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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 identifies for each vertex which edges to keep. For arbitrary meshes,
// this should use reductions. For regular meshes, this way is faster due to low bounded
// updegree.
//
// 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 vtk_m_worklet_contourtree_active_edge_transferrer_h
#define vtk_m_worklet_contourtree_active_edge_transferrer_h

#include <vtkm/worklet/WorkletMapField.h>

namespace vtkm
{
namespace worklet
{
namespace contourtree
{

// Worklet: set initial chain maximum value
template <typename DeviceAdapter>
class ActiveEdgeTransferrer : public vtkm::worklet::WorkletMapField
{
public:
  using ControlSignature =
    void(FieldIn<IdType> vertexID,              // (input) active vertex ID
         FieldIn<IdType> newPosition,           // (input) new position of edge in array
         FieldIn<IdType> newOutdegree,          // (input) the new updegree computed
         WholeArrayInOut<IdType> firstEdge,     // (i/o) first edge of each active vertex
         WholeArrayInOut<IdType> outdegree,     // (i/o) existing vertex updegrees
         WholeArrayInOut<IdType> chainExtremum, // (i/o) chain extremum for vertices
         WholeArrayInOut<IdType> edgeFar,       // (i/o) high end of each edge
         WholeArrayOut<IdType> newActiveEdges); // (output) new active edge list
  using ExecutionSignature = void(_1, _2, _3, _4, _5, _6, _7, _8);
  using InputDomain = _1;

  // Passed in constructor because of argument limit on operator
  using IdPortalType =
    typename vtkm::cont::ArrayHandle<vtkm::Id>::template ExecutionTypes<DeviceAdapter>::PortalConst;

  IdPortalType activeEdges; // (input) active edges
  IdPortalType prunesTo;    // (input) where a vertex prunes to

  // Constructor
  VTKM_EXEC_CONT
  ActiveEdgeTransferrer(IdPortalType ActiveEdges, IdPortalType PrunesTo)
    : activeEdges(ActiveEdges)
    , prunesTo(PrunesTo)
  {
  }

  // WARNING: POTENTIAL RISK FOR I/O
  // chainMaximum is safe for I/O here because:
  // 		we have previously eliminated maxima from the active vertex list
  //		we lookup chainMaximum of edgeHigh, which is guaranteed to be a maximum
  //		therefore, the chainMaximum entries edited are *NEVER* also accessed & v.v.
  //		edgeHigh is safe to edit, because each element only accesses it's own, and
  //		reads the current value before writing to it
  //		the same is true of firstEdge and updegree

  template <typename InOutFieldPortalType, typename OutFieldPortalType>
  VTKM_EXEC void operator()(const vtkm::Id& vertexID,
                            const vtkm::Id& newPosition,
                            const vtkm::Id& newOutdegree,
                            const InOutFieldPortalType& firstEdge,
                            const InOutFieldPortalType& outdegree,
                            const InOutFieldPortalType& chainExtremum,
                            const InOutFieldPortalType& edgeFar,
                            const OutFieldPortalType& newActiveEdges) const
  {
    // retrieve actual vertex ID & first edge
    vtkm::Id edgeFirst = firstEdge.Get(vertexID);

    // internal counter for # of edges
    vtkm::Id whichEdge = newPosition;

    // walk through the vertex edges, counting as we go
    for (vtkm::Id edge = 0; edge < outdegree.Get(vertexID); edge++)
    {
      // compute the index and edge ID of this edge
      vtkm::Id edgeIndex = edgeFirst + edge;
      vtkm::Id edgeID = activeEdges.Get(edgeIndex);

      // retrieve the vertex ID for the high end & update for pruning
      vtkm::Id highEnd = prunesTo.Get(chainExtremum.Get(edgeFar.Get(edgeID)));

      // we want to ignore edges that lead back to this vertex
      if (highEnd != vertexID)
      {
        // reset the high end of the edge, copying downwards
        edgeFar.Set(edgeID, highEnd);

        // and keep the edge around
        newActiveEdges.Set(whichEdge++, edgeID);

        // and reset the chain maximum for good measure
        chainExtremum.Set(vertexID, highEnd);
      }
    }

    // now reset the firstEdge variable for this vertex
    outdegree.Set(vertexID, newOutdegree);
    firstEdge.Set(vertexID, newPosition);
  }
}; // ActiveEdgeTransferrer
}
}
}

#endif
