vtk-m/vtkm/worklet/contourtree/JoinSuperArcFinder.h

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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:
//
// After the core join tree is constructed, we need to assign each vertex to a join superarc
// This was previously done with a set of rocking iterations, which burned extra memory and
// work.  The OpenMP version was therefore updated so that each vertex looped until it found
// it's destination arc.
//
// This functor implements that for use by a for_each call.
//
// 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_join_super_arc_finder_h
#define vtkm_worklet_contourtree_join_super_arc_finder_h

#include <vtkm/worklet/WorkletMapField.h>
#include <vtkm/worklet/contourtree/Types.h>
#include <vtkm/worklet/contourtree/VertexValueComparator.h>

namespace vtkm
{
namespace worklet
{
namespace contourtree
{

// Worklet for finding join superarc - expressed as a unary functor since it is not
// guaranteed to write back
// There will be no out-of-sequence writes, since:
// 1.  Critical points are already set and are simply skipped
// 2.  Regular points only read from critical points
// 3.  Regular points only write to critical points
//
template <typename T>
class JoinSuperArcFinder : public vtkm::worklet::WorkletMapField
{
public:
  using TagType = vtkm::List<T>;

  using ControlSignature = void(FieldIn vertex,           // (input) index into sorted edges
                                WholeArrayIn values,      // (input) data values
                                WholeArrayInOut saddles,  // (in out) saddles
                                WholeArrayInOut extrema); // (in out) maxima
  using ExecutionSignature = void(_1, _2, _3, _4);
  using InputDomain = _1;

  bool isJoinTree;

  // Constructor
  VTKM_EXEC_CONT
  JoinSuperArcFinder(bool IsJoinTree)
    : isJoinTree(IsJoinTree)
  {
  }

  template <typename InFieldPortalType, typename OutFieldPortalType>
  VTKM_EXEC void operator()(const vtkm::Id& vertex,
                            const InFieldPortalType& values,
                            const OutFieldPortalType& saddles,
                            const OutFieldPortalType& extrema) const
  {
    VertexValueComparator<InFieldPortalType> lessThan(values);

    // local copies
    vtkm::Id saddle = saddles.Get(vertex);
    vtkm::Id extreme = extrema.Get(vertex);

    // now test for regular points
    if (saddle != extreme)
      return;

    // while loop
    while (lessThan(saddle, vertex, isJoinTree))
    { //  saddle above vertex
      extreme = extrema.Get(saddle);
      saddle = saddles.Get(saddle);
      // if we're in the trunk, break immediately
      if (saddle == NO_VERTEX_ASSIGNED)
        break;
    } // saddle above vertex

    // when done, write back to the array
    extrema.Set(vertex, extreme);
    saddles.Set(vertex, saddle);
  }
}; // JoinSuperArcFinder
}
}
}

#endif