ENH: revert regionSplit to older algorithm (issue #805)

- the algorithm was last used in OpenFOAM-2.4, after which it was
  replaced with a FaceCellWave version.

  Whereas the original (2.4.x) version exhibited performance
  degradation on very large meshes (with explicit constraints), the
  FaceCellWave version exhibited performance issues with large numbers
  of blocked faces.

  With large numbers of blocked faces, the FaceCellWave regionSplit
  could take between 10 to 100 times longer due to the slow
  propagation speed through blocked faces.

  The 2.4 regionSplit has been revamped to avoid local memory
  allocations, which appears to have been the source of the original
  performance issues on large meshes.

  For additional performance, intermediate renumbering is also avoided
  during the consolidation of regions over processor domains.

src/meshTools/regionSplit/regionSplit.H

@@ -2,8 +2,8 @@
   =========                 |
   \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
    \\    /   O peration     |
-    \\  /    A nd           | Copyright (C) 2011-2016 OpenFOAM Foundation
-     \\/     M anipulation  |
+    \\  /    A nd           | Copyright (C) 2011-2013 OpenFOAM Foundation
+     \\/     M anipulation  | Copyright (C) 2018 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -103,12 +103,11 @@ Description
 
     \endverbatim
 
-
     Can optionally keep all regions local to the processor.
 
-    Note: does not walk across cyclicAMI/cyclicACMI - since not 'coupled()'
-          at the patch level.
-
+Note
+    does not walk across cyclicAMI/cyclicACMI - since these are not
+    \c coupled() at the patch level.
 
 SourceFiles
     regionSplit.C
@@ -128,6 +127,8 @@ SourceFiles
 namespace Foam
 {
 
+// Forward declarations
+
 class polyMesh;
 
 /*---------------------------------------------------------------------------*\
@@ -141,21 +142,52 @@ class regionSplit
 {
     // Private data
 
-        mutable autoPtr<globalIndex> globalNumberingPtr_;
+        autoPtr<globalIndex> globalNumberingPtr_;
+
+        //- Temporary list of cells that have changed
+        mutable DynamicList<label> changedCells_;
+
+        //- Temporary list of faces that have changed
+        mutable DynamicList<label> changedFaces_;
 
 
     // Private Member Functions
 
-        //- Calculate region split in non-compact (global) numbering.
-        void calcNonCompactRegionSplit
+        //- Update faceRegion data between (non-processor) coupled faces.
+        void updateFacePair
+        (
+            const label face0,
+            const label face1,
+            labelList& faceRegion,
+            DynamicList<label>& facesChanged
+        ) const;
+
+        //- Given a seed cell label, fill cellRegion/faceRegion with markValue
+        //- for contiguous region around it
+        void fillSeedMask
+        (
+            const List<labelPair>& explicitConnections,
+            const label seedCellID,
+            const label markValue,
+            labelList& cellRegion,
+            labelList& faceRegion
+        ) const;
+
+
+        //- Calculate the local region split.
+        //  \return number of processor-local regions,
+        //      without consolidation between procesors
+        label calcLocalRegionSplit
         (
-            const globalIndex& globalFaces,
             const boolList& blockedFace,
             const List<labelPair>& explicitConnections,
             labelList& cellRegion
         ) const;
 
-        //- Calculate global region split. Return globalIndex.
+
+        //- Calculate the local region split.
+        //  \return number of processor-local regions,
+        //      without consolidation between procesors
         autoPtr<globalIndex> calcRegionSplit
         (
             const bool doGlobalRegions,
@@ -176,27 +208,27 @@ public:
         //- Construct from mesh
         regionSplit
         (
-            const polyMesh&,
+            const polyMesh& mesh,
             const bool doGlobalRegions = Pstream::parRun()
         );
 
         //- Construct from mesh and whether face is blocked
-        //  \note blockedFace has to be consistent across coupled faces!
+        //  \note blockedFace must be consistent across coupled faces!
         regionSplit
         (
-            const polyMesh&,
+            const polyMesh& mesh,
             const boolList& blockedFace,
             const bool doGlobalRegions = Pstream::parRun()
         );
 
-        //- Construct from mesh and whether face is blocked.
-        //  Additional explicit connections between normal boundary faces.
-        //  \note blockedFace has to be consistent across coupled faces!
+        //- Construct from mesh and whether face is blocked, with additional
+        //- explicit connections between normal boundary faces.
+        //  \note blockedFace must be consistent across coupled faces!
         regionSplit
         (
-            const polyMesh&,
+            const polyMesh& mesh,
             const boolList& blockedFace,
-            const List<labelPair>&,
+            const List<labelPair>& explicitConnections,
             const bool doGlobalRegions = Pstream::parRun()
         );
 
@@ -220,6 +252,19 @@ public:
         {
             return globalNumbering().size();
         }
+
+
+        //- Manually consolidate the regions globally by swapping information
+        //  between processor domains and reducing the regions accordingly.
+        //
+        //  \return number of local regions after reduction.
+        autoPtr<globalIndex> reduceRegions
+        (
+            const label numLocalRegions,
+            const boolList& blockedFace,
+            labelList& cellRegion
+        ) const;
+
 };