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//  This code is based on the algorithm presented in the paper:
//  “Parallel Peak Pruning for Scalable SMP Contour Tree Computation.”
//  Hamish Carr, Gunther Weber, Christopher Sewell, and James Ahrens.
//  Proceedings of the IEEE Symposium on Large Data Analysis and Visualization
//  (LDAV), October 2016, Baltimore, Maryland.

//=======================================================================================
//
// COMMENTS:
//
// This functor replaces a parallel loop examining neighbours - again, for arbitrary
// meshes, it needs to be a reduction, but for regular meshes, it's faster this way.
//
// Any vector needed by the functor for lookup purposes will be passed as a parameter to
// the constructor and saved, with the actual function call being the operator ()
//
// Vectors marked I/O are intrinsically risky unless there is an algorithmic guarantee
// that the read/writes are completely independent - which for our case actually occurs
// The I/O vectors should therefore be justified in comments both here & in caller
//
//=======================================================================================

#ifndef vtkm_worklet_contourtree_governing_saddle_finder_h
#define vtkm_worklet_contourtree_governing_saddle_finder_h

#include <vtkm/worklet/WorkletMapField.h>

namespace vtkm
{
namespace worklet
{
namespace contourtree
{

// Worklet for setting initial chain maximum value
class GoverningSaddleFinder : public vtkm::worklet::WorkletMapField
{
public:
  using ControlSignature = void(FieldIn edgeNo,           // (input) index into sorted edges
                                WholeArrayIn edgeSorter,  // (input) sorted edge index
                                WholeArrayIn edgeFar,     // (input) high ends of edges
                                WholeArrayIn edgeNear,    // (input) low ends of edges
                                WholeArrayOut prunesTo,   // (output) where vertex is pruned to
                                WholeArrayOut outdegree); // (output) updegree of vertex
  using ExecutionSignature = void(_1, _2, _3, _4, _5, _6);
  using InputDomain = _1;

  // Constructor
  VTKM_EXEC_CONT
  GoverningSaddleFinder() {}

  template <typename InFieldPortalType, typename OutFieldPortalType>
  VTKM_EXEC void operator()(const vtkm::Id& edgeNo,
                            const InFieldPortalType& edgeSorter,
                            const InFieldPortalType& edgeFar,
                            const InFieldPortalType& edgeNear,
                            const OutFieldPortalType& prunesTo,
                            const OutFieldPortalType& outdegree) const
  {
    // default to true
    bool isBestSaddleEdge = true;

    // retrieve the edge ID
    vtkm::Id edge = edgeSorter.Get(edgeNo);

    // edge no. 0 is always best, so skip it
    if (edgeNo != 0)
    {
      // retrieve the previous edge
      vtkm::Id prevEdge = edgeSorter.Get(edgeNo - 1);
      // if the previous edge has the same far end
      if (edgeFar.Get(prevEdge) == edgeFar.Get(edge))
        isBestSaddleEdge = false;
    }

    if (isBestSaddleEdge)
    { // found an extremum
      // retrieve the near end as the saddle
      vtkm::Id saddle = edgeNear.Get(edge);
      // and the far end as the extremum
      vtkm::Id extreme = edgeFar.Get(edge);

      // set the extremum to point to the saddle in the chainExtremum array
      prunesTo.Set(extreme, saddle);

      // and set the outdegree to 0
      outdegree.Set(extreme, 0);
    } // found a extremum
  }   // operator()

}; // GoverningSaddleFinder
}
}
}

#endif
