Implement extents support

Adds a variable `GlobalPointIndexStart` to `CellSetStructured`.
Adding this to the cell-set, instead of the coordinate system, enables this
feature for different types of datasets like uniform grid, rectilinear, etc.,
with this one change.
The extents can be computed using `GlobalPointIndexStart` and `PointDimensions`.

vtkm/cont/CellSetStructured.h

@@ -71,10 +71,20 @@ public:
     this->Structure.SetPointDimensions(dimensions);
   }
 
+  void SetGlobalPointIndexStart(SchedulingRangeType start)
+  {
+    this->Structure.SetGlobalPointIndexStart(start);
+  }
+
   SchedulingRangeType GetPointDimensions() const { return this->Structure.GetPointDimensions(); }
 
   SchedulingRangeType GetCellDimensions() const { return this->Structure.GetCellDimensions(); }
 
+  SchedulingRangeType GetGlobalPointIndexStart() const
+  {
+    return this->Structure.GetGlobalPointIndexStart();
+  }
+
   vtkm::IdComponent GetNumberOfPointsInCell(vtkm::Id vtkmNotUsed(cellIndex) = 0) const
   {
     return this->Structure.GetNumberOfPointsInCell();
@@ -145,16 +155,20 @@ public:
   {
     diy::save(bb, cs.GetName());
     diy::save(bb, cs.GetPointDimensions());
+    diy::save(bb, cs.GetGlobalPointIndexStart());
   }
 
   static VTKM_CONT void load(BinaryBuffer& bb, Type& cs)
   {
     std::string name;
     diy::load(bb, name);
-    typename Type::SchedulingRangeType dims;
+    typename Type::SchedulingRangeType dims, start;
     diy::load(bb, dims);
+    diy::load(bb, start);
+
     cs = Type(name);
     cs.SetPointDimensions(dims);
+    cs.SetGlobalPointIndexStart(start);
   }
 };
 

vtkm/exec/ConnectivityStructured.h

@@ -119,6 +119,12 @@ public:
     return this->Internals.GetPointDimensions();
   }
 
+  VTKM_EXEC_CONT
+  SchedulingRangeType GetGlobalPointIndexStart() const
+  {
+    return this->Internals.GetGlobalPointIndexStart();
+  }
+
   friend class ConnectivityStructured<ToTopology, FromTopology, Dimension>;
 
 private:

vtkm/internal/ConnectivityStructuredInternals.h

@@ -47,6 +47,9 @@ public:
   VTKM_EXEC_CONT
   void SetPointDimensions(vtkm::Id dimensions) { this->PointDimensions = dimensions; }
 
+  VTKM_EXEC_CONT
+  void SetGlobalPointIndexStart(vtkm::Id start) { this->GlobalPointIndexStart = start; }
+
   VTKM_EXEC_CONT
   vtkm::Id GetPointDimensions() const { return this->PointDimensions; }
 
@@ -65,6 +68,9 @@ public:
     return this->GetNumberOfPoints();
   }
 
+  VTKM_EXEC_CONT
+  SchedulingRangeType GetGlobalPointIndexStart() const { return this->GlobalPointIndexStart; }
+
   static constexpr vtkm::IdComponent NUM_POINTS_IN_CELL = 2;
   static constexpr vtkm::IdComponent MAX_CELL_TO_POINT = 2;
 
@@ -147,7 +153,8 @@ public:
   }
 
 private:
-  vtkm::Id PointDimensions;
+  vtkm::Id PointDimensions = 0;
+  vtkm::Id GlobalPointIndexStart = 0;
 };
 
 //2 D specialization.
@@ -160,6 +167,9 @@ public:
   VTKM_EXEC_CONT
   void SetPointDimensions(vtkm::Id2 dims) { this->PointDimensions = dims; }
 
+  VTKM_EXEC_CONT
+  void SetGlobalPointIndexStart(vtkm::Id2 start) { this->GlobalPointIndexStart = start; }
+
   VTKM_EXEC_CONT
   const vtkm::Id2& GetPointDimensions() const { return this->PointDimensions; }
 
@@ -181,6 +191,9 @@ public:
     return this->GetPointDimensions();
   }
 
+  VTKM_EXEC_CONT
+  const vtkm::Id2& GetGlobalPointIndexStart() const { return this->GlobalPointIndexStart; }
+
   static constexpr vtkm::IdComponent NUM_POINTS_IN_CELL = 4;
   static constexpr vtkm::IdComponent MAX_CELL_TO_POINT = 4;
 
@@ -305,7 +318,8 @@ public:
   }
 
 private:
-  vtkm::Id2 PointDimensions;
+  vtkm::Id2 PointDimensions = { 0, 0 };
+  vtkm::Id2 GlobalPointIndexStart = { 0, 0 };
 };
 
 //3 D specialization.
@@ -323,6 +337,9 @@ public:
     this->CellDim01 = (dims[0] - 1) * (dims[1] - 1);
   }
 
+  VTKM_EXEC_CONT
+  void SetGlobalPointIndexStart(vtkm::Id3 start) { this->GlobalPointIndexStart = start; }
+
   VTKM_EXEC_CONT
   const vtkm::Id3& GetPointDimensions() const { return this->PointDimensions; }
 
@@ -344,6 +361,9 @@ public:
     return this->GetPointDimensions();
   }
 
+  VTKM_EXEC_CONT
+  const vtkm::Id3& GetGlobalPointIndexStart() const { return this->GlobalPointIndexStart; }
+
   static constexpr vtkm::IdComponent NUM_POINTS_IN_CELL = 8;
   static constexpr vtkm::IdComponent MAX_CELL_TO_POINT = 8;
 
@@ -498,9 +518,10 @@ public:
   }
 
 private:
-  vtkm::Id3 PointDimensions;
-  vtkm::Id3 CellDimensions;
-  vtkm::Id CellDim01;
+  vtkm::Id3 PointDimensions = { 0, 0, 0 };
+  vtkm::Id3 GlobalPointIndexStart = { 0, 0, 0 };
+  vtkm::Id3 CellDimensions = { 0, 0, 0 };
+  vtkm::Id CellDim01 = 0;
 };
 
 // We may want to generalize this class depending on how ConnectivityExplicit