From bd57627955a68bde0cf3eb1bd28a188d81951ebc Mon Sep 17 00:00:00 2001
From: Mark Olesen <Mark.Olesen@keysight.com>
Date: Thu, 28 Aug 2025 14:33:18 +0200
Subject: [PATCH] ENH: extend bitSet functionality

- num_blocks(), test_set() as per boost
- broadcast(), reduceAnd(), reduceOr() to simplify parallel operations
- matrix-like output for PackedList::writeList()

BUG: Pstream::broadcastList() missing resize on sub-ranks

- latent bug since it was unused in any OpenFOAM code
---
 applications/test/PackedList/Make/files       |   2 +-
 ...{Test-PackedList.C => Test-PackedList.cxx} |   0
 applications/test/bitSet1/Test-bitSet1.cxx    |   3 +
 applications/test/bitSet2/Test-bitSet2.cxx    |   4 +-
 .../parallel-chunks/Test-parallel-chunks.cxx  |  22 +-
 .../parallel-comm0/Test-parallel-comm0.cxx    |  52 ++--
 .../parallel-comm3a/Test-parallel-comm3a.cxx  |   4 +-
 .../parallel-comm3b/Test-parallel-comm3b.cxx  |   4 +-
 .../test/parallel-nbx2/Test-parallel-nbx2.cxx |   4 +-
 .../containers/Bits/PackedList/PackedList.H   |  28 +-
 .../containers/Bits/PackedList/PackedListI.H  |  32 +--
 .../containers/Bits/PackedList/PackedListIO.C |  36 ++-
 src/OpenFOAM/containers/Bits/bitSet/bitSet.C  | 239 +++++++++++++++++-
 src/OpenFOAM/containers/Bits/bitSet/bitSet.H  |  75 +++++-
 src/OpenFOAM/containers/Bits/bitSet/bitSetI.H |  32 ++-
 src/OpenFOAM/db/IOstreams/Pstreams/Pstream.H  |   3 +
 .../IOstreams/Pstreams/PstreamBroadcast.txx   |  10 +
 17 files changed, 433 insertions(+), 117 deletions(-)
 rename applications/test/PackedList/{Test-PackedList.C => Test-PackedList.cxx} (100%)

diff --git a/applications/test/PackedList/Make/files b/applications/test/PackedList/Make/files
index 93a37615fc..ec391bad6e 100644
--- a/applications/test/PackedList/Make/files
+++ b/applications/test/PackedList/Make/files
@@ -1,3 +1,3 @@
-Test-PackedList.C
+Test-PackedList.cxx
 
 EXE = $(FOAM_USER_APPBIN)/Test-PackedList
diff --git a/applications/test/PackedList/Test-PackedList.C b/applications/test/PackedList/Test-PackedList.cxx
similarity index 100%
rename from applications/test/PackedList/Test-PackedList.C
rename to applications/test/PackedList/Test-PackedList.cxx
diff --git a/applications/test/bitSet1/Test-bitSet1.cxx b/applications/test/bitSet1/Test-bitSet1.cxx
index 08d5f47a2a..4c5df5ac32 100644
--- a/applications/test/bitSet1/Test-bitSet1.cxx
+++ b/applications/test/bitSet1/Test-bitSet1.cxx
@@ -142,6 +142,9 @@ int main(int argc, char *argv[])
             Info<< "got: " << bset1 << nl
                 << "and: " << bset2 << nl
                 << "and: " << bset3 << nl;
+
+            Info<< "==";
+            bset3.writeList(Info, 10) << nl;  // matrix-like output
         }
     }
 
diff --git a/applications/test/bitSet2/Test-bitSet2.cxx b/applications/test/bitSet2/Test-bitSet2.cxx
index 6b0675b836..98a4c59489 100644
--- a/applications/test/bitSet2/Test-bitSet2.cxx
+++ b/applications/test/bitSet2/Test-bitSet2.cxx
@@ -137,11 +137,11 @@ inline bool compare
     const std::string& expected
 )
 {
-    const List<unsigned int>& store = bitset.storage();
+    const auto& store = bitset.storage();
 
     std::string has;
 
-    for (label blocki=0; blocki < bitset.nBlocks(); ++blocki)
+    for (label blocki=0; blocki < bitset.num_blocks(); ++blocki)
     {
         has += toString(store[blocki]);
     }
diff --git a/applications/test/parallel-chunks/Test-parallel-chunks.cxx b/applications/test/parallel-chunks/Test-parallel-chunks.cxx
index 1975699889..78b1dce67e 100644
--- a/applications/test/parallel-chunks/Test-parallel-chunks.cxx
+++ b/applications/test/parallel-chunks/Test-parallel-chunks.cxx
@@ -185,9 +185,9 @@ int main(int argc, char *argv[])
     }
     broadcast_chunks<labelList, label>(input1);
 
-    Pstream::maxCommsSize = 33;
+    UPstream::maxCommsSize = 33;
 
-    args.readIfPresent("comms-size", Pstream::maxCommsSize);
+    args.readIfPresent("comms-size", UPstream::maxCommsSize);
 
     broadcast_chunks<labelList, label>(input1);
 
@@ -197,11 +197,11 @@ int main(int argc, char *argv[])
         PstreamBuffers pBufs;
 
         labelList sendData;
-        if (Pstream::master())
+        if (UPstream::master())
         {
             sendData = identity(500);
 
-            for (const int proci : Pstream::subProcs())
+            for (const int proci : UPstream::subProcs())
             {
                 UOPstream os(proci, pBufs);
                 os << sendData;
@@ -211,7 +211,7 @@ int main(int argc, char *argv[])
         Info<< "call finishedSends()" << endl;
         pBufs.finishedScatters();
 
-        if (!Pstream::master())
+        if (UPstream::is_subrank())
         {
             UIPstream is(UPstream::masterNo(), pBufs);
             is >> sendData;
@@ -225,11 +225,11 @@ int main(int argc, char *argv[])
         labelListList recvBufs(UPstream::nProcs());
         labelList recvSizes;
 
-        if (Pstream::master())
+        if (UPstream::master())
         {
-            for (const int proci : Pstream::allProcs())
+            for (const int proci : UPstream::allProcs())
             {
-                if (proci != Pstream::myProcNo())
+                if (proci != UPstream::myProcNo())
                 {
                     sendBufs[proci] = identity(500);
                 }
@@ -253,11 +253,11 @@ int main(int argc, char *argv[])
         Map<labelList> recvBufs;
         Map<label> recvSizes;
 
-        if (Pstream::master())
+        if (UPstream::master())
         {
-            for (const int proci : Pstream::allProcs())
+            for (const int proci : UPstream::allProcs())
             {
-                if (proci != Pstream::myProcNo())
+                if (proci != UPstream::myProcNo())
                 {
                     sendBufs(proci) = identity(500);
                 }
diff --git a/applications/test/parallel-comm0/Test-parallel-comm0.cxx b/applications/test/parallel-comm0/Test-parallel-comm0.cxx
index 9185ff4cb4..107f8855f4 100644
--- a/applications/test/parallel-comm0/Test-parallel-comm0.cxx
+++ b/applications/test/parallel-comm0/Test-parallel-comm0.cxx
@@ -110,21 +110,25 @@ int main(int argc, char *argv[])
             << " (self) reduced " << selfVal << nl;
 
         // Identical size on all procs
-        bitSet procUsed(nProcs);
-
-        if ((myRank % 4) == 0)
         {
-            procUsed.set(myRank);
+            bitSet localUsed(nProcs);
+            localUsed.set(myRank, ((myRank % 4) == 0));
+
+            Pout<< "local  procUsed " << localUsed << nl;
+            localUsed.reduceOr(UPstream::worldComm, false);
+            Pout<< "reduce procUsed " << localUsed << nl;
+        }
+
+        // With allGather
+        {
+            bitSet procUsed
+            (
+                bitSet::allGather((myRank % 4) == 0)
+            );
+
+            Pout<< "allGather: " << procUsed << nl;
         }
 
-        Pout<< "local  procUsed " << procUsed << nl;
-        reduce
-        (
-            procUsed.data(),
-            procUsed.size_data(),
-            bitOrOp<unsigned int>()
-        );
-        Pout<< "reduce procUsed " << procUsed << nl;
 
         // Identical size on all procs
         // encode as 0:empty, 1:uniform, 2:nonuniform, 3:mixed
@@ -147,12 +151,26 @@ int main(int argc, char *argv[])
         }
 
         Pout<< "local  uniform " << uniformity << nl;
-        reduce
+        // reduce with op<..>()
+        #if 1
+        Foam::reduce
         (
             uniformity.data(),
-            uniformity.size_data(),
-            bitOrOp<unsigned int>()
+            uniformity.num_blocks(),
+            bitOrOp<unsigned int>(),
+            UPstream::msgType(),  // ignored
+            UPstream::worldComm
         );
+        #else
+        // Direct call to MPI_Allreduce
+        UPstream::mpiAllReduce
+        (
+            uniformity.data(),
+            uniformity.num_blocks(),
+            UPstream::opCodes::op_bit_or,
+            UPstream::worldComm
+        );
+        #endif
         Pout<< "reduce uniform " << uniformity << nl;
     }
 
@@ -160,8 +178,8 @@ int main(int argc, char *argv[])
     {
         Pair<label> val
         (
-            Pstream::myProcNo(UPstream::commWorld()),
-            Pstream::myProcNo(UPstream::commWorld())
+            UPstream::myProcNo(UPstream::commWorld()),
+            UPstream::myProcNo(UPstream::commWorld())
         );
 
         Pair<label> worldVal = val;
diff --git a/applications/test/parallel-comm3a/Test-parallel-comm3a.cxx b/applications/test/parallel-comm3a/Test-parallel-comm3a.cxx
index c0eb29074f..b208c1e59b 100644
--- a/applications/test/parallel-comm3a/Test-parallel-comm3a.cxx
+++ b/applications/test/parallel-comm3a/Test-parallel-comm3a.cxx
@@ -79,7 +79,7 @@ int main(int argc, char *argv[])
 
     #include "setRootCase.H"
 
-    if (!Pstream::parRun())
+    if (!UPstream::parRun())
     {
         Info<< "\nWarning: not parallel - skipping further tests\n" << endl;
         return 0;
@@ -97,7 +97,7 @@ int main(int argc, char *argv[])
     DynamicList<MPI_Request> recvRequests(10);
 
 
-    if (!Pstream::master())
+    if (UPstream::is_subrank())
     {
         // Send some random length to master
 
diff --git a/applications/test/parallel-comm3b/Test-parallel-comm3b.cxx b/applications/test/parallel-comm3b/Test-parallel-comm3b.cxx
index ea64880146..bdf511334f 100644
--- a/applications/test/parallel-comm3b/Test-parallel-comm3b.cxx
+++ b/applications/test/parallel-comm3b/Test-parallel-comm3b.cxx
@@ -76,7 +76,7 @@ int main(int argc, char *argv[])
 
     #include "setRootCase.H"
 
-    if (!Pstream::parRun())
+    if (!UPstream::parRun())
     {
         Info<< "\nWarning: not parallel - skipping further tests\n" << endl;
         return 0;
@@ -96,7 +96,7 @@ int main(int argc, char *argv[])
     // Map request indices to procs
     Map<label> recvFromProc(20);
 
-    if (!Pstream::master())
+    if (UPstream::is_subrank())
     {
         // Send some random length to master
 
diff --git a/applications/test/parallel-nbx2/Test-parallel-nbx2.cxx b/applications/test/parallel-nbx2/Test-parallel-nbx2.cxx
index 9f656517c6..0c315393c6 100644
--- a/applications/test/parallel-nbx2/Test-parallel-nbx2.cxx
+++ b/applications/test/parallel-nbx2/Test-parallel-nbx2.cxx
@@ -52,7 +52,7 @@ int main(int argc, char *argv[])
 
     const bool optNonBlocking = args.found("non-blocking");
 
-    if (!Pstream::parRun())
+    if (!UPstream::parRun())
     {
         Info<< "\nWarning: not parallel - skipping further tests\n" << endl;
         return 0;
@@ -73,7 +73,7 @@ int main(int argc, char *argv[])
     DynamicList<UPstream::Request> sendRequests(10);
     DynamicList<UPstream::Request> recvRequests(10);
 
-    if (!Pstream::master())
+    if (UPstream::is_subrank())
     {
         // Send some random length to master
 
diff --git a/src/OpenFOAM/containers/Bits/PackedList/PackedList.H b/src/OpenFOAM/containers/Bits/PackedList/PackedList.H
index aa6c755962..377aa6ac05 100644
--- a/src/OpenFOAM/containers/Bits/PackedList/PackedList.H
+++ b/src/OpenFOAM/containers/Bits/PackedList/PackedList.H
@@ -6,7 +6,7 @@
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
     Copyright (C) 2011-2016 OpenFOAM Foundation
-    Copyright (C) 2017-2023 OpenCFD Ltd.
+    Copyright (C) 2017-2025 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -422,20 +422,20 @@ public:
     // Low-level access
 
         //- The number of internal storage blocks
-        inline label nBlocks() const;
+        inline label num_blocks() const noexcept;
 
         //- Return the underlying storage blocks
-        inline const List<unsigned int>& storage() const;
+        const List<block_type>& storage() const noexcept { return blocks_; }
 
         //- Return the underlying storage blocks
         //  Manipulate with utmost caution
-        inline List<unsigned int>& storage();
+        List<block_type>& storage() noexcept { return blocks_; }
 
         //- A const pointer to the raw storage
-        inline const unsigned int* cdata() const noexcept;
+        const block_type* cdata() const noexcept { return blocks_.cdata(); }
 
         //- A pointer to the raw storage
-        inline unsigned int* data() noexcept;
+        block_type* data() noexcept { return blocks_.data(); }
 
         //- A const pointer to the raw storage, reinterpreted as byte data
         inline const char* cdata_bytes() const noexcept;
@@ -443,15 +443,15 @@ public:
         //- A pointer to the raw storage, reinterpreted as byte data
         inline char* data_bytes() noexcept;
 
-        //- The number of integer blocks addressed in the raw storage
+        //- The number of integer blocks addressed in the raw storage.
+        //- Same as num_blocks().
         inline std::streamsize size_data() const noexcept;
 
-        //- The number of bytes used in the raw storage
-        //- including any unused padding.
+        //- The number of bytes addressed in the raw storage
+        //- including any padding.
         inline std::streamsize size_bytes() const noexcept;
 
-        //- The number of bytes used in the raw storage
-        //- including any unused padding.
+        //- Same as size_bytes()
         inline std::streamsize byteSize() const noexcept;
 
 
@@ -465,7 +465,7 @@ public:
 
         //- Write List, with line-breaks in ASCII when length exceeds shortLen.
         //  Using '0' suppresses line-breaks entirely.
-        Ostream& writeList(Ostream& os, const label shortLen=0) const;
+        Ostream& writeList(Ostream& os, label shortLen=0) const;
 
         //- Write as a dictionary entry with keyword
         void writeEntry(const word& keyword, Ostream& os) const;
@@ -529,10 +529,10 @@ public:
         public:
 
             //- Copy construct
-            reference(const reference&) = default;
+            reference(const reference&) noexcept = default;
 
             //- Move construct
-            reference(reference&&) = default;
+            reference(reference&&) noexcept = default;
 
             //- Value assignment
             inline void operator=(const reference& other);
diff --git a/src/OpenFOAM/containers/Bits/PackedList/PackedListI.H b/src/OpenFOAM/containers/Bits/PackedList/PackedListI.H
index a8e7f93dd9..1c96982316 100644
--- a/src/OpenFOAM/containers/Bits/PackedList/PackedListI.H
+++ b/src/OpenFOAM/containers/Bits/PackedList/PackedListI.H
@@ -609,40 +609,12 @@ inline void Foam::PackedList<Width>::shrink_to_fit()
 
 
 template<unsigned Width>
-inline Foam::List<unsigned int>& Foam::PackedList<Width>::storage()
-{
-    return blocks_;
-}
-
-
-template<unsigned Width>
-inline const Foam::List<unsigned int>& Foam::PackedList<Width>::storage() const
-{
-    return blocks_;
-}
-
-
-template<unsigned Width>
-inline Foam::label Foam::PackedList<Width>::nBlocks() const
+inline Foam::label Foam::PackedList<Width>::num_blocks() const noexcept
 {
     return num_blocks(size());
 }
 
 
-template<unsigned Width>
-inline const unsigned int* Foam::PackedList<Width>::cdata() const noexcept
-{
-    return blocks_.cdata();
-}
-
-
-template<unsigned Width>
-inline unsigned int* Foam::PackedList<Width>::data() noexcept
-{
-    return blocks_.data();
-}
-
-
 template<unsigned Width>
 inline const char* Foam::PackedList<Width>::cdata_bytes() const noexcept
 {
@@ -667,7 +639,7 @@ inline std::streamsize Foam::PackedList<Width>::size_data() const noexcept
 template<unsigned Width>
 inline std::streamsize Foam::PackedList<Width>::size_bytes() const noexcept
 {
-    return size_data() * sizeof(block_type);
+    return num_blocks(size()) * sizeof(block_type);
 }
 
 
diff --git a/src/OpenFOAM/containers/Bits/PackedList/PackedListIO.C b/src/OpenFOAM/containers/Bits/PackedList/PackedListIO.C
index 74fdaf4137..723c439db7 100644
--- a/src/OpenFOAM/containers/Bits/PackedList/PackedListIO.C
+++ b/src/OpenFOAM/containers/Bits/PackedList/PackedListIO.C
@@ -5,7 +5,7 @@
     \\  /    A nd           | www.openfoam.com
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
-    Copyright (C) 2018-2022 OpenCFD Ltd.
+    Copyright (C) 2018-2025 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -178,12 +178,14 @@ template<unsigned Width>
 Foam::Ostream& Foam::PackedList<Width>::writeList
 (
     Ostream& os,
-    const label shortLen
+    label shortLen
 ) const
 {
     const PackedList<Width>& list = *this;
     const label len = list.size();
 
+    if (shortLen < 0) shortLen = 1;  // <- sanity
+
     if (os.format() == IOstreamOption::BINARY)
     {
         // Binary (always contiguous)
@@ -209,7 +211,7 @@ Foam::Ostream& Foam::PackedList<Width>::writeList
         os << len << token::BEGIN_LIST;
 
         // Contents
-        for (label i=0; i < len; ++i)
+        for (label i = 0; i < len; ++i)
         {
             if (i) os << token::SPACE;
             os << label(list.get(i));
@@ -226,9 +228,33 @@ Foam::Ostream& Foam::PackedList<Width>::writeList
         os << nl << len << nl << token::BEGIN_LIST << nl;
 
         // Contents
-        for (label i=0; i < len; ++i)
+        if (shortLen <= 1)
         {
-            os << label(list.get(i)) << nl;
+            // simple multi-line
+            for (label i = 0; i < len; ++i)
+            {
+                os << label(list.get(i)) << nl;
+            }
+        }
+        else
+        {
+            // 'matrix' of values
+            label line = 0;
+
+            for (label i = 0; i < len; ++i, ++line)
+            {
+                if (line == shortLen)
+                {
+                    os << nl;
+                    line = 0;
+                }
+                else if (line)
+                {
+                    os << token::SPACE;
+                }
+                os << label(list.get(i));
+            }
+            if (line) os << nl;
         }
 
         // End delimiter
diff --git a/src/OpenFOAM/containers/Bits/bitSet/bitSet.C b/src/OpenFOAM/containers/Bits/bitSet/bitSet.C
index 4773994130..a90cdad5c5 100644
--- a/src/OpenFOAM/containers/Bits/bitSet/bitSet.C
+++ b/src/OpenFOAM/containers/Bits/bitSet/bitSet.C
@@ -5,7 +5,7 @@
     \\  /    A nd           | www.openfoam.com
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
-    Copyright (C) 2018-2022 OpenCFD Ltd.
+    Copyright (C) 2018-2025 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -28,14 +28,19 @@ License
 #include "bitSet.H"
 #include "labelRange.H"
 #include "IOstreams.H"
+#include "UPstream.H"
+#include "addToRunTimeSelectionTable.H"
 
 // * * * * * * * * * * * * * * Static Data Members * * * * * * * * * * * * * //
 
 namespace Foam
 {
     defineTypeNameAndDebug(bitSet, 0);
-}
 
+    // TBD: add IO support of compound type?
+    // defineNamedCompoundTypeName(bitSet, List<1>);
+    // addNamedCompoundToRunTimeSelectionTable(bitSet, bitSet, List<1>);
+}
 
 // * * * * * * * * * * * * Protected Member Functions  * * * * * * * * * * * //
 
@@ -76,7 +81,7 @@ Foam::bitSet& Foam::bitSet::minusEq(const bitSet& other)
 
 Foam::bitSet& Foam::bitSet::andEq(const bitSet& other)
 {
-    if (&other == this)
+    if (FOAM_UNLIKELY(&other == this))
     {
         // Self '&=' : no-op
 
@@ -273,19 +278,24 @@ Foam::bitSet::bitSet(const labelRange& range)
 
 void Foam::bitSet::assign(const UList<bool>& bools)
 {
-    const label len = bools.size();
+    fill(false);
+    resize(bools.size());
 
-    clear();
-    resize(len);
+    unsigned bitIdx = 0u;
+    auto* packed = blocks_.data();
 
-    // Could also handle block-wise (in the future?)
-
-    // Set according to indices that are true.
-    for (label i = 0; i < len; ++i)
+    // Set according to indices that are true
+    for (const auto b : bools)
     {
-        if (bools[i])
+        if (b)
         {
-            set(i);
+            *packed |= (1u << bitIdx);
+        }
+
+        if (++bitIdx >= PackedList<1>::elem_per_block)
+        {
+            bitIdx = 0u;
+            ++packed;
         }
     }
 }
@@ -540,4 +550,209 @@ Foam::List<bool> Foam::bitSet::values() const
 }
 
 
+// * * * * * * * * * * * * * *  Parallel Functions * * * * * * * * * * * * * //
+
+void Foam::bitSet::broadcast(int communicator, bool syncSizes)
+{
+    if (communicator < 0)
+    {
+        communicator = UPstream::worldComm;
+    }
+
+    if (!UPstream::is_parallel(communicator))
+    {
+        return;
+    }
+
+    int64_t len(size());
+
+    if (syncSizes)
+    {
+        UPstream::broadcast(&len, 1, communicator);
+
+        if (UPstream::is_subrank(communicator))
+        {
+            fill(false);
+            resize(len);
+        }
+    }
+
+    if (len)
+    {
+        // Only broadcast non-empty
+        UPstream::broadcast(this->data(), this->num_blocks(), communicator);
+    }
+}
+
+
+void Foam::bitSet::reduceAnd(int communicator, bool syncSizes)
+{
+    if (communicator < 0)
+    {
+        communicator = UPstream::worldComm;
+    }
+
+    if (!UPstream::is_parallel(communicator))
+    {
+        return;
+    }
+
+    const label origSize(size());
+
+    if (syncSizes)
+    {
+        // Operation is an intersection
+        // - common size may be smaller than the original size
+        int64_t commonSize(size());
+
+        UPstream::mpiAllReduce
+        (
+            &commonSize,
+            1,
+            UPstream::opCodes::op_min,
+            communicator
+        );
+        resize(commonSize);
+    }
+
+    if (!empty())
+    {
+        UPstream::mpiAllReduce
+        (
+            this->data(),
+            this->num_blocks(),
+            UPstream::opCodes::op_bit_and,
+            communicator
+        );
+
+        clear_trailing_bits();  // safety
+    }
+
+    // Undo side effects from the reduction
+    if (syncSizes)
+    {
+        resize(origSize);
+    }
+}
+
+
+void Foam::bitSet::reduceOr(int communicator, bool syncSizes)
+{
+    if (communicator < 0)
+    {
+        communicator = UPstream::worldComm;
+    }
+
+    if (!UPstream::is_parallel(communicator))
+    {
+        return;
+    }
+
+    // const label origSize(size());
+
+    if (syncSizes)
+    {
+        // Operation can increase the addressed size
+
+        // Extend size based on the addressed length.
+        // This is greedy, but produces consistent sizing
+        int64_t commonSize(size());
+
+        // Alternative: Extend size based on the bits used.
+        // - tighter, but inconsistent sizes result
+        // // label commonSize(find_last()+1);
+
+        UPstream::mpiAllReduce
+        (
+            &commonSize,
+            1,
+            UPstream::opCodes::op_max,
+            communicator
+        );
+
+        extend(commonSize);
+    }
+
+    if (!empty())
+    {
+        UPstream::mpiAllReduce
+        (
+            this->data(),
+            this->num_blocks(),
+            UPstream::opCodes::op_bit_or,
+            communicator
+        );
+
+        clear_trailing_bits();  // safety
+    }
+}
+
+
+Foam::bitSet Foam::bitSet::gatherValues(bool localValue, int communicator)
+{
+    if (communicator < 0)
+    {
+        communicator = UPstream::worldComm;
+    }
+
+    bitSet allValues;
+
+    if (!UPstream::is_parallel(communicator))
+    {
+        // non-parallel: return own value
+        // TBD: only when UPstream::is_rank(communicator) as well?
+        allValues.resize(1);
+        allValues.set(0, localValue);
+    }
+    else
+    {
+        List<bool> bools;
+        if (UPstream::master(communicator))
+        {
+            bools.resize(UPstream::nProcs(communicator), false);
+        }
+
+        UPstream::mpiGather
+        (
+            &localValue,    // Send
+            bools.data(),   // Recv
+            1,              // Num send/recv data per rank
+            communicator
+        );
+
+        // Transcribe to bitSet (on master)
+        allValues.assign(bools);
+    }
+
+    return allValues;
+}
+
+
+// Note that for allGather()
+// - MPI_Gather of individual bool values and broadcast the packed result
+// - this avoids bit_or on 32bit values everywhere, since we know a priori
+//   that each rank only contributes 1bit of info
+
+Foam::bitSet Foam::bitSet::allGather(bool localValue, int communicator)
+{
+    if (communicator < 0)
+    {
+        communicator = UPstream::worldComm;
+    }
+
+    bitSet allValues(bitSet::gatherValues(localValue, communicator));
+
+    if (UPstream::is_parallel(communicator))
+    {
+        // Identical size on all ranks
+        allValues.resize(UPstream::nProcs(communicator));
+
+        // Sizes are consistent - broadcast without resizing
+        allValues.broadcast(communicator, false);
+    }
+
+    return allValues;
+}
+
+
 // ************************************************************************* //
diff --git a/src/OpenFOAM/containers/Bits/bitSet/bitSet.H b/src/OpenFOAM/containers/Bits/bitSet/bitSet.H
index b1e4c7fdd4..ff65c5c1c5 100644
--- a/src/OpenFOAM/containers/Bits/bitSet/bitSet.H
+++ b/src/OpenFOAM/containers/Bits/bitSet/bitSet.H
@@ -5,7 +5,7 @@
     \\  /    A nd           | www.openfoam.com
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
-    Copyright (C) 2018-2023 OpenCFD Ltd.
+    Copyright (C) 2018-2025 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -46,7 +46,6 @@ See also
 #ifndef Foam_bitSet_H
 #define Foam_bitSet_H
 
-#include "className.H"
 #include "PackedList.H"
 
 // * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
@@ -111,7 +110,11 @@ protected:
 
 public:
 
-    // Forward declaration of access classes
+    //- A bitSet acts like a packed boolList
+    typedef bool value_type;
+
+
+    // Forward Declarations
 
         class reference;
         class const_iterator;
@@ -331,6 +334,13 @@ public:
         //      set(pos) on individual bits.
         void set(const labelRange& range);
 
+        //- Test for \em True value at specified position
+        //- and change the value at that position.
+        //  Does auto-vivify for non-existent, non-zero entries.
+        //
+        //  \note Method name compatibility with std::bitset
+        bool test_set(const label i, const bool val = true);
+
 
     // Unsetting single or multiple values
 
@@ -475,10 +485,10 @@ public:
         public:
 
             //- Copy construct
-            reference(const reference&) = default;
+            reference(const reference&) noexcept = default;
 
             //- Move construct
-            reference(reference&&) = default;
+            reference(reference&&) noexcept = default;
 
             //- Flip the bit at the position, no range-checking
             inline void flip();
@@ -605,6 +615,61 @@ public:
         }
 
 
+    // Parallel Operations
+
+        //- Broadcast the contents
+        void broadcast
+        (
+            //! The UPstream communicator (default is worldComm)
+            int communicator = -1,
+            //! False: sizes already consistent. True: adjust sizes.
+            bool syncSizes = true
+        );
+
+        //- Inplace \c bit_and parallel reduction
+        void reduceAnd
+        (
+            //! The UPstream communicator (default is worldComm)
+            int communicator = -1,
+            //! False: sizes already consistent. True: adjust (shrink) sizes.
+            bool syncSizes = true
+        );
+
+        //- Inplace \c bit_or parallel reduction
+        void reduceOr
+        (
+            //! The UPstream communicator (default is worldComm)
+            int communicator = -1,
+            //! False: sizes already consistent. True: adjust (extend) sizes.
+            bool syncSizes = true
+        );
+
+        //- Gather individual values into bitSet locations.
+        //  On master, the resulting bitSet has size == nProcs,
+        //  otherwise zero length.
+        //  \n
+        //  For \b non-parallel :
+        //  the length of the returned set is 1 with localValue.
+        static bitSet gatherValues
+        (
+            //! The processor-local value
+            bool localValue,
+            //! The UPstream communicator (default is worldComm)
+            int communicator = -1
+        );
+
+        //- Allgather individual values into bitSet locations.
+        //  The resulting bitSet has size nProcs, identical on all ranks.
+        //  Behaves like Pstream::allGatherValues() but returning a bitSet.
+        static bitSet allGather
+        (
+            //! The processor-local value
+            bool localValue,
+            //! The UPstream communicator (default is worldComm)
+            int communicator = -1
+        );
+
+
     // Housekeeping
 
         //- Same as contains()
diff --git a/src/OpenFOAM/containers/Bits/bitSet/bitSetI.H b/src/OpenFOAM/containers/Bits/bitSet/bitSetI.H
index f035d0c99f..25226ebdcd 100644
--- a/src/OpenFOAM/containers/Bits/bitSet/bitSetI.H
+++ b/src/OpenFOAM/containers/Bits/bitSet/bitSetI.H
@@ -5,7 +5,7 @@
     \\  /    A nd           | www.openfoam.com
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
-    Copyright (C) 2018-2022 OpenCFD Ltd.
+    Copyright (C) 2018-2025 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -482,23 +482,27 @@ inline void Foam::bitSet::fill(const bool val)
 {
     if (empty())
     {
-        return;  // Trivial case
+        return;
     }
-
-    const label nblocks = num_blocks(size());
-
-    // Fill value for complete blocks
-    const unsigned int blockval = (val ? ~0u : 0u);
-
-    for (label blocki=0; blocki < nblocks; ++blocki)
-    {
-        blocks_[blocki] = blockval;
-    }
-
-    if (val)
+    else if (val)
     {
+        std::fill_n(blocks_.data(), num_blocks(), ~0u);
         clear_trailing_bits();
     }
+    else
+    {
+        // or: PackedList<1>::reset();
+        // or: blocks_ = 0u;
+        std::fill_n(blocks_.data(), num_blocks(), 0u);
+    }
+}
+
+
+inline bool Foam::bitSet::test_set(const label i, const bool val)
+{
+    bool old = test(i);
+    PackedList<1>::set(i, val);
+    return old;
 }
 
 
diff --git a/src/OpenFOAM/db/IOstreams/Pstreams/Pstream.H b/src/OpenFOAM/db/IOstreams/Pstreams/Pstream.H
index a2b2781752..06580c58c9 100644
--- a/src/OpenFOAM/db/IOstreams/Pstreams/Pstream.H
+++ b/src/OpenFOAM/db/IOstreams/Pstreams/Pstream.H
@@ -51,6 +51,9 @@ SourceFiles
 namespace Foam
 {
 
+// Forward Declarations
+class bitSet;
+
 /*---------------------------------------------------------------------------*\
                            Class Pstream Declaration
 \*---------------------------------------------------------------------------*/
diff --git a/src/OpenFOAM/db/IOstreams/Pstreams/PstreamBroadcast.txx b/src/OpenFOAM/db/IOstreams/Pstreams/PstreamBroadcast.txx
index 447d268544..f5c6b27c62 100644
--- a/src/OpenFOAM/db/IOstreams/Pstreams/PstreamBroadcast.txx
+++ b/src/OpenFOAM/db/IOstreams/Pstreams/PstreamBroadcast.txx
@@ -149,6 +149,11 @@ void Foam::Pstream::broadcastList
     {
         return;
     }
+    else if constexpr (std::is_same_v<bitSet, ListType>)
+    {
+        // Specialized handling implemented within bitSet itself
+        list.broadcast(communicator);
+    }
     else if constexpr (is_contiguous_v<typename ListType::value_type>)
     {
         // List data are contiguous
@@ -166,6 +171,11 @@ void Foam::Pstream::broadcastList
             communicator
         );
 
+        if (UPstream::is_subrank(communicator))
+        {
+            list.resize_nocopy(len);
+        }
+
         if (len)
         {
             // Only broadcast non-empty content