Ghost cells generator: point exchange optim in structured grids

Points at the interface between blocks were being exchange on structured
data (including points position and point data). This exchange is
unnecessary, so this commit removes it.

Author: yohannbearzi

Parallel/DIY/vtkDIYGhostUtilities.cxx

@@ -2594,20 +2594,47 @@ vtkSmartPointer<vtkIdList> ComputeInterfacePointIdsForStructuredData(unsigned ch
   kmin = std::max(extent[4], localExtent[4]);
   kmax = std::min(extent[5], localExtent[5]);
 
-  // We give ownership of the non ghost version of a point to the most right / back / top grid.
-  // We do that by removing the most right / back / top layer of points of the intersection between
-  // the 2 input extents.
-  if (adjacencyMask & Adjacency::Right)
-  {
-    --imax;
+  constexpr unsigned char LR = Adjacency::Right | Adjacency::Left;
+  constexpr unsigned char BF = Adjacency::Back | Adjacency::Front;
+  constexpr unsigned char TB = Adjacency::Top | Adjacency::Bottom;
+
+  // Points on the interface do not need to be exchanged, so we shrink the extent at those
+  // interfaces.
+  // Since input mask can have had a bitwise not operator performed, we weed out couples 11
+  // (they should not exist anyway: you cannot be adjacent to Right and Left at the same time, for
+  // instance).
+  if ((adjacencyMask & LR) != LR)
+  {
+    if (adjacencyMask & Adjacency::Right)
+    {
+      --imax;
+    }
+    if (adjacencyMask & Adjacency::Left)
+    {
+      ++imin;
+    }
   }
-  if (adjacencyMask & Adjacency::Back)
+  if ((adjacencyMask & BF) != BF)
   {
-    --jmax;
+    if (adjacencyMask & Adjacency::Back)
+    {
+      --jmax;
+    }
+    if (adjacencyMask & Adjacency::Front)
+    {
+      ++jmin;
+    }
   }
-  if (adjacencyMask & Adjacency::Top)
+  if ((adjacencyMask & TB) != TB)
   {
-    --kmax;
+    if (adjacencyMask & Adjacency::Top)
+    {
+      --kmax;
+    }
+    if (adjacencyMask & Adjacency::Bottom)
+    {
+      ++kmin;
+    }
   }
 
   const int* gridExtent = grid->GetExtent();
@@ -2668,9 +2695,10 @@ vtkSmartPointer<vtkIdList> ComputeOutputInterfacePointIdsForStructuredData(
   ExtentType localExtent
     { gridExtent[0], gridExtent[1], gridExtent[2], gridExtent[3], gridExtent[4], gridExtent[5] };
 
-  // We do a bit shift on adjacencyMask to have the same adjacency mask as in the Input version of
-  // this function. It produces an axial symmetry on each dimension having an adjacency.
-  return ComputeInterfacePointIdsForStructuredData(adjacencyMask << 1, localExtent, extent, grid);
+  // We apply a bitwise NOT opeartion on adjacencyMask to have the same adjacency mask as
+  // in the Input version of this function. It produces an axial symmetry on each dimension
+  // having an adjacency.
+  return ComputeInterfacePointIdsForStructuredData(~(adjacencyMask | 0x40), localExtent, extent, grid);
 }
 
 //----------------------------------------------------------------------------