globalIndex.H

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/*---------------------------------------------------------------------------*\
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  \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
   \\    /   O peration     |
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     \\/     M anipulation  |
-------------------------------------------------------------------------------
    Copyright (C) 2011-2017 OpenFOAM Foundation
    Copyright (C) 2018-2025 OpenCFD Ltd.
-------------------------------------------------------------------------------
License
    This file is part of OpenFOAM.

    OpenFOAM is free software: you can redistribute it and/or modify it
    under the terms of the GNU General Public License as published by
    the Free Software Foundation, either version 3 of the License, or
    (at your option) any later version.

    OpenFOAM is distributed in the hope that it will be useful, but WITHOUT
    ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
    FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
    for more details.

    You should have received a copy of the GNU General Public License
    along with OpenFOAM.  If not, see <http://www.gnu.org/licenses/>.

Class
    Foam::globalIndex

Description
    Calculates a unique integer (label so might not have enough room - 2G max)
    for processor + local index. E.g.

    globalIndex globalFaces(mesh.nFaces());
    label globalFacei = globalFaces.toGlobal(facei);

SourceFiles
    globalIndex.C
    globalIndexI.H
    globalIndex.txx

\*---------------------------------------------------------------------------*/

#ifndef Foam_globalIndex_H
#define Foam_globalIndex_H

#include "Pstream.H"
#include "CompactListList.H"
#include "DynamicList.H"

// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

namespace Foam
{

// Forward Declarations
class globalIndex;

Istream& operator>>(Istream& is, globalIndex& gi);
Ostream& operator<<(Ostream& os, const globalIndex& gi);

/*---------------------------------------------------------------------------*\
                         Class globalIndex Declaration
\*---------------------------------------------------------------------------*/

class globalIndex
{
    // Private Data

        //- Offset (addressing) table - like CompactListList
        labelList offsets_;


    // Private Member Functions

        //- Sort and bin. validBins contains bins with non-zero size.
        static CompactListList<label>
        bin
        (
            const labelUList& offsets,
            const labelUList& globalIds,
            labelList& order,
            DynamicList<label>& validBins
        );

        // Cannot use non-blocking for non-contiguous data.
        // template<class Type>
        // inline static UPstream::commsTypes getCommsType
        // (
        //     UPstream::commsTypes commsType
        // ) noexcept;

        //- Report overflow at specified (non-negative) index
        static void reportOverflowAndExit
        (
            const label idx,
            const label prevOffset = -1,  // The last valid offset value
            const label count = 0         // The count to add to prevOffset
        );

public:

    // Public Data Types

        //- Dispatch tag:
        //- Construct with a single (local size) entry, no communication.
        struct gatherNone{};

        //- Dispatch tag:
        //- Construct 'one-sided' from local sizes,
        //- using gather but no broadcast.
        struct gatherOnly{};

        //- Dispatch tag:
        //- Construct 'one-sided' from the non-master local sizes
        //- using gather but no broadcast.
        struct gatherNonLocal{};


    // Static Member Functions

        //- Return a null globalIndex (reference to a nullObject).
        //- Behaves like an empty globalIndex
        static const globalIndex& null() noexcept
        {
            return NullObjectRef<globalIndex>();
        }


    // Constructors

        //- Default construct (empty)
        globalIndex() noexcept = default;

        //- Copy construct from a list of offsets.
        //- No communication required
        inline explicit globalIndex(const labelUList& listOffsets);

        //- Move construct from a list of offsets.
        //- No communication required
        inline explicit globalIndex(labelList&& listOffsets);

        //- Construct from a list of sizes and calculate the offsets.
        //- No communication required
        inline globalIndex
        (
            const globalIndex::gatherNone,
            const labelUList& localSizes
        );

        //- Construct from local size, using gather/broadcast
        //- with default/specified communicator if parallel.
        inline explicit globalIndex
        (
            const label localSize,
            const label comm = UPstream::worldComm,   
            const bool parallel = UPstream::parRun()  
        );

        //- Construct with a single (local size) entry, no communication
        inline globalIndex
        (
            const globalIndex::gatherNone,
            const label localSize,
            const label comm = -1   
        );

        //- Construct 'one-sided' from local sizes.
        //- Uses UPstream::listGatherValues, but no broadcast.
        //- Will be empty on non-master processes.
        //
        //  \note can be used when Pstream::parRun() is false.
        inline globalIndex
        (
            const globalIndex::gatherOnly,
            const label localSize,
            const label comm = UPstream::worldComm  
        );

        //- Construct 'one-sided' from the non-master local sizes
        //- (ie, master size is treated as 0).
        //- Uses UPstream::listGatherValues, but no broadcast.
        //- Will be empty on non-master processes.
        //
        //  \note can be used when Pstream::parRun() is false.
        inline globalIndex
        (
            const globalIndex::gatherNonLocal,
            const label localSize,
            const label comm = UPstream::worldComm  
        );

        //- Construct from Istream.
        //- No communication required
        explicit globalIndex(Istream& is);


    // Member Functions

        //- Check for default constructed or total-size == 0
        inline bool empty() const noexcept;

        //- The number of items covered by the offsets
        inline label length() const noexcept;

        //- Global sum of localSizes. Same as totalSize()
        FOAM_DEPRECATED_STRICT(2022-10, "totalSize() - unambiguous")
        inline label size() const;

        //- The span size covered by the offsets, zero if empty
        inline label span() const noexcept;

        //- The total addressed size, which corresponds to the
        //- end offset and also the sum of all localSizes.
        inline label totalSize() const noexcept;

        //- The local sizes. Same as localSizes()
        inline labelList sizes() const;

        //- The local starts
        inline const labelUList localStarts() const;

        //- The local sizes
        labelList localSizes() const;

        //- Return start/size ranges for all data
        List<labelRange> ranges() const;

        //- Global max of localSizes
        inline label maxSize() const;


    // Access

        //- Const-access to the offsets
        inline const labelList& offsets() const noexcept;

        //- Write-access to the offsets, for changing after construction
        inline labelList& offsets() noexcept;


    // Dimensions

        //- True if local-only content (ie, nProcs == 1).
        //- Such content is often created with gatherNone.
        inline bool single() const noexcept;

        //- The number of processors covered by the offsets,
        //- same as the primary length()
        inline label nProcs() const noexcept;

        //- Range of process indices for all addressed offsets (processes)
        inline labelRange allProcs() const noexcept;

        //- Range of process indices for addressed sub-offsets (processes)
        inline labelRange subProcs() const noexcept;

        //- The value corresponding to the first offset
        inline label begin_value() const noexcept;

        //- The value corresponding to the last offset (end offset),
        //- which is 1 beyond the end of the range.
        inline label end_value() const noexcept;

        //- The first offset range. It is (0,0) if globalIndex is empty
        labelRange front() const;

        //- The last offset range. It is (0,0) if globalIndex is empty
        labelRange back() const;


    // Edit

        //- Reset to be empty (no offsets)
        inline void clear();

        //- Change the number of entries (nProcs) in the offsets table.
        //- Extending will fill with empty local sizes.
        void resize(const label n);

        //- Reset from local size, using gather/broadcast
        //- with default/specified communicator if parallel.
        void reset
        (
            const label localSize,
            const label comm = UPstream::worldComm,   
            const bool parallel = UPstream::parRun()  
        );

        //- Reset offsets from a list of local sizes,
        //- with optional check for label overflow.
        //- No communication required
        void reset
        (
            const labelUList& counts,
            const bool checkOverflow = false
        );

        //- Reset to a single (local size) entry, no communication
        inline void reset
        (
            const globalIndex::gatherNone,
            const label localSize,
            const label comm = -1   
        );

        //- Reset as 'one-sided' from local sizes.
        //- Uses UPstream::listGatherValues, but no broadcast.
        //- Will be empty on non-master processes.
        //
        //  \note can be used when Pstream::parRun() is false.
        inline void reset
        (
            const globalIndex::gatherOnly,
            const label localSize,
            const label comm = UPstream::worldComm  
        );

        //- Reset as 'one-sided' from the non-master local sizes
        //- (ie, master size is treated as 0).
        //- Uses UPstream::listGatherValues, but no broadcast.
        //- Will be empty on non-master processes.
        //
        //  \note can be used when Pstream::parRun() is false.
        inline void reset
        (
            const globalIndex::gatherNonLocal,
            const label localSize,
            const label comm = UPstream::worldComm  
        );

        //- Reset the globalIndex. Same as copy assignment.
        inline void reset(const globalIndex& gi);

        //- Alter local size for given processor
        void setLocalSize(const label proci, const label len);


    // Queries and renumbering

        //- True if contained within the offsets range
        inline bool contains(const label i) const noexcept;

        //- Start of proci data
        inline label localStart(const label proci) const;

        //- End of proci data
        inline label localEnd(const label proci) const;

        //- Size of proci data
        inline label localSize(const label proci) const;

        //- The max of localSizes, excluding the specified processor
        label maxNonLocalSize(const label proci) const;

        //- Return start/size range of proci data
        inline labelRange range(const label proci) const;

        //- Is on processor proci
        inline bool isLocal(const label proci, const label i) const;

        //- From local to global on proci
        inline label toGlobal(const label proci, const label i) const;

        //- From local to global on proci
        inline labelList toGlobal
        (
            const label proci,
            const labelUList& labels
        ) const;

        //- From local to global index on proci (inplace)
        inline void inplaceToGlobal
        (
            const label proci,
            labelUList& labels
        ) const;


        //- From global to local on proci
        inline label toLocal(const label proci, const label i) const;

        //- Find processor with specified global id.
        //- Check proci first, followed by binary search.
        //  \return the processor number or -1 (not found)
        inline label findProc(const label proci, const label i) const;

        //- Find processor above proci with specified global id - binary search.
        //  \return the processor above proci or -1 otherwise
        //  (including for out-of-range indices)
        inline label findProcAbove(const label proci, const label i) const;

        //- Find processor below proci with specified global id - binary search.
        //  Binary search.
        //  \return the processor below proci or -1 otherwise
        //  (including for out-of-range indices)
        inline label findProcBelow(const label proci, const label i) const;

        //- Which processor does global id come from?
        //- Checks proci first (assumed to occur reasonably frequently)
        //- followed by a binary search.
        //- Fatal for out-of-range indices
        inline label whichProcID(const label proci, const label i) const;


    // Queries relating to myProcNo (Caution: uses UPstream::worldComm)

        //- Local start on myProcNo()
        inline label localStart() const;

        //- Local end on myProcNo()
        inline label localEnd() const;

        //- Local size on myProcNo()
        inline label localSize() const;

        //- The max of localSizes, excluding current (myProcNo) rank
        inline label maxNonLocalSize() const;

        //- Return start/size range of local (myProcNo) data
        inline labelRange range() const;

        //- Is on local processor
        inline bool isLocal(const label i) const;

        //- From local to global index
        inline label toGlobal(const label i) const;

        //- From local to global index
        inline labelList toGlobal(const labelUList& labels) const;

        //- From local to global index (inplace)
        inline void inplaceToGlobal(labelUList& labels) const;

        //- From global to local on current processor.
        //  FatalError if not on local processor.
        inline label toLocal(const label i) const;

        //- Which processor does global id come from?
        //  Uses myProcNo for the initial local check.
        inline label whichProcID(const label i) const;


    // Iteration

        //- Forward input iterator with const access that is used to
        //- iterate across the globalIndex offsets() table.
        //  The de-referenced value is the range() with (start, size),
        //  but it also provides separate index, start, size information.
        class const_iterator
        {
            //- The parent class for which this is an iterator
            const globalIndex* parent_;

            //- The index into the parent
            label index_;

        public:

        // Constructors

            //- Construct from globalIndex list at given index
            explicit const_iterator
            (
                const globalIndex* globalIdx,
                const label i = 0
            ) noexcept;


        // Member Operators

            //- The index into the arrays
            inline label index() const noexcept;

            //- The local start
            inline label start() const;

            //- The local size
            inline label size() const;

            //- The local range
            inline labelRange range() const;

            //- The local range
            inline labelRange operator*() const;

            inline const_iterator& operator++();
            inline const_iterator operator++(int);

            inline const_iterator& operator--();
            inline const_iterator operator--(int);

            inline bool operator==(const const_iterator& iter) const noexcept;
            inline bool operator!=(const const_iterator& iter) const noexcept;
        };


        //- A const_iterator set to the beginning
        inline const_iterator cbegin() const noexcept;

        //- A const_iterator set to beyond the end
        inline const const_iterator cend() const noexcept;

        //- A const_iterator set to the beginning
        inline const_iterator begin() const noexcept;

        //- A const_iterator set to beyond the end
        inline const const_iterator end() const noexcept;

        //- Return const_iterator at offset proci from begin,
        //- clamped to [0,nProcs] range
        inline const_iterator cbegin(const label proci) const noexcept;

        //- Return const_iterator at offset proci from begin,
        //- clamped to [0,nProcs] range
        inline const_iterator begin(const label proci) const noexcept;

        // FUTURE?
        // //- Return start/size range of proci data
        // labelRange operator[](const label proci) const
        // {
        //     return this->range(proci);
        // }


    // Helper Functions

        //- Calculate globally-consistent local range (offset/size)
        //- based on the local input size(s).
        //  Effectively the same as globalIndex(localSize).range()
        //  but without constructing an extra intermediate list of offsets
        static labelRange calcRange
        (
            const label localSize,
            const label comm = UPstream::worldComm,  
            const bool checkOverflow = false
        );

        //- Calculate globally-consistent local start offset
        //- based on the local input size(s).
        //  Effectively the same as globalIndex(localSize).localStart()
        //  but without constructing an extra intermediate list of offsets
        static label calcOffset
        (
            const label localSize,
            const label comm = UPstream::worldComm,  
            const bool checkOverflow = false
        );

        //- Calculate offsets from a list of local sizes,
        //- with optional check for label overflow
        static labelList calcOffsets
        (
            const labelUList& counts,
            const bool checkOverflow = false
        );

        //- Calculate offsets from an indirect list of local sizes,
        //- with optional check for label overflow
        template<class Addr>
        static labelList calcOffsets
        (
            const IndirectListBase<label, Addr>& counts,
            const bool checkOverflow = false
        );

        //- Calculate offsets from list of lists,
        //- with optional check for label overflow
        template<class SubListType>
        static labelList calcListOffsets
        (
            const List<SubListType>& lists,
            const bool checkOverflow = false
        );

        //- Calculate ranges (offset/size) from a list of local sizes,
        //- with optional check for label overflow
        static List<labelRange> calcRanges
        (
            const labelUList& counts,
            const bool checkOverflow = false
        );

        //- Split the top-level offsets into inter-node and local-node
        //- components suitable to a two-stage hierarchy.
        bool splitNodeOffsets
        (
            labelList& interNodeOffsets,
            labelList& localNodeOffsets,
            const label communicator = UPstream::worldComm,
            const bool absoluteLocalNodeOffsets = false
        ) const;


    // Misc low-level gather routines

        //- Collect single values in processor order on master (== procIDs[0]).
        //  Handles contiguous/non-contiguous data.
        //  non-zero output field (master only)
        template<class ProcIDsContainer, class Type>
        [[nodiscard]]
        static List<Type> listGatherValues
        (
            const label comm,           
            const ProcIDsContainer& procIDs,
            const Type& localValue,
            const int tag = UPstream::msgType(),
            UPstream::commsTypes commsType = UPstream::commsTypes::nonBlocking
        );

        //- Collect data in processor order on master (== procIDs[0]).
        //  Handles contiguous/non-contiguous data, skips empty fields.
        template<class ProcIDsContainer, class Type>
        static void gather
        (
            const labelUList& offsets,  
            const label comm,           
            const ProcIDsContainer& procIDs,
            const UList<Type>& fld,     
            UList<Type>& allFld,
            const int tag = UPstream::msgType(),
            UPstream::commsTypes commsType = UPstream::commsTypes::nonBlocking
        );

        //- Collect indirect data in processor order on master
        //  Handles contiguous/non-contiguous data, skips empty fields.
        template<class ProcIDsContainer, class Type, class Addr>
        static void gather
        (
            const labelUList& offsets,  
            const label comm,           
            const ProcIDsContainer& procIDs,
            const IndirectListBase<Type, Addr>& fld,  
            UList<Type>& allFld,
            const int tag = UPstream::msgType(),
            UPstream::commsTypes commsType = UPstream::commsTypes::nonBlocking
        );

        //- Inplace collect in processor order on master (== procIDs[0]).
        template<class ProcIDsContainer, class Type>
        static void gatherInplace
        (
            const labelUList& offsets,  
            const label comm,           
            const ProcIDsContainer& procIDs,
            List<Type>& fld,            
            const int tag = UPstream::msgType(),
            UPstream::commsTypes commsType = UPstream::commsTypes::nonBlocking
        );

        //- Collect data in processor order on master (== procIDs[0]).
        //  \note the globalIndex offsets needed on master only.
        template<class ProcIDsContainer, class Type>
        void gather
        (
            const label comm,           
            const ProcIDsContainer& procIDs,
            const UList<Type>& fld,     
            List<Type>& allFld,
            const int tag = UPstream::msgType(),
            UPstream::commsTypes commsType = UPstream::commsTypes::nonBlocking
        ) const;

        //- Inplace collect in processor order on master (== procIDs[0]).
        //  \note the globalIndex offsets needed on master only.
        template<class ProcIDsContainer, class Type>
        void gatherInplace
        (
            const label comm,           
            const ProcIDsContainer& procIDs,
            List<Type>& fld,            
            const int tag = UPstream::msgType(),
            UPstream::commsTypes commsType = UPstream::commsTypes::nonBlocking
        ) const
        {
            gatherInplace(offsets_, comm, procIDs, fld, tag, commsType);
        }


    // Gather

        //- Collect data in processor order on master
        //- (in serial: performs a simple copy).
        //  Communication with default/specified communicator, message tag.
        template<class Type>
        void gather
        (
            const UList<Type>& sendData,
            List<Type>& allData,
            const int tag = UPstream::msgType(),
            UPstream::commsTypes commsType = UPstream::commsTypes::nonBlocking,
            const label comm = UPstream::worldComm  
        ) const;

        //- Collect data indirectly in processor order on master
        //- (in serial: performs a simple copy).
        //  Communication with default/specified communicator, message tag.
        template<class Type, class Addr>
        void gather
        (
            const IndirectListBase<Type, Addr>& sendData,
            List<Type>& allData,
            const int tag = UPstream::msgType(),
            UPstream::commsTypes commsType = UPstream::commsTypes::nonBlocking,
            const label comm = UPstream::worldComm  
        ) const;

        //- Collect data in processor order on master
        //- (in serial: performs a simple copy).
        //  Communication with default/specified communicator, message tag.
        //
        //  \return output (master), zero-sized on non-master
        template<class Type, class OutputContainer = List<Type>>
        [[nodiscard]]
        OutputContainer gather
        (
            const UList<Type>& sendData,  
            const int tag = UPstream::msgType(),
            UPstream::commsTypes commsType = UPstream::commsTypes::nonBlocking,
            const label comm = UPstream::worldComm  
        ) const;

        //- Collect data indirectly in processor order on master.
        //  Communication with default/specified communicator, message tag.
        //
        //  \return output (master), zero-sized on non-master
        template<class Type, class Addr, class OutputContainer = List<Type>>
        [[nodiscard]]
        OutputContainer gather
        (
            const IndirectListBase<Type, Addr>& sendData, 
            const int tag = UPstream::msgType(),
            UPstream::commsTypes commsType = UPstream::commsTypes::nonBlocking,
            const label comm = UPstream::worldComm  
        ) const;

        //- Inplace collect data in processor order on master
        //- (in serial: a no-op).
        //  Communication with default/specified communicator, message tag.
        //  After the gather, the field is zero-sized on non-master.
        template<class Type>
        void gatherInplace
        (
            List<Type>& fld,  
            const int tag = UPstream::msgType(),
            UPstream::commsTypes commsType = UPstream::commsTypes::nonBlocking,
            const label comm = UPstream::worldComm  
        ) const;

        //- Use MPI_Gatherv call for contiguous data when possible
        //- (in serial: performs a simple copy).
        //  Communication with default/specified communicator.
        //  \attention The nProcs for globalIndex and communicator
        //      must match!!
        template<class Type, class OutputContainer = List<Type>>
        void mpiGather
        (
            const UList<Type>& sendData,
            OutputContainer& allData,
            const label comm = UPstream::worldComm,  

            // For fallback routines:
            UPstream::commsTypes commsType = UPstream::commsTypes::nonBlocking,
            const int tag = UPstream::msgType()
        ) const;

        //- Use MPI_Gatherv call for contiguous data when possible
        //- (in serial: performs a simple copy).
        //  Communication with default/specified communicator.
        //  \attention The nProcs for globalIndex and communicator
        //      must match!!
        //
        //  \return output (master), zero-sized on non-master
        template<class Type, class OutputContainer = List<Type>>
        [[nodiscard]]
        OutputContainer mpiGather
        (
            const UList<Type>& sendData,
            const label comm = UPstream::worldComm,  

            // For fallback routines:
            UPstream::commsTypes commsType = UPstream::commsTypes::nonBlocking,
            const int tag = UPstream::msgType()
        ) const;

        //- Use MPI_Gatherv call to inplace collect contiguous data
        //- when possible.
        //- (in serial: a no-op).
        //  Communication with default/specified communicator.
        //  \attention The nProcs for globalIndex and communicator
        //      must match!!
        //
        //  After the gather, the field is zero-sized on non-master.
        template<class Type>
        void mpiGatherInplace
        (
            List<Type>& fld,
            const label comm = UPstream::worldComm,  

            // For fallback routines:
            UPstream::commsTypes commsType = UPstream::commsTypes::nonBlocking,
            const int tag = UPstream::msgType()
        ) const;


    // Gather Operations

        //- Use MPI_Gatherv call to collect contiguous data when possible
        //- (in serial: performs a simple copy).
        //  Communication with default/specified communicator.
        //
        //  The allData is output (master), zero-sized on non-master
        template<class Type, class OutputContainer = List<Type>>
        static void mpiGatherOp
        (
            const UList<Type>& sendData,
            OutputContainer& allData,
            const label comm = UPstream::worldComm,  

            // For fallback routines:
            UPstream::commsTypes commsType = UPstream::commsTypes::nonBlocking,
            const int tag = UPstream::msgType()
        );

        //- Use MPI_Gatherv call to collect contiguous data when possible
        //- (in serial: performs a simple copy).
        //  Communication with default/specified communicator.
        //
        //  \return output (master), zero-sized on non-master
        template<class Type, class OutputContainer = List<Type>>
        [[nodiscard]]
        static OutputContainer mpiGatherOp
        (
            const UList<Type>& sendData,
            const label comm = UPstream::worldComm,  

            // For fallback routines:
            UPstream::commsTypes commsType = UPstream::commsTypes::nonBlocking,
            const int tag = UPstream::msgType()
        );

        //- Use MPI_Gatherv call to inplace collect contiguous data
        //- when possible.
        //- (in serial: a no-op).
        //  Communication with default/specified communicator.
        //
        //  After the gather, the field is zero-sized on non-master.
        template<class Type>
        static void mpiGatherInplaceOp
        (
            List<Type>& fld,
            const label comm = UPstream::worldComm,  

            // For fallback routines:
            UPstream::commsTypes commsType = UPstream::commsTypes::nonBlocking,
            const int tag = UPstream::msgType()
        );

        //- Collect data in processor order on master
        //- (in serial: performs a simple copy).
        //  Communication with default/specified communicator, message tag.
        template<class Type>
        static void gatherOp
        (
            const UList<Type>& sendData,
            List<Type>& allData,
            const int tag = UPstream::msgType(),
            UPstream::commsTypes commsType = UPstream::commsTypes::nonBlocking,
            const label comm = UPstream::worldComm  
        );

        //- Collect data in processor order on master
        //- (in serial: performs a simple copy).
        //  Communication with default/specified communicator, message tag.
        template<class Type, class Addr>
        static void gatherOp
        (
            const IndirectListBase<Type, Addr>& sendData,
            List<Type>& allData,
            const int tag = UPstream::msgType(),
            UPstream::commsTypes commsType = UPstream::commsTypes::nonBlocking,
            const label comm = UPstream::worldComm  
        );

        //- Collect and return data in processor order on master
        //- (in serial: performs a simple copy).
        //  Communication with default/specified communicator, message tag.
        //
        //  \return output (master), zero-sized on non-master
        template<class Type, class OutputContainer = List<Type>>
        [[nodiscard]]
        static OutputContainer gatherOp
        (
            const UList<Type>& sendData,
            const int tag = UPstream::msgType(),
            UPstream::commsTypes commsType = UPstream::commsTypes::nonBlocking,
            const label comm = UPstream::worldComm  
        );

        //- Collect and return data in processor order on master
        //- (in serial: performs a simple copy).
        //  Communication with default/specified communicator, message tag.
        //
        //  \return output (master), zero-sized on non-master
        template<class Type, class Addr, class OutputContainer = List<Type>>
        [[nodiscard]]
        static OutputContainer gatherOp
        (
            const IndirectListBase<Type, Addr>& sendData,
            const int tag = UPstream::msgType(),
            UPstream::commsTypes commsType = UPstream::commsTypes::nonBlocking,
            const label comm = UPstream::worldComm  
        );

        //- Inplace collect data in processor order on master
        //- (in serial: a no-op).
        //  Communication with default/specified communicator, message tag.
        //
        //  After the gather, the field is zero-sized on non-master.
        template<class Type>
        static void gatherInplaceOp
        (
            List<Type>& fld,
            const int tag = UPstream::msgType(),
            UPstream::commsTypes commsType = UPstream::commsTypes::nonBlocking,
            const label comm = UPstream::worldComm  
        );


    // Scatter

        //- Distribute data in processor order.
        //  Requires fld to be correctly sized!
        //  Communication with default/specified communicator, message tag.
        template<class ProcIDsContainer, class Type>
        static void scatter
        (
            const labelUList& offsets,  
            const label comm,           
            const ProcIDsContainer& procIDs,
            const UList<Type>& allFld,
            UList<Type>& fld,
            const int tag = UPstream::msgType(),
            UPstream::commsTypes commsType = UPstream::commsTypes::nonBlocking
        );

        //- Distribute data in processor order.
        //  Requires fld to be correctly sized!
        //  Communication with default/specified communicator, message tag.
        //  \note the globalIndex offsets needed on master only.
        template<class ProcIDsContainer, class Type>
        void scatter
        (
            const label comm,           
            const ProcIDsContainer& procIDs,
            const UList<Type>& allFld,
            UList<Type>& fld,
            const int tag = UPstream::msgType(),
            UPstream::commsTypes commsType = UPstream::commsTypes::nonBlocking
        ) const
        {
            scatter(offsets_, comm, procIDs, allFld, fld, tag, commsType);
        }

        //- Distribute data in processor order.
        //  Requires fld to be correctly sized!
        //  Communication with default/specified communicator, message tag.
        //  \note the globalIndex offsets needed on master only.
        template<class Type>
        void scatter
        (
            const UList<Type>& allData,
            UList<Type>& localData,
            const int tag = UPstream::msgType(),
            UPstream::commsTypes commsType = UPstream::commsTypes::nonBlocking,
            const label comm = UPstream::worldComm  
        ) const;

        //- Distribute data in processor order
        //- (in serial: performs a simple copy).
        //  Communication with default/specified communicator, message tag.
        //  \note the globalIndex offsets needed on master only.
        template<class Type, class OutputContainer = List<Type>>
        [[nodiscard]]
        OutputContainer scatter
        (
            const UList<Type>& allData,
            const int tag = UPstream::msgType(),
            UPstream::commsTypes commsType = UPstream::commsTypes::nonBlocking,
            const label comm = UPstream::worldComm  
        ) const;


    // Scatter

        //- Get (potentially remote) data.
        //- Elements required given as global indices
        //  Communication with default/specified communicator, message tag.
        template<class Type, class CombineOp>
        void get
        (
            List<Type>& allFld,
            const labelUList& globalIds,
            const CombineOp& cop,
            const label comm = UPstream::worldComm,  
            const int tag = UPstream::msgType()
        ) const;


    // Member Operators

        //- Compare for equality - uses the offsets
        bool operator==(const globalIndex& rhs) const
        {
            return (this->offsets() == rhs.offsets());
        }

        //- Compare for inequality - uses the offsets
        bool operator!=(const globalIndex& rhs) const
        {
            return !(*this == rhs);
        }

        //- Compare for less-than - uses the offsets
        bool operator<(const globalIndex& rhs) const
        {
            return (this->offsets() < rhs.offsets());
        }


    // IOstream Operators

        friend Istream& operator>>(Istream& is, globalIndex& gi);
        friend Ostream& operator<<(Ostream& os, const globalIndex& gi);


    // Housekeeping

        //- Construct from local size, using gather/broadcast
        //- with default/specified communicator if parallel.
        FOAM_DEPRECATED_FOR(2022-03, "construct without message tag")
        globalIndex
        (
            const label localSize,
            const int tag,          // message tag (unused)
            const label comm,       // communicator
            const bool parallel     // use parallel comms
        )
        {
            reset(localSize, comm, parallel);
        }

        //- Reset from local size, using gather/broadcast
        //- with default/specified communicator if parallel.
        FOAM_DEPRECATED_FOR(2022-03, "reset without message tag")
        void reset
        (
            const label localSize,
            const int tag,          // message tag (unused)
            const label comm,       // communicator
            const bool parallel     // use parallel comms
        )
        {
            reset(localSize, comm, parallel);
        }

        //- Prefer localStart() to avoid confusing with offsets()
        FOAM_DEPRECATED_FOR(2022-02, "use localStart()")
        label offset(const label proci) const { return localStart(proci); }
};


// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

} // End namespace Foam

// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

#include "globalIndexI.H"

// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

#ifdef NoRepository
    #include "globalIndex.txx"
#endif

// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

#endif

// ************************************************************************* //