaultDistanceCalculator_t> m_dist_calc;
+  };
+
   /** @brief Flight distance chi2 to the given vertex.
    *
    *  Note that if the given data object contains a vertex link then that will