text: /*
   * (C) Crown Copyright 2021 Met Office
   *
   * This software is licensed under the terms of the Apache Licence Version 2.0
   * which can be obtained at http://www.apache.org/licenses/LICENSE-2.0.
   */
  
  #include "eckit/config/Resource.h"
  
  #include "atlas/array/ArrayView.h"
  #include "atlas/array/MakeView.h"
  #include "atlas/interpolation/method/PointIndex2.h"
  #include "atlas/mesh/HybridElements.h"
  #include "atlas/runtime/Trace.h"
  #include "atlas/util/Topology.h"
  
  namespace atlas {
  namespace interpolation {
  namespace method {
  
  ElemIndex2* create_element2D_kdtree(const Mesh& mesh, const Field& field_centres) {
      ATLAS_TRACE();
      const array::ArrayView<const double, 2> centres = array::make_view<double, 2>(field_centres);
      const array::ArrayView<const int, 1> flags      = array::make_view<int, 1>(mesh.cells().flags());
      auto include_element                            = [&](unsigned int e) {
          using util::Topology;
          return not Topology::view(flags(e)).check(Topology::INVALID);
      };
  
      static bool fastBuildKDTrees = eckit::Resource<bool>("$ATLAS_FAST_BUILD_KDTREES", true);
  
      ElemIndex2* tree      = new ElemIndex2();
      const size_t nb_cells = centres.shape(0);
  
      if (fastBuildKDTrees) {
          std::vector<ElemIndex2::Value> p;
          p.reserve(nb_cells);
  
          for (unsigned int j = 0; j < nb_cells; ++j) {
              if (include_element(j)) {
                  p.emplace_back(ElemIndex2::Point(centres(j, LON), centres(j, LAT)), ElemIndex2::Payload(j));
              }
          }
  
          tree->build(p.begin(), p.end());
      }
      else {
          for (unsigned int j = 0; j < nb_cells; ++j) {
              if (include_element(j)) {
                  tree->insert(
                      ElemIndex2::Value(ElemIndex2::Point(centres(j, LON), centres(j, LAT)), ElemIndex2::Payload(j)));
              }
          }
      }
      return tree;
  }
  
  }  // namespace method
  }  // namespace interpolation
  }  // namespace atlas
  