From be30598e3d80babbda8462f3e6d022e7ae52060c Mon Sep 17 00:00:00 2001
From: Mark Olesen <Mark.Olesen@esi-group.com>
Date: Mon, 3 Mar 2025 11:13:07 +0100
Subject: [PATCH] ENH: add UPstream::localNode_parentProcs() method

- returns the (start/size) range of the commLocalNode ranks in terms
  of the (const) world communicator processors. This allows mapping
  back into lists defined in terms of the world ranks.
---
 .../test/nodeTopology/Test-nodeTopology.cxx   |  7 ++++
 src/OpenFOAM/db/IOstreams/Pstreams/UPstream.C | 32 +++++++++++++++++++
 src/OpenFOAM/db/IOstreams/Pstreams/UPstream.H |  7 ++--
 3 files changed, 44 insertions(+), 2 deletions(-)

diff --git a/applications/test/nodeTopology/Test-nodeTopology.cxx b/applications/test/nodeTopology/Test-nodeTopology.cxx
index 9909d51d4f..58b3d56cde 100644
--- a/applications/test/nodeTopology/Test-nodeTopology.cxx
+++ b/applications/test/nodeTopology/Test-nodeTopology.cxx
@@ -115,6 +115,13 @@ int main(int argc, char *argv[])
     );
 
 
+    if (UPstream::parRun())
+    {
+        const auto& procs = UPstream::localNode_parentProcs();
+        Perr<< "local processors: [" << procs.min()
+            << ".." << procs.max() << ']' << endl;
+    }
+
     // Generate the graph
     if (UPstream::master(UPstream::worldComm))
     {
diff --git a/src/OpenFOAM/db/IOstreams/Pstreams/UPstream.C b/src/OpenFOAM/db/IOstreams/Pstreams/UPstream.C
index 4ba0e1f4ab..d3b8e489e3 100644
--- a/src/OpenFOAM/db/IOstreams/Pstreams/UPstream.C
+++ b/src/OpenFOAM/db/IOstreams/Pstreams/UPstream.C
@@ -791,6 +791,38 @@ const Foam::List<int>& Foam::UPstream::interNode_offsets()
 }
 
 
+const Foam::UPstream::rangeType& Foam::UPstream::localNode_parentProcs()
+{
+    static UPstream::rangeType singleton;
+
+    if (singleton.empty())
+    {
+        // The inter-node offsets [in const world comm] also include a
+        // startup guard
+        const auto& offsets = UPstream::interNode_offsets();
+
+        const auto nodei =
+            Foam::findLower
+            (
+                offsets,
+                // My place within const world comm
+                UPstream::myProcNo(constWorldComm_)+1
+            );
+
+        if (nodei >= 0)
+        {
+            singleton.reset
+            (
+                offsets[nodei],
+                offsets[nodei+1] - offsets[nodei]
+            );
+        }
+    }
+
+    return singleton;
+}
+
+
 // * * * * * * * * * * * * * * Static Data Members * * * * * * * * * * * * * //
 
 bool Foam::UPstream::parRun_(false);
diff --git a/src/OpenFOAM/db/IOstreams/Pstreams/UPstream.H b/src/OpenFOAM/db/IOstreams/Pstreams/UPstream.H
index d0e23c298c..a18d46128e 100644
--- a/src/OpenFOAM/db/IOstreams/Pstreams/UPstream.H
+++ b/src/OpenFOAM/db/IOstreams/Pstreams/UPstream.H
@@ -1232,6 +1232,9 @@ public:
         //- Processor offsets corresponding to the inter-node communicator
         static const List<int>& interNode_offsets();
 
+        //- The range (start/size) of the commLocalNode ranks in terms of the
+        //- (const) world communicator processors.
+        static const rangeType& localNode_parentProcs();
 
         //- Linear communication schedule (special case) for given communicator
         static const commsStructList& linearCommunication(int communicator);
@@ -1240,8 +1243,8 @@ public:
         static const commsStructList& treeCommunication(int communicator);
 
         //- Communication schedule for all-to-master (proc 0) as
-        //- linear/tree/none with switching based on UPstream::nProcsSimpleSum
-        //- and the is_parallel() state and the optional \c linear parameter.
+        //- linear/tree/none with switching based on UPstream::nProcsSimpleSum,
+        //- the is_parallel() state and the optional \c linear parameter.
         static const commsStructList& whichCommunication
         (
             const int communicator,