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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 implements chain-doubling (pointer-doubling), but minimises memory writeback
// by testing whether we've hit the end of the chain already
//
// 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_chain_doubler_h
#define vtkm_worklet_contourtree_chain_doubler_h

#include <vtkm/worklet/WorkletMapField.h>

namespace vtkm
{
namespace worklet
{
namespace contourtree
{

// Functor for doing chain doubling
// Unary because it takes the index of the element to process, and is not guaranteed to
// write back
// moreover, we aren't worried about out-of-sequence writes, since the worst that happens
// is that an element gets pointer-tripled in the iteration. It will still converge to the
// same destination.
class ChainDoubler : public vtkm::worklet::WorkletMapField
{
public:
  using ControlSignature = void(FieldIn vertexID, WholeArrayInOut chains);
  using ExecutionSignature = void(_1, _2);
  using InputDomain = _1;

  // Constructor
  VTKM_EXEC_CONT
  ChainDoubler() {}

  template <typename InOutFieldPortalType>
  VTKM_EXEC void operator()(const vtkm::Id& vertexID, const InOutFieldPortalType& chains) const
  {
    vtkm::Id next = chains.Get(vertexID);
    vtkm::Id doubleNext = chains.Get(next);

    if (next != doubleNext)
      chains.Set(vertexID, doubleNext);
  }
}; // ChainDoubler
}
}
}

#endif