RDFSnapshotHelpers.cxx

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/**
 \file RDFSnapshotHelpers.cxx
 \ingroup dataframe
 \author Enrico Guiraud, CERN
 \author Danilo Piparo, CERN
 \date 2016-12
 \author Vincenzo Eduardo Padulano
 \author Stephan Hageboeck
 \date 2025-06
*/
 
/*************************************************************************
 * Copyright (C) 1995-2025, Rene Brun and Fons Rademakers.               *
 * All rights reserved.                                                  *
 *                                                                       *
 * For the licensing terms see $ROOTSYS/LICENSE.                         *
 * For the list of contributors see $ROOTSYS/README/CREDITS.             *
 *************************************************************************/
 
#include <ROOT/RDF/SnapshotHelpers.hxx>
 
#include <ROOT/REntry.hxx>
#include <ROOT/RFieldToken.hxx>
#include <ROOT/RNTuple.hxx>
#include <ROOT/RNTupleDS.hxx>
#include <ROOT/RNTupleFillContext.hxx>
#include <ROOT/RNTupleParallelWriter.hxx>
#include <ROOT/RTTreeDS.hxx>
#include <ROOT/TBufferMerger.hxx>
 
#include <TBranchObject.h>
#include <TClassEdit.h>
#include <TDictionary.h>
#include <TDataType.h>
#include <TFile.h>
#include <TLeaf.h>
#include <TTreeReader.h>
 
#include <algorithm>
#include <type_traits>
#include <utility>
 
using ROOT::Internal::RDF::RBranchData;
// Maintaining the following allows for faster vector resize:
static_assert(std::is_nothrow_move_assignable_v<RBranchData>);
static_assert(std::is_nothrow_move_constructible_v<RBranchData>);
 
namespace {
 
void AssertNoNullBranchAddresses(const std::vector<RBranchData> &branches)
{
   std::vector<TBranch *> branchesWithNullAddress;
   for (const auto &branchData : branches) {
      if (branchData.fOutputBranch->GetAddress() == nullptr)
         branchesWithNullAddress.push_back(branchData.fOutputBranch);
   }
 
   if (branchesWithNullAddress.empty())
      return;
 
   // otherwise build error message and throw
   std::vector<std::string> missingBranchNames;
   std::transform(branchesWithNullAddress.begin(), branchesWithNullAddress.end(),
                  std::back_inserter(missingBranchNames), [](TBranch *b) { return b->GetName(); });
   std::string msg = "RDataFrame::Snapshot:";
   if (missingBranchNames.size() == 1) {
      msg += " branch " + missingBranchNames[0] +
             " is needed as it provides the size for one or more branches containing dynamically sized arrays, but "
             "it is";
   } else {
      msg += " branches ";
      for (const auto &bName : missingBranchNames)
         msg += bName + ", ";
      msg.resize(msg.size() - 2); // remove last ", "
      msg += " are needed as they provide the size of other branches containing dynamically sized arrays, but they are";
   }
   msg += " not part of the set of branches that are being written out.";
   throw std::runtime_error(msg);
}
 
TBranch *SearchForBranch(TTree *inputTree, const std::string &branchName)
{
   if (inputTree) {
      if (auto *getBranchRes = inputTree->GetBranch(branchName.c_str()))
         return getBranchRes;
 
      // try harder
      if (auto *findBranchRes = inputTree->FindBranch(branchName.c_str()))
         return findBranchRes;
   }
   return nullptr;
}
 
std::vector<RBranchData>::iterator CreateCStyleArrayBranch(TTree &outputTree, std::vector<RBranchData> &outputBranches,
                                                           std::vector<RBranchData>::iterator thisBranch,
                                                           TBranch *inputBranch, int basketSize, void *address)
{
   if (!inputBranch)
      return thisBranch;
   const auto STLKind = TClassEdit::IsSTLCont(inputBranch->GetClassName());
   if (STLKind == ROOT::ESTLType::kSTLvector || STLKind == ROOT::ESTLType::kROOTRVec)
      return thisBranch;
   // must construct the leaflist for the output branch and create the branch in the output tree
   const auto *leaf = static_cast<TLeaf *>(inputBranch->GetListOfLeaves()->UncheckedAt(0));
   if (!leaf)
      return thisBranch;
   const auto bname = leaf->GetName();
   auto *sizeLeaf = leaf->GetLeafCount();
   const auto sizeLeafName = sizeLeaf ? std::string(sizeLeaf->GetName()) : std::to_string(leaf->GetLenStatic());
 
   // We proceed only if branch is a fixed-or-variable-sized array
   if (sizeLeaf || leaf->GetLenStatic() > 1) {
      if (sizeLeaf) {
         // The array branch `bname` has dynamic size stored in leaf `sizeLeafName`, so we need to ensure that it's
         // in the output tree.
         auto sizeLeafIt =
            std::find_if(outputBranches.begin(), outputBranches.end(),
                         [&sizeLeafName](const RBranchData &bd) { return bd.fOutputBranchName == sizeLeafName; });
         if (sizeLeafIt == outputBranches.end()) {
            // The size leaf is not part of the output branches yet, so emplace an empty slot for it.
            // This means that iterators need to be updated in case the container reallocates.
            const auto indexBeforeEmplace = std::distance(outputBranches.begin(), thisBranch);
            outputBranches.emplace_back("", sizeLeafName, /*isDefine=*/false, /*typeID=*/nullptr);
            thisBranch = outputBranches.begin() + indexBeforeEmplace;
            sizeLeafIt = outputBranches.end() - 1;
         }
         if (!sizeLeafIt->fOutputBranch) {
            // The size leaf was emplaced, but not initialised yet
            const auto sizeTypeStr = ROOT::Internal::RDF::TypeName2ROOTTypeName(sizeLeaf->GetTypeName());
            // Use original basket size for existing branches otherwise use custom basket size.
            const auto bufSize = (basketSize > 0) ? basketSize : sizeLeaf->GetBranch()->GetBasketSize();
            // The null branch address is a placeholder. It will be set when SetBranchesHelper is called for
            // `sizeLeafName`
            auto *outputBranch = outputTree.Branch(sizeLeafName.c_str(), static_cast<void *>(nullptr),
                                                   (sizeLeafName + '/' + sizeTypeStr).c_str(), bufSize);
            sizeLeafIt->fOutputBranch = outputBranch;
         }
      }
 
      const auto btype = leaf->GetTypeName();
      const auto rootbtype = ROOT::Internal::RDF::TypeName2ROOTTypeName(btype);
      if (rootbtype == ' ') {
         Warning("Snapshot",
                 "RDataFrame::Snapshot: could not correctly construct a leaflist for C-style array in column %s. The "
                 "leaf is of type '%s'. This column will not be written out.",
                 bname, btype);
         return thisBranch;
      }
 
      const auto leaflist = std::string(bname) + "[" + sizeLeafName + "]/" + rootbtype;
      // Use original basket size for existing branches and new basket size for new branches
      const auto bufSize = (basketSize > 0) ? basketSize : inputBranch->GetBasketSize();
      void *addressForBranch = [address]() -> void * {
         if (address) {
            // Address here points to a ROOT::RVec<std::byte> coming from RTreeUntypedArrayColumnReader. We know we need
            // its buffer, so we cast it and extract the address of the buffer
            auto *rawRVec = reinterpret_cast<ROOT::RVec<std::byte> *>(address);
            return rawRVec->data();
         }
         return nullptr;
      }();
      thisBranch->fOutputBranch =
         outputTree.Branch(thisBranch->fOutputBranchName.c_str(), addressForBranch, leaflist.c_str(), bufSize);
      thisBranch->fOutputBranch->SetTitle(inputBranch->GetTitle());
      thisBranch->fIsCArray = true;
   }
 
   return thisBranch;
}
 
void SetBranchAddress(TBranch *inputBranch, RBranchData &branchData, void *valueAddress)
{
   const static TClassRef TBOClRef("TBranchObject");
   if (inputBranch && inputBranch->IsA() == TBOClRef) {
      branchData.fOutputBranch->SetAddress(reinterpret_cast<void **>(inputBranch->GetAddress()));
   } else if (branchData.fOutputBranch->IsA() != TBranch::Class()) {
      // This is a relatively rare case of a fixed-size array getting redefined
      branchData.fBranchAddressForCArrays = valueAddress;
      branchData.fOutputBranch->SetAddress(&branchData.fBranchAddressForCArrays);
   } else {
      void *correctAddress = [valueAddress, isCArray = branchData.fIsCArray]() -> void * {
         if (isCArray) {
            // Address here points to a ROOT::RVec<std::byte> coming from RTreeUntypedArrayColumnReader. We know we
            // need its buffer, so we cast it and extract the address of the buffer
            auto *rawRVec = reinterpret_cast<ROOT::RVec<std::byte> *>(valueAddress);
            return rawRVec->data();
         }
         return valueAddress;
      }();
      branchData.fOutputBranch->SetAddress(correctAddress);
      branchData.fBranchAddressForCArrays = valueAddress;
   }
}
 
void CreateFundamentalTypeBranch(TTree &outputTree, RBranchData &bd, void *valueAddress, int bufSize)
{
   // Logic taken from
   // TTree::BranchImpRef(
   // const char* branchname, TClass* ptrClass, EDataType datatype, void* addobj, Int_t bufsize, Int_t splitlevel)
   auto rootTypeChar = ROOT::Internal::RDF::TypeID2ROOTTypeName(*bd.fInputTypeID);
   if (rootTypeChar == ' ') {
      Warning("Snapshot",
              "RDataFrame::Snapshot: could not correctly construct a leaflist for fundamental type in column %s. This "
              "column will not be written out.",
              bd.fOutputBranchName.c_str());
      return;
   }
   std::string leafList{bd.fOutputBranchName + '/' + rootTypeChar};
   bd.fOutputBranch = outputTree.Branch(bd.fOutputBranchName.c_str(), valueAddress, leafList.c_str(), bufSize);
}
 
/// Ensure that an object with the input name can be written to the target file. This means checking that the
/// TFile can be opened in the mode specified in `opts`, deleting any pre-existing objects with the same name in case
/// `opts.fOverwriteIfExists = true`, or throwing an error otherwise.
void EnsureValidSnapshotOutput(const ROOT::RDF::RSnapshotOptions &opts, const std::string &objName,
                               const std::string &fileName)
{
   TString fileMode = opts.fMode;
   fileMode.ToLower();
   if (fileMode != "update")
      return;
 
   // output file opened in "update" mode: must check whether target object name is already present in file
   std::unique_ptr<TFile> outFile{TFile::Open(fileName.c_str(), "UPDATE_WITHOUT_GLOBALREGISTRATION")};
   if (!outFile || outFile->IsZombie())
      throw std::invalid_argument("Snapshot: cannot open file \"" + fileName + "\" in update mode");
 
   // Object is not present in the file, we are good
   if (!outFile->GetKey(objName.c_str()))
      return;
 
   // object called objName is already present in the file
   if (opts.fOverwriteIfExists) {
      if (auto existingTree = outFile->Get<TTree>(objName.c_str()); existingTree) {
         // Special case for TTree. TTree::Delete invalidates the 'this' pointer, so we don't wrap it in a
         // std::unique_ptr.
         existingTree->Delete("all");
      } else {
         // Ensure deletion of object and all its cycles.
         outFile->Delete((objName + ";*").c_str());
      }
   } else {
      const std::string msg = "Snapshot: object \"" + objName + "\" already present in file \"" + fileName +
                              "\". If you want to delete the original object and write another, please set the "
                              "'fOverwriteIfExists' option to true in RSnapshotOptions.";
      throw std::invalid_argument(msg);
   }
}
 
void SetBranchesHelper(TTree *inputTree, TTree &outputTree,
                       std::vector<ROOT::Internal::RDF::RBranchData> &allBranchData, std::size_t currentIndex,
                       int basketSize, void *valueAddress)
{
   auto branchData = allBranchData.begin() + currentIndex;
   auto *inputBranch = branchData->fIsDefine ? nullptr : SearchForBranch(inputTree, branchData->fInputBranchName);
 
   if (branchData->fOutputBranch && valueAddress) {
      // The output branch was already created, we just need to (re)set its address
      SetBranchAddress(inputBranch, *branchData, valueAddress);
      return;
   }
 
   // Respect the original bufsize and splitlevel arguments
   // In particular, by keeping splitlevel equal to 0 if this was the case for `inputBranch`, we avoid
   // writing garbage when unsplit objects cannot be written as split objects (e.g. in case of a polymorphic
   // TObject branch, see https://bit.ly/2EjLMId ).
   // A user-provided basket size value takes precedence.
   const auto bufSize = (basketSize > 0) ? basketSize : (inputBranch ? inputBranch->GetBasketSize() : 32000);
   const auto splitLevel = inputBranch ? inputBranch->GetSplitLevel() : 99;
 
   auto *dictionary = TDictionary::GetDictionary(*branchData->fInputTypeID);
   if (dynamic_cast<TDataType *>(dictionary)) {
      // Branch of fundamental type
      CreateFundamentalTypeBranch(outputTree, *branchData, valueAddress, bufSize);
      return;
   }
 
   if (!branchData->fIsDefine) {
      // Cases where we need a leaflist (e.g. C-style arrays)
      // We only enter this code path if the input value does not come from a Define/Redefine. In those cases, it is
      // not allowed to create a column of C-style array type, so that can't happen when writing the TTree. This is
      // currently what prevents writing the wrong branch output type in a scenario where the input branch of the TTree
      // is a C-style array and then the user is Redefining it with some other type (e.g. a ROOT::RVec).
      branchData = CreateCStyleArrayBranch(outputTree, allBranchData, branchData, inputBranch, bufSize, valueAddress);
   }
   if (branchData->fOutputBranch) {
      // A branch was created in the previous function call
      if (valueAddress) {
         // valueAddress here points to a ROOT::RVec<std::byte> coming from RTreeUntypedArrayColumnReader. We know we
         // need its buffer, so we cast it and extract the address of the buffer
         auto *rawRVec = reinterpret_cast<ROOT::RVec<std::byte> *>(valueAddress);
         branchData->fBranchAddressForCArrays = rawRVec->data();
      }
      return;
   }
 
   if (auto *classPtr = dynamic_cast<TClass *>(dictionary)) {
      // Case of unsplit object with polymorphic type
      if (inputBranch && dynamic_cast<TBranchObject *>(inputBranch) && valueAddress)
         branchData->fOutputBranch =
            ROOT::Internal::TreeUtils::CallBranchImp(outputTree, branchData->fOutputBranchName.c_str(), classPtr,
                                                     inputBranch->GetAddress(), bufSize, splitLevel);
      // General case, with valid address
      else if (valueAddress)
         branchData->fOutputBranch = ROOT::Internal::TreeUtils::CallBranchImpRef(
            outputTree, branchData->fOutputBranchName.c_str(), classPtr, TDataType::GetType(*branchData->fInputTypeID),
            valueAddress, bufSize, splitLevel);
      // No value was passed, we're just creating a hollow branch to populate the dataset schema
      else
         branchData->fOutputBranch =
            outputTree.Branch(branchData->fOutputBranchName.c_str(), classPtr->GetName(), nullptr, bufSize);
      return;
   }
 
   // We are not aware of other cases
   throw std::logic_error(
      "RDataFrame::Snapshot: something went wrong when creating a TTree branch, please report this as a bug.");
}
 
auto GetSnapshotCompressionSettings(const ROOT::RDF::RSnapshotOptions &options)
{
   using CompAlgo = ROOT::RCompressionSetting::EAlgorithm::EValues;
   using OutputFormat = ROOT::RDF::ESnapshotOutputFormat;
 
   if (options.fOutputFormat == OutputFormat::kTTree || options.fOutputFormat == OutputFormat::kDefault) {
      // The default compression settings for TTree is 101
      if (options.fCompressionAlgorithm == CompAlgo::kUndefined) {
         return ROOT::CompressionSettings(CompAlgo::kZLIB, 1);
      }
      return ROOT::CompressionSettings(options.fCompressionAlgorithm, options.fCompressionLevel);
   } else if (options.fOutputFormat == OutputFormat::kRNTuple) {
      // The default compression settings for RNTuple is 505
      if (options.fCompressionAlgorithm == CompAlgo::kUndefined) {
         return ROOT::CompressionSettings(CompAlgo::kZSTD, 5);
      }
      return ROOT::CompressionSettings(options.fCompressionAlgorithm, options.fCompressionLevel);
   } else {
      throw std::invalid_argument("RDataFrame::Snapshot: unrecognized output format");
   }
}
 
std::string ModeWithoutGlobalRegistration(const std::string &mode)
{
   if (mode.find("_WITHOUT_GLOBALREGISTRATION") != std::string::npos) {
      return mode;
   }
   return mode + "_WITHOUT_GLOBALREGISTRATION";
}
 
} // namespace
 
ROOT::Internal::RDF::RBranchData::RBranchData(std::string inputBranchName, std::string outputBranchName, bool isDefine,
                                              const std::type_info *typeID)
   : fInputBranchName{std::move(inputBranchName)},
     fOutputBranchName{std::move(outputBranchName)},
     fInputTypeID{typeID},
     fIsDefine{isDefine}
{
   auto *dictionary = TDictionary::GetDictionary(*fInputTypeID);
   if (auto datatype = dynamic_cast<TDataType *>(dictionary); datatype) {
      fTypeData = FundamentalType(datatype->Size());
   } else if (auto tclass = dynamic_cast<TClass *>(dictionary); tclass) {
      fTypeData = EmptyDynamicType{tclass};
   }
}
 
/// @brief Return a pointer to an empty instance of the type represented by this branch.
/// For fundamental types, this is simply an 8-byte region of zeroes. For classes, it is an instance created with
/// TClass::New.
/// @param pointerToPointer Return a pointer to a pointer, so it can be used directly in TTree::SetBranchAddress().
void *ROOT::Internal::RDF::RBranchData::EmptyInstance(bool pointerToPointer)
{
   if (auto fundamental = std::get_if<FundamentalType>(&fTypeData); fundamental) {
      assert(!pointerToPointer); // Not used for fundamental types
      return fundamental->fBytes.data();
   }
 
   auto &dynamic = std::get<EmptyDynamicType>(fTypeData);
   if (!dynamic.fEmptyInstance) {
      auto *dictionary = TDictionary::GetDictionary(*fInputTypeID);
      assert(dynamic_cast<TDataType *>(dictionary) ==
             nullptr); // TDataType should be handled by writing into the local buffer
 
      auto tclass = dynamic_cast<TClass *>(dictionary);
      assert(tclass);
      dynamic.fEmptyInstance = std::shared_ptr<void>{tclass->New(), tclass->GetDestructor()};
   }
 
   if (pointerToPointer) {
      // Make TTree happy (needs a pointer to pointer):
      dynamic.fRawPtrToEmptyInstance = dynamic.fEmptyInstance.get();
      return &dynamic.fRawPtrToEmptyInstance;
   } else {
      return dynamic.fEmptyInstance.get();
   }
}
 
/// Point the branch address to an empty instance of the type represented by this branch
/// or write null bytes into the space used by the fundamental type.
/// This is used in case of variations, when certain defines/actions don't execute. We
/// nevertheless need to write something, so we point the branch to an empty instance.
void ROOT::Internal::RDF::RBranchData::ClearBranchContents()
{
   if (!fOutputBranch)
      return;
 
   if (auto fundamental = std::get_if<FundamentalType>(&fTypeData); fundamental) {
      fundamental->fBytes.fill(std::byte{0});
   } else {
      // TTree expects pointer to pointer, to figure out who allocates the object:
      fOutputBranch->SetAddress(EmptyInstance(/*pointerToPointer=*/true));
   }
}
 
ROOT::Internal::RDF::UntypedSnapshotTTreeHelper::UntypedSnapshotTTreeHelper(
   std::string_view filename, std::string_view dirname, std::string_view treename, const ColumnNames_t &vbnames,
   const ColumnNames_t &bnames, const RSnapshotOptions &options, std::vector<bool> &&isDefine,
   ROOT::Detail::RDF::RLoopManager *loopManager, ROOT::Detail::RDF::RLoopManager *inputLM,
   const std::vector<const std::type_info *> &colTypeIDs)
   : fFileName(filename),
     fDirName(dirname),
     fTreeName(treename),
     fOptions(options),
     fOutputLoopManager(loopManager),
     fInputLoopManager(inputLM)
{
   EnsureValidSnapshotOutput(fOptions, fTreeName, fFileName);
 
   auto outputBranchNames = ReplaceDotWithUnderscore(bnames);
   fBranchData.reserve(vbnames.size());
   for (unsigned int i = 0; i < vbnames.size(); ++i) {
      fBranchData.emplace_back(vbnames[i], std::move(outputBranchNames[i]), isDefine[i], colTypeIDs[i]);
   }
}
 
// Define special member methods here where the definition of all the data member types is available
ROOT::Internal::RDF::UntypedSnapshotTTreeHelper::UntypedSnapshotTTreeHelper(
   ROOT::Internal::RDF::UntypedSnapshotTTreeHelper &&) noexcept = default;
ROOT::Internal::RDF::UntypedSnapshotTTreeHelper &ROOT::Internal::RDF::UntypedSnapshotTTreeHelper::operator=(
   ROOT::Internal::RDF::UntypedSnapshotTTreeHelper &&) noexcept = default;
 
ROOT::Internal::RDF::UntypedSnapshotTTreeHelper::~UntypedSnapshotTTreeHelper()
{
   if (!fTreeName.empty() /*not moved from*/ && !fOutputFile /* did not run */ && fOptions.fLazy) {
      const auto fileOpenMode = [&]() {
         TString checkupdate = fOptions.fMode;
         checkupdate.ToLower();
         return checkupdate == "update" ? "updated" : "created";
      }();
      Warning("Snapshot",
              "A lazy Snapshot action was booked but never triggered. The tree '%s' in output file '%s' was not %s. "
              "In case it was desired instead, remember to trigger the Snapshot operation, by storing "
              "its result in a variable and for example calling the GetValue() method on it.",
              fTreeName.c_str(), fFileName.c_str(), fileOpenMode);
   }
}
 
void ROOT::Internal::RDF::UntypedSnapshotTTreeHelper::InitTask(TTreeReader *, unsigned int)
{
   // We ask the input RLoopManager if it has a TTree. We cannot rely on getting this information when constructing
   // this action helper, since the TTree might change e.g. when ChangeSpec is called in-between distributed tasks.
   if (auto treeDS = dynamic_cast<ROOT::Internal::RDF::RTTreeDS *>(fInputLoopManager->GetDataSource()))
      fInputTree = treeDS->GetTree();
   fBranchAddressesNeedReset = true;
}
 
void ROOT::Internal::RDF::UntypedSnapshotTTreeHelper::Exec(unsigned int, const std::vector<void *> &values)
{
   if (!fBranchAddressesNeedReset) {
      UpdateCArraysPtrs(values);
   } else {
      SetBranches(values);
      fBranchAddressesNeedReset = false;
   }
 
   fOutputTree->Fill();
}
 
void ROOT::Internal::RDF::UntypedSnapshotTTreeHelper::UpdateCArraysPtrs(const std::vector<void *> &values)
{
   // This code deals with branches which hold C arrays of variable size. It can happen that the buffers
   // associated to those is re-allocated. As a result the value of the pointer can change therewith
   // leaving associated to the branch of the output tree an invalid pointer.
   // With this code, we set the value of the pointer in the output branch anew when needed.
   assert(values.size() == fBranchData.size());
   auto nValues = values.size();
   for (decltype(nValues) i{}; i < nValues; i++) {
      if (fBranchData[i].fIsCArray) {
         // valueAddress here points to a ROOT::RVec<std::byte> coming from RTreeUntypedArrayColumnReader. We know we
         // need its buffer, so we cast it and extract the address of the buffer
         auto *rawRVec = reinterpret_cast<ROOT::RVec<std::byte> *>(values[i]);
         if (auto *data = rawRVec->data(); fBranchData[i].fBranchAddressForCArrays != data) {
            fBranchData[i].fOutputBranch->SetAddress(data);
            fBranchData[i].fBranchAddressForCArrays = data;
         }
      }
   }
}
 
void ROOT::Internal::RDF::UntypedSnapshotTTreeHelper::SetBranches(const std::vector<void *> &values)
{
   // create branches in output tree
   assert(fBranchData.size() == values.size());
   for (std::size_t i = 0; i < fBranchData.size(); i++) { // fBranchData can grow due to insertions
      SetBranchesHelper(fInputTree, *fOutputTree, fBranchData, i, fOptions.fBasketSize, values[i]);
   }
   AssertNoNullBranchAddresses(fBranchData);
}
 
void ROOT::Internal::RDF::UntypedSnapshotTTreeHelper::SetEmptyBranches(TTree *inputTree, TTree &outputTree)
{
   void *dummyValueAddress{};
   for (std::size_t i = 0; i < fBranchData.size(); i++) { // fBranchData can grow due to insertions
      SetBranchesHelper(inputTree, outputTree, fBranchData, i, fOptions.fBasketSize, dummyValueAddress);
   }
}
 
void ROOT::Internal::RDF::UntypedSnapshotTTreeHelper::Initialize()
{
   fOutputFile.reset(TFile::Open(fFileName.c_str(), ModeWithoutGlobalRegistration(fOptions.fMode).c_str(),
                                 /*ftitle=*/"", GetSnapshotCompressionSettings(fOptions)));
   if (!fOutputFile)
      throw std::runtime_error("Snapshot: could not create output file " + fFileName);
 
   TDirectory *outputDir = fOutputFile.get();
   if (!fDirName.empty()) {
      TString checkupdate = fOptions.fMode;
      checkupdate.ToLower();
      if (checkupdate == "update")
         outputDir = fOutputFile->mkdir(fDirName.c_str(), "", true); // do not overwrite existing directory
      else
         outputDir = fOutputFile->mkdir(fDirName.c_str());
   }
 
   fOutputTree = std::make_unique<TTree>(fTreeName.c_str(), fTreeName.c_str(), fOptions.fSplitLevel, /*dir=*/outputDir);
 
   if (fOptions.fAutoFlush)
      fOutputTree->SetAutoFlush(fOptions.fAutoFlush);
}
 
void ROOT::Internal::RDF::UntypedSnapshotTTreeHelper::Finalize()
{
   assert(fOutputTree != nullptr);
   assert(fOutputFile != nullptr);
 
   // There were no entries to fill the TTree with (either the input TTree was empty or no event passed after
   // filtering). We have already created an empty TTree, now also create the branches to preserve the schema
   if (fOutputTree->GetEntries() == 0) {
      SetEmptyBranches(fInputTree, *fOutputTree);
   }
   // use AutoSave to flush TTree contents because TTree::Write writes in gDirectory, not in fDirectory
   fOutputTree->AutoSave("flushbaskets");
   // must destroy the TTree first, otherwise TFile will delete it too leading to a double delete
   fOutputTree.reset();
   fOutputFile->Close();
 
   // Now connect the data source to the loop manager so it can be used for further processing
   auto fullTreeName = fDirName.empty() ? fTreeName : fDirName + '/' + fTreeName;
   fOutputLoopManager->SetDataSource(std::make_unique<ROOT::Internal::RDF::RTTreeDS>(fullTreeName, fFileName));
}
 
/**
 * \brief Create a new UntypedSnapshotTTreeHelper with a different output file name
 *
 * \param newName A type-erased string with the output file name
 * \return UntypedSnapshotTTreeHelper
 *
 * This MakeNew implementation is tied to the cloning feature of actions
 * of the computation graph. In particular, cloning a Snapshot node usually
 * also involves changing the name of the output file, otherwise the cloned
 * Snapshot would overwrite the same file.
 */
ROOT::Internal::RDF::UntypedSnapshotTTreeHelper
ROOT::Internal::RDF::UntypedSnapshotTTreeHelper::MakeNew(void *newName, std::string_view)
{
   const std::string finalName = *reinterpret_cast<const std::string *>(newName);
   std::vector<std::string> inputBranchNames;
   std::vector<std::string> outputBranchNames;
   std::vector<bool> isDefine;
   std::vector<const std::type_info *> inputColumnTypeIDs;
   for (const auto &bd : fBranchData) {
      if (bd.fInputBranchName.empty())
         break;
      inputBranchNames.push_back(bd.fInputBranchName);
      outputBranchNames.push_back(bd.fOutputBranchName);
      isDefine.push_back(bd.fIsDefine);
      inputColumnTypeIDs.push_back(bd.fInputTypeID);
   }
 
   return ROOT::Internal::RDF::UntypedSnapshotTTreeHelper{finalName,
                                                          fDirName,
                                                          fTreeName,
                                                          std::move(inputBranchNames),
                                                          std::move(outputBranchNames),
                                                          fOptions,
                                                          std::move(isDefine),
                                                          fOutputLoopManager,
                                                          fInputLoopManager,
                                                          inputColumnTypeIDs};
}
 
ROOT::Internal::RDF::UntypedSnapshotTTreeHelperMT::UntypedSnapshotTTreeHelperMT(
   unsigned int nSlots, std::string_view filename, std::string_view dirname, std::string_view treename,
   const ColumnNames_t &vbnames, const ColumnNames_t &bnames, const RSnapshotOptions &options,
   std::vector<bool> &&isDefine, ROOT::Detail::RDF::RLoopManager *loopManager, ROOT::Detail::RDF::RLoopManager *inputLM,
   const std::vector<const std::type_info *> &colTypeIDs)
   : fNSlots(nSlots),
     fOutputFiles(fNSlots),
     fOutputTrees(fNSlots),
     fBranchAddressesNeedReset(fNSlots, 1),
     fInputTrees(fNSlots),
     fFileName(filename),
     fDirName(dirname),
     fTreeName(treename),
     fOptions(options),
     fOutputLoopManager(loopManager),
     fInputLoopManager(inputLM)
{
   EnsureValidSnapshotOutput(fOptions, fTreeName, fFileName);
 
   auto outputBranchNames = ReplaceDotWithUnderscore(bnames);
   fBranchData.reserve(fNSlots);
   for (unsigned int slot = 0; slot < fNSlots; ++slot) {
      fBranchData.emplace_back();
      auto &thisSlot = fBranchData.back();
      thisSlot.reserve(vbnames.size());
      for (unsigned int i = 0; i < vbnames.size(); ++i) {
         thisSlot.emplace_back(vbnames[i], outputBranchNames[i], isDefine[i], colTypeIDs[i]);
      }
   }
}
 
// Define special member methods here where the definition of all the data member types is available
ROOT::Internal::RDF::UntypedSnapshotTTreeHelperMT::UntypedSnapshotTTreeHelperMT(
   ROOT::Internal::RDF::UntypedSnapshotTTreeHelperMT &&) noexcept = default;
ROOT::Internal::RDF::UntypedSnapshotTTreeHelperMT &ROOT::Internal::RDF::UntypedSnapshotTTreeHelperMT::operator=(
   ROOT::Internal::RDF::UntypedSnapshotTTreeHelperMT &&) noexcept = default;
 
ROOT::Internal::RDF::UntypedSnapshotTTreeHelperMT::~UntypedSnapshotTTreeHelperMT()
{
   if (!fTreeName.empty() /*not moved from*/ && fOptions.fLazy && !fOutputFiles.empty() &&
       std::all_of(fOutputFiles.begin(), fOutputFiles.end(), [](const auto &f) { return !f; }) /* never run */) {
      const auto fileOpenMode = [&]() {
         TString checkupdate = fOptions.fMode;
         checkupdate.ToLower();
         return checkupdate == "update" ? "updated" : "created";
      }();
      Warning("Snapshot",
              "A lazy Snapshot action was booked but never triggered. The tree '%s' in output file '%s' was not %s. "
              "In case it was desired instead, remember to trigger the Snapshot operation, by storing "
              "its result in a variable and for example calling the GetValue() method on it.",
              fTreeName.c_str(), fFileName.c_str(), fileOpenMode);
   }
}
 
void ROOT::Internal::RDF::UntypedSnapshotTTreeHelperMT::InitTask(TTreeReader *r, unsigned int slot)
{
   ::TDirectory::TContext c; // do not let tasks change the thread-local gDirectory
   if (!fOutputFiles[slot]) {
      // first time this thread executes something, let's create a TBufferMerger output directory
      fOutputFiles[slot] = fMerger->GetFile();
   }
   TDirectory *treeDirectory = fOutputFiles[slot].get();
   if (!fDirName.empty()) {
      // call returnExistingDirectory=true since MT can end up making this call multiple times
      treeDirectory = fOutputFiles[slot]->mkdir(fDirName.c_str(), "", true);
   }
   // re-create output tree as we need to create its branches again, with new input variables
   // TODO we could instead create the output tree and its branches, change addresses of input variables in each task
   fOutputTrees[slot] =
      std::make_unique<TTree>(fTreeName.c_str(), fTreeName.c_str(), fOptions.fSplitLevel, /*dir=*/treeDirectory);
   fOutputTrees[slot]->SetBit(TTree::kEntriesReshuffled);
   // TODO can be removed when RDF supports interleaved TBB task execution properly, see ROOT-10269
   fOutputTrees[slot]->SetImplicitMT(false);
   if (fOptions.fAutoFlush)
      fOutputTrees[slot]->SetAutoFlush(fOptions.fAutoFlush);
   if (r) {
      // We could be getting a task-local TTreeReader from the TTreeProcessorMT.
      fInputTrees[slot] = r->GetTree();
   } else if (auto treeDS = dynamic_cast<ROOT::Internal::RDF::RTTreeDS *>(fInputLoopManager->GetDataSource())) {
      fInputTrees[slot] = treeDS->GetTree();
   }
   fBranchAddressesNeedReset[slot] = 1; // reset first event flag for this slot
}
 
void ROOT::Internal::RDF::UntypedSnapshotTTreeHelperMT::FinalizeTask(unsigned int slot)
{
   if (fOutputTrees[slot]->GetEntries() > 0)
      fOutputFiles[slot]->Write();
   for (auto &branchData : fBranchData[slot])
      branchData.ClearBranchPointers(); // The branch pointers will go stale below
   // clear now to avoid concurrent destruction of output trees and input tree (which has them listed as fClones)
   fOutputTrees[slot].reset(nullptr);
}
 
void ROOT::Internal::RDF::UntypedSnapshotTTreeHelperMT::Exec(unsigned int slot, const std::vector<void *> &values)
{
   if (fBranchAddressesNeedReset[slot] == 0) {
      UpdateCArraysPtrs(slot, values);
   } else {
      SetBranches(slot, values);
      fBranchAddressesNeedReset[slot] = 0;
   }
   fOutputTrees[slot]->Fill();
   auto entries = fOutputTrees[slot]->GetEntries();
   auto autoFlush = fOutputTrees[slot]->GetAutoFlush();
   if ((autoFlush > 0) && (entries % autoFlush == 0))
      fOutputFiles[slot]->Write();
}
 
void ROOT::Internal::RDF::UntypedSnapshotTTreeHelperMT::UpdateCArraysPtrs(unsigned int slot,
                                                                          const std::vector<void *> &values)
{
   // This code deals with branches which hold C arrays of variable size. It can happen that the buffers
   // associated to those is re-allocated. As a result the value of the pointer can change therewith
   // leaving associated to the branch of the output tree an invalid pointer.
   // With this code, we set the value of the pointer in the output branch anew when needed.
   assert(values.size() == fBranchData[slot].size());
   auto nValues = values.size();
   for (decltype(nValues) i{}; i < nValues; i++) {
      auto &branchData = fBranchData[slot][i];
      if (branchData.fIsCArray) {
         // valueAddress here points to a ROOT::RVec<std::byte> coming from RTreeUntypedArrayColumnReader. We know we
         // need its buffer, so we cast it and extract the address of the buffer
         auto *rawRVec = reinterpret_cast<ROOT::RVec<std::byte> *>(values[i]);
         if (auto *data = rawRVec->data(); branchData.fBranchAddressForCArrays != data) {
            // reset the branch address
            branchData.fOutputBranch->SetAddress(data);
            branchData.fBranchAddressForCArrays = data;
         }
      }
   }
}
 
void ROOT::Internal::RDF::UntypedSnapshotTTreeHelperMT::SetBranches(unsigned int slot,
                                                                    const std::vector<void *> &values)
{
   // create branches in output tree
   auto &branchData = fBranchData[slot];
   assert(branchData.size() == values.size());
   for (std::size_t i = 0; i < branchData.size(); i++) { // branchData can grow due to insertions
      SetBranchesHelper(fInputTrees[slot], *fOutputTrees[slot], branchData, i, fOptions.fBasketSize, values[i]);
   }
 
   AssertNoNullBranchAddresses(branchData);
}
 
void ROOT::Internal::RDF::UntypedSnapshotTTreeHelperMT::SetEmptyBranches(TTree *inputTree, TTree &outputTree)
{
   void *dummyValueAddress{};
   auto &branchData = fBranchData.front();
   for (std::size_t i = 0; i < branchData.size(); i++) { // branchData can grow due to insertions
      SetBranchesHelper(inputTree, outputTree, branchData, i, fOptions.fBasketSize, dummyValueAddress);
   }
}
 
void ROOT::Internal::RDF::UntypedSnapshotTTreeHelperMT::Initialize()
{
   auto outFile =
      std::unique_ptr<TFile>{TFile::Open(fFileName.c_str(), ModeWithoutGlobalRegistration(fOptions.fMode).c_str(),
                                         /*ftitle=*/fFileName.c_str(), GetSnapshotCompressionSettings(fOptions))};
   if (!outFile)
      throw std::runtime_error("Snapshot: could not create output file " + fFileName);
   fOutputFile = outFile.get();
   fMerger = std::make_unique<ROOT::TBufferMerger>(std::move(outFile));
}
 
void ROOT::Internal::RDF::UntypedSnapshotTTreeHelperMT::Finalize()
{
 
   for (auto &file : fOutputFiles) {
      if (file) {
         file->Write();
         file->Close();
      }
   }
 
   // If there were no entries to fill the TTree with (either the input TTree was empty or no event passed after
   // filtering), create an empty TTree in the output file and create the branches to preserve the schema
   auto fullTreeName = fDirName.empty() ? fTreeName : fDirName + '/' + fTreeName;
   assert(fOutputFile && "Missing output file in Snapshot finalization.");
   // Use GetKey to avoid having to deal with memory management of the object in the file
   if (!fOutputFile->GetKey(fullTreeName.c_str())) {
 
      // First find in which directory we need to write the output TTree
      TDirectory *treeDirectory = fOutputFile;
      if (!fDirName.empty()) {
         treeDirectory = fOutputFile->mkdir(fDirName.c_str(), "", true);
      }
      ::TDirectory::TContext c{treeDirectory};
 
      // Create the output TTree and create the user-requested branches
      auto outTree =
         std::make_unique<TTree>(fTreeName.c_str(), fTreeName.c_str(), fOptions.fSplitLevel, /*dir=*/treeDirectory);
      TTree *inputTree{};
      if (auto treeDS = dynamic_cast<ROOT::Internal::RDF::RTTreeDS *>(fInputLoopManager->GetDataSource()))
         inputTree = treeDS->GetTree();
      SetEmptyBranches(inputTree, *outTree);
 
      fOutputFile->Write();
   }
 
   // flush all buffers to disk by destroying the TBufferMerger
   fOutputFiles.clear();
   fMerger.reset();
 
   // Now connect the data source to the loop manager so it can be used for further processing
   fOutputLoopManager->SetDataSource(std::make_unique<ROOT::Internal::RDF::RTTreeDS>(fullTreeName, fFileName));
}
 
/**
 * \brief Create a new UntypedSnapshotTTreeHelperMT with a different output file name
 *
 * \param newName A type-erased string with the output file name
 * \return UntypedSnapshotTTreeHelperMT
 *
 * This MakeNew implementation is tied to the cloning feature of actions
 * of the computation graph. In particular, cloning a Snapshot node usually
 * also involves changing the name of the output file, otherwise the cloned
 * Snapshot would overwrite the same file.
 */
ROOT::Internal::RDF::UntypedSnapshotTTreeHelperMT
ROOT::Internal::RDF::UntypedSnapshotTTreeHelperMT::MakeNew(void *newName, std::string_view)
{
   const std::string finalName = *reinterpret_cast<const std::string *>(newName);
   std::vector<std::string> inputBranchNames;
   std::vector<std::string> outputBranchNames;
   std::vector<bool> isDefine;
   std::vector<const std::type_info *> inputColumnTypeIDs;
   for (const auto &bd : fBranchData.front()) {
      if (bd.fInputBranchName.empty())
         break;
      inputBranchNames.push_back(bd.fInputBranchName);
      outputBranchNames.push_back(bd.fOutputBranchName);
      isDefine.push_back(bd.fIsDefine);
      inputColumnTypeIDs.push_back(bd.fInputTypeID);
   }
 
   return ROOT::Internal::RDF::UntypedSnapshotTTreeHelperMT{fNSlots,
                                                            finalName,
                                                            fDirName,
                                                            fTreeName,
                                                            std::move(inputBranchNames),
                                                            std::move(outputBranchNames),
                                                            fOptions,
                                                            std::move(isDefine),
                                                            fOutputLoopManager,
                                                            fInputLoopManager,
                                                            std::move(inputColumnTypeIDs)};
}
 
ROOT::Internal::RDF::UntypedSnapshotRNTupleHelper::UntypedSnapshotRNTupleHelper(
   unsigned int nSlots, std::string_view filename, std::string_view dirname, std::string_view ntuplename,
   const ColumnNames_t &vfnames, const ColumnNames_t &fnames, const RSnapshotOptions &options,
   ROOT::Detail::RDF::RLoopManager *inputLM, ROOT::Detail::RDF::RLoopManager *outputLM,
   const std::vector<const std::type_info *> &colTypeIDs)
   : fFileName(filename),
     fDirName(dirname),
     fNTupleName(ntuplename),
     fOptions(options),
     fInputLoopManager(inputLM),
     fOutputLoopManager(outputLM),
     fInputFieldNames(vfnames),
     fOutputFieldNames(ReplaceDotWithUnderscore(fnames)),
     fNSlots(nSlots),
     fFillContexts(nSlots),
     fEntries(nSlots),
     fInputColumnTypeIDs(colTypeIDs)
{
   EnsureValidSnapshotOutput(fOptions, fNTupleName, fFileName);
}
 
// Define special member methods here where the definition of all the data member types is available
ROOT::Internal::RDF::UntypedSnapshotRNTupleHelper::UntypedSnapshotRNTupleHelper(
   ROOT::Internal::RDF::UntypedSnapshotRNTupleHelper &&) noexcept = default;
ROOT::Internal::RDF::UntypedSnapshotRNTupleHelper &ROOT::Internal::RDF::UntypedSnapshotRNTupleHelper::operator=(
   ROOT::Internal::RDF::UntypedSnapshotRNTupleHelper &&) noexcept = default;
 
ROOT::Internal::RDF::UntypedSnapshotRNTupleHelper::~UntypedSnapshotRNTupleHelper()
{
   if (!fNTupleName.empty() /* not moved from */ && !fOutputFile /* did not run */ && fOptions.fLazy)
      Warning("Snapshot", "A lazy Snapshot action was booked but never triggered.");
}
 
void ROOT::Internal::RDF::UntypedSnapshotRNTupleHelper::Initialize()
{
   auto model = ROOT::RNTupleModel::CreateBare();
   auto nFields = fOutputFieldNames.size();
   fFieldTokens.resize(nFields);
   for (decltype(nFields) i = 0; i < nFields; i++) {
      // Need to retrieve the type of every field to create as a string
      // If the input type for a field does not have RTTI, internally we store it as the tag UseNativeDataType. When
      // that is detected, we need to ask the data source which is the type name based on the on-disk information.
      const auto typeName = *fInputColumnTypeIDs[i] == typeid(ROOT::Internal::RDF::UseNativeDataType)
                               ? ROOT::Internal::RDF::GetTypeNameWithOpts(*fInputLoopManager->GetDataSource(),
                                                                          fInputFieldNames[i], fOptions.fVector2RVec)
                               : ROOT::Internal::RDF::TypeID2TypeName(*fInputColumnTypeIDs[i]);
 
      // Cardinality fields are read-only, so instead we snapshot them as their inner type.
      if (typeName.substr(0, 25) == "ROOT::RNTupleCardinality<") {
         // Get "T" from "ROOT::RNTupleCardinality<T>".
         std::string cardinalityType = typeName.substr(25, typeName.size() - 26);
         Warning("Snapshot",
                 "Column \"%s\" is a read-only \"%s\" column. It will be snapshot as its inner type \"%s\" instead.",
                 fInputFieldNames[i].c_str(), typeName.c_str(), cardinalityType.c_str());
         model->AddField(ROOT::RFieldBase::Create(fOutputFieldNames[i], cardinalityType).Unwrap());
      } else {
         model->AddField(ROOT::RFieldBase::Create(fOutputFieldNames[i], typeName).Unwrap());
      }
      fFieldTokens[i] = model->GetToken(fOutputFieldNames[i]);
   }
   model->Freeze();
 
   ROOT::RNTupleWriteOptions writeOptions;
   writeOptions.SetCompression(GetSnapshotCompressionSettings(fOptions));
   writeOptions.SetInitialUnzippedPageSize(fOptions.fInitialUnzippedPageSize);
   writeOptions.SetMaxUnzippedPageSize(fOptions.fMaxUnzippedPageSize);
   writeOptions.SetApproxZippedClusterSize(fOptions.fApproxZippedClusterSize);
   writeOptions.SetMaxUnzippedClusterSize(fOptions.fMaxUnzippedClusterSize);
   writeOptions.SetEnablePageChecksums(fOptions.fEnablePageChecksums);
   writeOptions.SetEnableSamePageMerging(fOptions.fEnableSamePageMerging);
 
   fOutputFile.reset(TFile::Open(fFileName.c_str(), ModeWithoutGlobalRegistration(fOptions.fMode).c_str()));
   if (!fOutputFile)
      throw std::runtime_error("Snapshot: could not create output file " + fFileName);
 
   TDirectory *outputDir = fOutputFile.get();
   if (!fDirName.empty()) {
      TString checkupdate = fOptions.fMode;
      checkupdate.ToLower();
      if (checkupdate == "update")
         outputDir = fOutputFile->mkdir(fDirName.c_str(), "", true); // do not overwrite existing directory
      else
         outputDir = fOutputFile->mkdir(fDirName.c_str());
   }
 
   // The RNTupleParallelWriter has exclusive access to the underlying TFile, no further synchronization is needed for
   // calls to Fill() (in Exec) and FlushCluster() (in FinalizeTask).
   fWriter = ROOT::RNTupleParallelWriter::Append(std::move(model), fNTupleName, *outputDir, writeOptions);
}
 
void ROOT::Internal::RDF::UntypedSnapshotRNTupleHelper::InitTask(TTreeReader *, unsigned int slot)
{
   if (!fFillContexts[slot]) {
      fFillContexts[slot] = fWriter->CreateFillContext();
      fEntries[slot] = fFillContexts[slot]->GetModel().CreateBareEntry();
   }
}
 
void ROOT::Internal::RDF::UntypedSnapshotRNTupleHelper::Exec(unsigned int slot, const std::vector<void *> &values)
{
   auto &fillContext = fFillContexts[slot];
   auto &outputEntry = fEntries[slot];
   assert(values.size() == fFieldTokens.size());
   for (decltype(values.size()) i = 0; i < values.size(); i++) {
      outputEntry->BindRawPtr(fFieldTokens[i], values[i]);
   }
   fillContext->Fill(*outputEntry);
}
 
void ROOT::Internal::RDF::UntypedSnapshotRNTupleHelper::FinalizeTask(unsigned int slot)
{
   // In principle we would not need to flush a cluster here, but we want to benefit from parallelism for compression.
   // NB: RNTupleFillContext::FlushCluster() is a nop if there is no new entry since the last flush.
   fFillContexts[slot]->FlushCluster();
}
 
void ROOT::Internal::RDF::UntypedSnapshotRNTupleHelper::Finalize()
{
   // First clear and destroy all entries, which were created from the RNTupleFillContexts.
   fEntries.clear();
   fFillContexts.clear();
   // Then destroy the RNTupleParallelWriter and write the metadata.
   fWriter.reset();
   // We can now set the data source of the loop manager for the RDataFrame that is returned by the Snapshot call.
   fOutputLoopManager->SetDataSource(std::make_unique<ROOT::RDF::RNTupleDS>(fDirName + "/" + fNTupleName, fFileName));
}
 
/**
 * Create a new UntypedSnapshotRNTupleHelper with a different output file name.
 *
 * \param[in] newName A type-erased string with the output file name
 * \return UntypedSnapshotRNTupleHelper
 *
 * This MakeNew implementation is tied to the cloning feature of actions
 * of the computation graph. In particular, cloning a Snapshot node usually
 * also involves changing the name of the output file, otherwise the cloned
 * Snapshot would overwrite the same file.
 */
ROOT::Internal::RDF::UntypedSnapshotRNTupleHelper
ROOT::Internal::RDF::UntypedSnapshotRNTupleHelper::MakeNew(void *newName)
{
   const std::string finalName = *reinterpret_cast<const std::string *>(newName);
   return UntypedSnapshotRNTupleHelper{
      fNSlots,           finalName, fDirName,          fNTupleName,        fInputFieldNames,
      fOutputFieldNames, fOptions,  fInputLoopManager, fOutputLoopManager, fInputColumnTypeIDs};
}
 
/*
 * ------------------------------------
 * Snapshot with systematic variations
 * ------------------------------------
 */
namespace ROOT::Internal::RDF {
/// An object to store an output file and a tree in one common place to share them between instances
/// of Snapshot with systematic uncertainties.
struct SnapshotOutputWriter {
   std::unique_ptr<TFile> fFile;
   std::unique_ptr<TTree> fTree;
   std::string fDirectoryName;
   RLoopManager *fOutputLoopManager;
 
   // Bitmasks to indicate whether syst. uncertainties have been computed. Bound to TBranches, so need to be stable in
   // memory.
   struct Bitmask {
      std::string branchName;
      std::bitset<64> bitset{};
      std::unique_ptr<uint64_t> branchBuffer{new uint64_t{}};
   };
   std::vector<Bitmask> fBitMasks;
 
   std::unordered_map<std::string, unsigned int> fBranchToVariationMapping;
   // The corresponding ROOT dictionary is declared in core/clingutils/src
   std::unordered_map<std::string, std::pair<std::string, unsigned int>> fBranchToBitmaskMapping;
   unsigned int fNBits = 0;
 
   SnapshotOutputWriter(TFile *file) : fFile{file} { assert(fFile); }
   ~SnapshotOutputWriter()
   {
      if (!fBranchToBitmaskMapping.empty()) {
         fFile->WriteObject(&fBranchToBitmaskMapping,
                            (std::string{"R_rdf_column_to_bitmask_mapping_"} + fTree->GetName()).c_str());
      }
      if (fTree) {
         // use AutoSave to flush TTree contents because TTree::Write writes in gDirectory, not in fDirectory
         fTree->AutoSave("flushbaskets");
 
         // Now connect the data source to the loop manager so it can be used for further processing
         std::string tree = fTree->GetName();
         if (!fDirectoryName.empty())
            tree = fDirectoryName + '/' + tree;
         std::string file = fFile->GetName();
 
         fTree.reset();
         fFile.reset();
 
         if (fOutputLoopManager)
            fOutputLoopManager->SetDataSource(std::make_unique<ROOT::Internal::RDF::RTTreeDS>(tree, file));
      }
   }
   SnapshotOutputWriter(SnapshotOutputWriter const &) = delete; // Anyway deleted because of the unique_ptrs
   SnapshotOutputWriter &operator=(SnapshotOutputWriter const &) = delete;
   SnapshotOutputWriter(SnapshotOutputWriter &&) noexcept =
      delete; // Can be done, but need to make move-from object safe to destruct
   SnapshotOutputWriter &operator=(SnapshotOutputWriter &&) noexcept = delete;
 
   /// Register a branch and corresponding systematic uncertainty.
   /// This will create an entry in the mapping from branch names to bitmasks, so the corresponding
   /// column can be masked if it doesn't contain valid entries. This mapping is written next to the
   /// tree into the output file.
   void RegisterBranch(std::string const &branchName, unsigned int variationIndex)
   {
      if (auto it = fBranchToVariationMapping.find(branchName); it != fBranchToVariationMapping.end()) {
         if (variationIndex != it->second) {
            throw std::logic_error("Branch " + branchName +
                                   " is being registered with different variation index than the expected one: " +
                                   std::to_string(variationIndex));
         }
         return;
      }
 
      // Neither branch nor systematic are known, so a new entry needs to be created
      fNBits = std::max(fNBits, variationIndex);
      const auto vectorIndex = variationIndex / 64u;
      const auto bitIndex = variationIndex % 64u;
 
      // Create bitmask branches as long as necessary to capture the bit
      while (vectorIndex >= fBitMasks.size()) {
         std::string bitmaskBranchName =
            std::string{"R_rdf_mask_"} + fTree->GetName() + '_' + std::to_string(fBitMasks.size());
         fBitMasks.push_back(Bitmask{bitmaskBranchName});
         fTree->Branch(bitmaskBranchName.c_str(), fBitMasks.back().branchBuffer.get());
      }
 
      fBranchToVariationMapping[branchName] = variationIndex;
      fBranchToBitmaskMapping[branchName] = std::make_pair(fBitMasks[vectorIndex].branchName, bitIndex);
   }
 
   /// Clear all bits, as if none of the variations passed its filter.
   void ClearMaskBits()
   {
      for (auto &mask : fBitMasks)
         mask.bitset.reset();
   }
 
   /// Set a bit signalling that the variation at `index` passed its filter.
   void SetMaskBit(unsigned int index)
   {
      const auto vectorIndex = index / 64;
      const auto bitIndex = index % 64;
      fBitMasks[vectorIndex].bitset.set(bitIndex, true);
   }
 
   /// Test if any of the mask bits are set.
   bool MaskEmpty() const
   {
      return std::none_of(fBitMasks.begin(), fBitMasks.end(), [](Bitmask const &mask) { return mask.bitset.any(); });
   }
 
   /// Write the current event and the bitmask to the output dataset.
   void Write() const
   {
      if (!fTree)
         throw std::runtime_error("The TTree associated to the Snapshot action doesn't exist, any more.");
 
      for (auto const &mask : fBitMasks) {
         *mask.branchBuffer = mask.bitset.to_ullong();
      }
 
      fTree->Fill();
   }
};
 
} // namespace ROOT::Internal::RDF
 
ROOT::Internal::RDF::SnapshotHelperWithVariations::SnapshotHelperWithVariations(
   std::string_view filename, std::string_view dirname, std::string_view treename, const ColumnNames_t &vbnames,
   const ColumnNames_t &bnames, const RSnapshotOptions &options, std::vector<bool> &&isDefine,
   ROOT::Detail::RDF::RLoopManager *outputLoopMgr, ROOT::Detail::RDF::RLoopManager *inputLoopMgr,
   const std::vector<const std::type_info *> &colTypeIDs)
   : fOptions(options), fInputLoopManager{inputLoopMgr}, fOutputLoopManager{outputLoopMgr}
{
   EnsureValidSnapshotOutput(fOptions, std::string(treename), std::string(filename));
 
   TDirectory::TContext fileCtxt;
   fOutputHandle = std::make_shared<SnapshotOutputWriter>(
      TFile::Open(filename.data(), ModeWithoutGlobalRegistration(fOptions.fMode).c_str(), /*ftitle=*/"",
                  GetSnapshotCompressionSettings(fOptions)));
   if (!fOutputHandle->fFile)
      throw std::runtime_error(std::string{"Snapshot: could not create output file "} + std::string{filename});
 
   TDirectory *outputDir = fOutputHandle->fFile.get();
   if (!dirname.empty()) {
      fOutputHandle->fDirectoryName = dirname;
      TString checkupdate = fOptions.fMode;
      checkupdate.ToLower();
      if (checkupdate == "update")
         outputDir =
            fOutputHandle->fFile->mkdir(std::string{dirname}.c_str(), "", true); // do not overwrite existing directory
      else
         outputDir = fOutputHandle->fFile->mkdir(std::string{dirname}.c_str());
   }
 
   fOutputHandle->fTree = std::make_unique<TTree>(std::string{treename}.c_str(), std::string{treename}.c_str(),
                                                  fOptions.fSplitLevel, /*dir=*/outputDir);
   fOutputHandle->fOutputLoopManager = fOutputLoopManager;
   if (fOptions.fAutoFlush)
      fOutputHandle->fTree->SetAutoFlush(fOptions.fAutoFlush);
 
   auto outputBranchNames = ReplaceDotWithUnderscore(bnames);
 
   fBranchData.reserve(vbnames.size());
   for (unsigned int i = 0; i < vbnames.size(); ++i) {
      fOutputHandle->RegisterBranch(outputBranchNames[i], 0);
      fBranchData.emplace_back(vbnames[i], outputBranchNames[i], isDefine[i], colTypeIDs[i]);
   }
}
 
/// Register a new column as a variation of the column at `originalColumnIndex`, and clone its properties.
/// If a nominal column is registered here, it is written without changes, but it means that it will be masked
/// in case its selection cuts don't pass.
/// \param slot Task ID for MT runs.
/// \param columnIndex Index where the data of this column will be passed into the helper.
/// \param originalColumnIndex If the column being registered is a variation of a "nominal" column, this designates the
/// original.
///         Properties such as name and output type are cloned from the original.
/// \param variationName The variation that this column belongs to. If "nominal" is used, this column is considered as
/// the original.
void ROOT::Internal::RDF::SnapshotHelperWithVariations::RegisterVariedColumn(unsigned int /*slot*/,
                                                                             unsigned int columnIndex,
                                                                             unsigned int originalColumnIndex,
                                                                             unsigned int variationIndex,
                                                                             std::string const &variationName)
{
   if (columnIndex == originalColumnIndex) {
      // This is a nominal column, but it participates in variations.
      // It always needs to be written, but we still need to create a mask bit to mark when nominal is invalid.
      assert(variationIndex == 0);
      fBranchData[columnIndex].fVariationIndex = 0;
      fOutputHandle->RegisterBranch(fBranchData[columnIndex].fOutputBranchName, variationIndex);
   } else if (columnIndex >= fBranchData.size()) {
      // First task, need to create branches
      fBranchData.resize(columnIndex + 1);
      auto &bd = fBranchData[columnIndex];
      bd = fBranchData[originalColumnIndex];
      std::string newOutputName = bd.fOutputBranchName + "__" + variationName;
      std::replace(newOutputName.begin(), newOutputName.end(), ':', '_');
      bd.fOutputBranchName = std::move(newOutputName);
      bd.fVariationIndex = variationIndex;
 
      fOutputHandle->RegisterBranch(bd.fOutputBranchName, variationIndex);
   } else {
      assert(static_cast<unsigned int>(fBranchData[columnIndex].fVariationIndex) == variationIndex);
   }
}
 
/// Bind all output branches to RDF columns for the given slots.
void ROOT::Internal::RDF::SnapshotHelperWithVariations::InitTask(TTreeReader *, unsigned int)
{
   // We ask the input RLoopManager if it has a TTree. We cannot rely on getting this information when constructing
   // this action helper, since the TTree might change e.g. when ChangeSpec is called in-between distributed tasks.
   if (auto treeDS = dynamic_cast<ROOT::Internal::RDF::RTTreeDS *>(fInputLoopManager->GetDataSource()))
      fInputTree = treeDS->GetTree();
 
   // Create all output branches; and bind them to empty values
   for (std::size_t i = 0; i < fBranchData.size(); i++) { // fBranchData can grow due to insertions
      SetBranchesHelper(fInputTree, *fOutputHandle->fTree, fBranchData, i, fOptions.fBasketSize,
                        fBranchData[i].EmptyInstance(/*pointerToPointer=*/false));
   }
 
   AssertNoNullBranchAddresses(fBranchData);
}
 
/// Connect all output fields to the values pointed to by `values`, fill the output dataset,
/// call the Fill of the output tree, and clear the mask bits that show whether a variation was reached.
void ROOT::Internal::RDF::SnapshotHelperWithVariations::Exec(unsigned int /*slot*/, const std::vector<void *> &values,
                                                             std::vector<bool> const &filterPassed)
{
   // Rebind branch pointers to RDF values
   assert(fBranchData.size() == values.size());
   for (std::size_t i = 0; i < values.size(); i++) {
      const auto variationIndex = fBranchData[i].fVariationIndex;
      if (variationIndex < 0) {
         // Branch without variations, it always needs to be written
         SetBranchesHelper(fInputTree, *fOutputHandle->fTree, fBranchData, i, fOptions.fBasketSize, values[i]);
      } else {
         // Nominal will always be written, systematics only if needed
         if (variationIndex == 0 || filterPassed[variationIndex]) {
            const bool fundamentalType = fBranchData[i].WriteValueIfFundamental(values[i]);
            if (!fundamentalType) {
               SetBranchesHelper(fInputTree, *fOutputHandle->fTree, fBranchData, i, fOptions.fBasketSize, values[i]);
            }
         }
 
         if (filterPassed[variationIndex]) {
            fOutputHandle->SetMaskBit(variationIndex);
         }
      }
   }
 
   assert(!fOutputHandle->MaskEmpty()); // Exec should not have been called if nothing passes
 
   fOutputHandle->Write();
   fOutputHandle->ClearMaskBits();
   for (auto &branchData : fBranchData) {
      branchData.ClearBranchContents();
   }
}
 
void ROOT::Internal::RDF::SnapshotHelperWithVariations::Finalize()
{
   fOutputHandle.reset();
}