Writer.h

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/***********************************************************************************\
* (c) Copyright 2024 CERN for the benefit of the LHCb and ATLAS collaborations      *
*                                                                                   *
* This software is distributed under the terms of the Apache version 2 licence,     *
* copied verbatim in the file "LICENSE".                                            *
*                                                                                   *
* In applying this licence, CERN does not waive the privileges and immunities       *
* granted to it by virtue of its status as an Intergovernmental Organization        *
* or submit itself to any jurisdiction.                                             *
\***********************************************************************************/
#pragma once
 
#include <Gaudi/Algorithm.h>
#include <Gaudi/Functional/Consumer.h>
#include <Gaudi/details/BranchWrapper.h>
#include <GaudiKernel/StdArrayAsProperty.h>
#include <TFile.h>
#include <TTree.h>
#include <fmt/format.h>
#include <gsl/pointers>
#include <gsl/span>
#include <mutex>
#include <numeric>
#include <tuple>
#include <utility>
 
namespace Gaudi::NTuple {
 
  template <typename Signature>
  struct WriterMixin {
    WriterMixin() = 0; // This function has to be implemented with this speialization:
                       // WriterMixin<std::tuple<OUTPUTs...>( const INPUTs&... )>
  };
 
  template <typename... OUTPUTs, typename... INPUTs>
  struct WriterMixin<std::tuple<OUTPUTs...>( const INPUTs&... )> {
    TTree*                                        m_tree = nullptr;   // Pointer to the TTree being written to
    std::array<std::string, sizeof...( OUTPUTs )> m_branchNames;      // Names of the branches to be created
    mutable std::vector<details::BranchWrapper>   m_branchWrappers{}; // Container for BranchWrapper objects
 
    virtual std::tuple<OUTPUTs...> transform( const INPUTs&... inputs ) const = 0;
 
    // Initialize the TTree and creates branches
    void initTree( const std::unique_ptr<TFile>& file, const gsl::span<std::string, sizeof...( OUTPUTs )> branchNames,
                   const Gaudi::Algorithm& algRef ) {
      file->cd();
      m_tree = std::make_unique<TTree>( "WriterTree", "Tree of Writer Algorithm" ).release();
      if ( !m_tree ) {
        throw GaudiException( "Failed to create TTree. Ensure sufficient resources and permissions.", algRef.name(),
                              StatusCode::FAILURE );
      }
      m_branchWrappers.reserve( m_branchWrappers.size() + sizeof...( OUTPUTs ) );
      createBranchesForOutputs( branchNames, std::make_index_sequence<sizeof...( OUTPUTs )>{}, algRef );
    }
 
    // Create branches in the TTree based on the provided names and output data types
    template <std::size_t... Is>
    void createBranchesForOutputs( const gsl::span<std::string, sizeof...( OUTPUTs )> branchNames,
                                   const std::index_sequence<Is...>, const Gaudi::Algorithm& algRef ) const {
      ( ...,
        m_branchWrappers.emplace_back(
            m_tree, System::typeinfoName( typeid( typename std::tuple_element<Is, std::tuple<OUTPUTs...>>::type ) ),
            branchNames[Is], "", algRef ) );
    }
 
    // Fill the TTree with transformed data from the input
    void fillTree( const INPUTs&... inputs ) const {
      auto transformedData = transform( inputs... );
      std::apply(
          [&]( const auto&... elems ) {
            size_t index = 0;
            ( ..., m_branchWrappers[index++].setDataPtr( const_cast<void*>( static_cast<const void*>( &elems ) ) ) );
          },
          transformedData );
      m_tree->Fill();
    }
 
    // Write the TTree to the associated ROOT file
    void writeTree( const std::unique_ptr<TFile>& file, const Gaudi::Algorithm& algRef ) {
      file->cd();
      if ( m_tree->Write() <= 0 ) {
        throw GaudiException( "Failed to write TTree to ROOT file.", algRef.name(), StatusCode::FAILURE );
      }
      m_tree = nullptr;
      algRef.info() << "TTree written to TFile." << endmsg;
    }
 
    virtual ~WriterMixin() = default;
  };
 
  template <typename Signature, typename Traits_ = Gaudi::Functional::Traits::BaseClass_t<Gaudi::Algorithm>>
  struct Writer {
    Writer() = 0; // If you wish to not provide any transformation for your data, please use the NTuple::GenericWriter
                  // algorithm
  };
 
  template <typename... OUTPUTs, typename... INPUTs, typename Traits_>
  struct Writer<std::tuple<OUTPUTs...>( const INPUTs&... ), Traits_>
      : Gaudi::Functional::Consumer<void( const INPUTs&... ), Traits_>,
        WriterMixin<std::tuple<OUTPUTs...>( const INPUTs&... )> {
    using Consumer_t = Gaudi::Functional::Consumer<void( const INPUTs&... ), Traits_>;
    using Consumer_t::Consumer_t;
 
    Gaudi::Property<std::string> m_filename{ this, "TreeFilename", "ntuple_writer_ts_tree.root",
                                             "Filename for the TTree." }; // Filename for the TTree
    Gaudi::Property<std::array<std::string, sizeof...( OUTPUTs )>> m_branchNames{
        this, "BranchNames", {}, "Names of the tree branches." }; // Names for the tree branches
    std::unique_ptr<TFile> m_file = nullptr;                      // Pointer to the ROOT file
    mutable std::mutex     m_mtx;                                 // Mutex for thread-safe operations
 
    // Initialize the algorithm, set up the ROOT file and a TTree branch for each input location
    virtual StatusCode initialize() override {
      return Consumer_t::initialize().andThen( [this]() {
        m_file = std::make_unique<TFile>( m_filename.value().c_str(), "RECREATE" );
        if ( !m_file || m_file->IsZombie() ) {
          throw GaudiException(
              fmt::format( "Failed to open file '{}'. Check file path and permissions.", m_filename.value() ),
              this->name(), StatusCode::FAILURE );
        }
 
        this->initTree( m_file, m_branchNames.value(), *this );
 
        return StatusCode::SUCCESS;
      } );
    }
 
    // Execute the algorithm for each event, retrieving data from the event store and writing it to the TTree
    void operator()( const INPUTs&... args ) const override {
      std::lock_guard<std::mutex> lock( m_mtx );
      this->fillTree( args... );
    }
 
    // Finalize the algorithm by writing the TTree to the file and closing it
    virtual StatusCode finalize() override {
      this->writeTree( m_file, *this );
      return Consumer_t::finalize();
    }
  };
} // namespace Gaudi::NTuple