RNTupleReader.hxx

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/// \file ROOT/RNTupleReader.hxx
/// \ingroup NTuple ROOT7
/// \author Jakob Blomer <jblomer@cern.ch>
/// \date 2024-02-20
/// \warning This is part of the ROOT 7 prototype! It will change without notice. It might trigger earthquakes. Feedback
/// is welcome!
 
/*************************************************************************
 * Copyright (C) 1995-2024, Rene Brun and Fons Rademakers.               *
 * All rights reserved.                                                  *
 *                                                                       *
 * For the licensing terms see $ROOTSYS/LICENSE.                         *
 * For the list of contributors see $ROOTSYS/README/CREDITS.             *
 *************************************************************************/
 
#ifndef ROOT7_RNTupleReader
#define ROOT7_RNTupleReader
 
#include <ROOT/RConfig.hxx> // for R__unlikely
#include <ROOT/RError.hxx>
#include <ROOT/RNTupleDescriptor.hxx>
#include <ROOT/RNTupleMetrics.hxx>
#include <ROOT/RNTupleModel.hxx>
#include <ROOT/RNTupleReadOptions.hxx>
#include <ROOT/RNTupleUtil.hxx>
#include <ROOT/RNTupleView.hxx>
#include <ROOT/RPageStorage.hxx>
#include <ROOT/RSpan.hxx>
 
#include <iostream>
#include <iterator>
#include <memory>
#include <string>
#include <string_view>
 
namespace ROOT {
namespace Experimental {
 
class REntry;
class RNTuple;
 
/// Listing of the different options that can be printed by RNTupleReader::GetInfo()
enum class ENTupleInfo {
   kSummary,        // The ntuple name, description, number of entries
   kStorageDetails, // size on storage, page sizes, compression factor, etc.
   kMetrics,        // internals performance counters, requires that EnableMetrics() was called
};
 
// clang-format off
/**
\class ROOT::Experimental::RNTupleReader
\ingroup NTuple
\brief An RNTuple that is used to read data from storage
 
An input ntuple provides data from storage as C++ objects. The ntuple model can be created from the data on storage
or it can be imposed by the user. The latter case allows users to read into a specialized ntuple model that covers
only a subset of the fields in the ntuple. The ntuple model is used when reading complete entries.
Individual fields can be read as well by instantiating a tree view.
 
~~~ {.cpp}
#include <ROOT/RNTupleReader.hxx>
using ROOT::Experimental::RNTupleReader;
 
#include <iostream>
 
auto ntuple = RNTupleReader::Open("myNTuple", "some/file.root");
std::cout << "myNTuple has " << ntuple->GetNEntries() << " entries\n";
~~~
*/
// clang-format on
class RNTupleReader {
private:
   /// Set as the page source's scheduler for parallel page decompression if IMT is on
   /// Needs to be destructed after the pages source is destructed (an thus be declared before)
   std::unique_ptr<Internal::RPageStorage::RTaskScheduler> fUnzipTasks;
 
   std::unique_ptr<Internal::RPageSource> fSource;
   /// Needs to be destructed before fSource
   std::unique_ptr<RNTupleModel> fModel;
   /// We use a dedicated on-demand reader for Show() and Scan(). Printing data uses all the fields
   /// from the full model even if the analysis code uses only a subset of fields. The display reader
   /// is a clone of the original reader.
   std::unique_ptr<RNTupleReader> fDisplayReader;
   /// The ntuple descriptor in the page source is protected by a read-write lock. We don't expose that to the
   /// users of RNTupleReader::GetDescriptor().  Instead, if descriptor information is needed, we clone the
   /// descriptor.  Using the descriptor's generation number, we know if the cached descriptor is stale.
   /// Retrieving descriptor data from an RNTupleReader is supposed to be for testing and information purposes,
   /// not on a hot code path.
   std::unique_ptr<RNTupleDescriptor> fCachedDescriptor;
   Detail::RNTupleMetrics fMetrics;
 
   RNTupleReader(std::unique_ptr<RNTupleModel> model, std::unique_ptr<Internal::RPageSource> source);
   /// The model is generated from the ntuple metadata on storage.
   explicit RNTupleReader(std::unique_ptr<Internal::RPageSource> source);
 
   void ConnectModel(RNTupleModel &model);
   RNTupleReader *GetDisplayReader();
   void InitPageSource();
 
   DescriptorId_t RetrieveFieldId(std::string_view fieldName) const;
 
public:
   // Browse through the entries
   class RIterator {
   private:
      NTupleSize_t fIndex = kInvalidNTupleIndex;
 
   public:
      using iterator = RIterator;
      using iterator_category = std::forward_iterator_tag;
      using value_type = NTupleSize_t;
      using difference_type = NTupleSize_t;
      using pointer = NTupleSize_t *;
      using reference = NTupleSize_t &;
 
      RIterator() = default;
      explicit RIterator(NTupleSize_t index) : fIndex(index) {}
      ~RIterator() = default;
 
      iterator operator++(int) /* postfix */
      {
         auto r = *this;
         fIndex++;
         return r;
      }
      iterator &operator++() /* prefix */
      {
         ++fIndex;
         return *this;
      }
      reference operator*() { return fIndex; }
      pointer operator->() { return &fIndex; }
      bool operator==(const iterator &rh) const { return fIndex == rh.fIndex; }
      bool operator!=(const iterator &rh) const { return fIndex != rh.fIndex; }
   };
 
   /// Used to specify the underlying RNTuples in OpenFriends()
   struct ROpenSpec {
      std::string fNTupleName;
      std::string fStorage;
      RNTupleReadOptions fOptions;
 
      ROpenSpec() = default;
      ROpenSpec(std::string_view n, std::string_view s) : fNTupleName(n), fStorage(s) {}
   };
 
   /// Open an RNTuple for reading.
   ///
   /// Throws an RException if there is no RNTuple with the given name.
   ///
   /// **Example: open an RNTuple and print the number of entries**
   /// ~~~ {.cpp}
   /// #include <ROOT/RNTupleReader.hxx>
   /// using ROOT::Experimental::RNTupleReader;
   ///
   /// #include <iostream>
   ///
   /// auto ntuple = RNTupleReader::Open("myNTuple", "some/file.root");
   /// std::cout << "myNTuple has " << ntuple->GetNEntries() << " entries\n";
   /// ~~~
   static std::unique_ptr<RNTupleReader> Open(std::string_view ntupleName, std::string_view storage,
                                              const RNTupleReadOptions &options = RNTupleReadOptions());
   static std::unique_ptr<RNTupleReader>
   Open(RNTuple *ntuple, const RNTupleReadOptions &options = RNTupleReadOptions());
   /// The caller imposes a model, which must be compatible with the model found in the data on storage.
   static std::unique_ptr<RNTupleReader> Open(std::unique_ptr<RNTupleModel> model, std::string_view ntupleName,
                                              std::string_view storage,
                                              const RNTupleReadOptions &options = RNTupleReadOptions());
   static std::unique_ptr<RNTupleReader>
   Open(std::unique_ptr<RNTupleModel> model, RNTuple *ntuple, const RNTupleReadOptions &options = RNTupleReadOptions());
   /// Open RNTuples as one virtual, horizontally combined ntuple.  The underlying RNTuples must
   /// have an identical number of entries.  Fields in the combined RNTuple are named with the ntuple name
   /// as a prefix, e.g. myNTuple1.px and myNTuple2.pt (see tutorial ntpl006_friends)
   static std::unique_ptr<RNTupleReader> OpenFriends(std::span<ROpenSpec> ntuples);
   std::unique_ptr<RNTupleReader> Clone()
   {
      return std::unique_ptr<RNTupleReader>(new RNTupleReader(fSource->Clone()));
   }
   ~RNTupleReader();
 
   NTupleSize_t GetNEntries() const { return fSource->GetNEntries(); }
   const RNTupleModel &GetModel();
 
   /// Returns a cached copy of the page source descriptor. The returned pointer remains valid until the next call
   /// to LoadEntry or to any of the views returned from the reader.
   const RNTupleDescriptor &GetDescriptor();
 
   /// Prints a detailed summary of the ntuple, including a list of fields.
   ///
   /// **Example: print summary information to stdout**
   /// ~~~ {.cpp}
   /// #include <ROOT/RNTupleReader.hxx>
   /// using ROOT::Experimental::ENTupleInfo;
   /// using ROOT::Experimental::RNTupleReader;
   ///
   /// #include <iostream>
   ///
   /// auto ntuple = RNTupleReader::Open("myNTuple", "some/file.root");
   /// ntuple->PrintInfo();
   /// // or, equivalently:
   /// ntuple->PrintInfo(ENTupleInfo::kSummary, std::cout);
   /// ~~~
   /// **Example: print detailed column storage data to stderr**
   /// ~~~ {.cpp}
   /// #include <ROOT/RNTupleReader.hxx>
   /// using ROOT::Experimental::ENTupleInfo;
   /// using ROOT::Experimental::RNTupleReader;
   ///
   /// #include <iostream>
   ///
   /// auto ntuple = RNTupleReader::Open("myNTuple", "some/file.root");
   /// ntuple->PrintInfo(ENTupleInfo::kStorageDetails, std::cerr);
   /// ~~~
   ///
   /// For use of ENTupleInfo::kMetrics, see #EnableMetrics.
   void PrintInfo(const ENTupleInfo what = ENTupleInfo::kSummary, std::ostream &output = std::cout);
 
   /// Shows the values of the i-th entry/row, starting with 0 for the first entry. By default,
   /// prints the output in JSON format.
   /// Uses the visitor pattern to traverse through each field of the given entry.
   void Show(NTupleSize_t index, std::ostream &output = std::cout);
 
   /// Analogous to Fill(), fills the default entry of the model. Returns false at the end of the ntuple.
   /// On I/O errors, raises an exception.
   void LoadEntry(NTupleSize_t index)
   {
      // TODO(jblomer): can be templated depending on the factory method / constructor
      if (R__unlikely(!fModel)) {
         fModel = fSource->GetSharedDescriptorGuard()->CreateModel();
         ConnectModel(*fModel);
      }
      LoadEntry(index, fModel->GetDefaultEntry());
   }
   /// Fills a user provided entry after checking that the entry has been instantiated from the ntuple model
   void LoadEntry(NTupleSize_t index, REntry &entry) { entry.Read(index); }
 
   /// Returns an iterator over the entry indices of the RNTuple.
   ///
   /// **Example: iterate over all entries and print each entry in JSON format**
   /// ~~~ {.cpp}
   /// #include <ROOT/RNTupleReader.hxx>
   /// using ROOT::Experimental::ENTupleShowFormat;
   /// using ROOT::Experimental::RNTupleReader;
   ///
   /// #include <iostream>
   ///
   /// auto ntuple = RNTupleReader::Open("myNTuple", "some/file.root");
   /// for (auto i : ntuple->GetEntryRange()) {
   ///    ntuple->Show(i);
   /// }
   /// ~~~
   RNTupleGlobalRange GetEntryRange() { return RNTupleGlobalRange(0, GetNEntries()); }
 
   /// Provides access to an individual field that can contain either a scalar value or a collection, e.g.
   /// GetView<double>("particles.pt") or GetView<std::vector<double>>("particle").  It can as well be the index
   /// field of a collection itself, like GetView<NTupleSize_t>("particle").
   ///
   /// Raises an exception if there is no field with the given name.
   ///
   /// **Example: iterate over a field named "pt" of type `float`**
   /// ~~~ {.cpp}
   /// #include <ROOT/RNTupleReader.hxx>
   /// using ROOT::Experimental::RNTupleReader;
   ///
   /// #include <iostream>
   ///
   /// auto ntuple = RNTupleReader::Open("myNTuple", "some/file.root");
   /// auto pt = ntuple->GetView<float>("pt");
   ///
   /// for (auto i : ntuple->GetEntryRange()) {
   ///    std::cout << i << ": " << pt(i) << "\n";
   /// }
   /// ~~~
   template <typename T>
   RNTupleView<T, false> GetView(std::string_view fieldName)
   {
      return GetView<T>(RetrieveFieldId(fieldName));
   }
 
   template <typename T>
   RNTupleView<T, true> GetView(std::string_view fieldName, std::shared_ptr<T> objPtr)
   {
      return GetView<T>(RetrieveFieldId(fieldName), objPtr);
   }
 
   template <typename T>
   RNTupleView<T, false> GetView(DescriptorId_t fieldId)
   {
      return RNTupleView<T, false>(fieldId, fSource.get());
   }
 
   template <typename T>
   RNTupleView<T, true> GetView(DescriptorId_t fieldId, std::shared_ptr<T> objPtr)
   {
      return RNTupleView<T, true>(fieldId, fSource.get(), objPtr);
   }
 
   /// Raises an exception if:
   /// * there is no field with the given name or,
   /// * the field is not a collection
   RNTupleCollectionView GetCollectionView(std::string_view fieldName)
   {
      auto fieldId = fSource->GetSharedDescriptorGuard()->FindFieldId(fieldName);
      if (fieldId == kInvalidDescriptorId) {
         throw RException(R__FAIL("no field named '" + std::string(fieldName) + "' in RNTuple '" +
                                  fSource->GetSharedDescriptorGuard()->GetName() + "'"));
      }
      return GetCollectionView(fieldId);
   }
 
   RNTupleCollectionView GetCollectionView(DescriptorId_t fieldId)
   {
      return RNTupleCollectionView(fieldId, fSource.get());
   }
 
   RIterator begin() { return RIterator(0); }
   RIterator end() { return RIterator(GetNEntries()); }
 
   /// Enable performance measurements (decompression time, bytes read from storage, etc.)
   ///
   /// **Example: inspect the reader metrics after loading every entry**
   /// ~~~ {.cpp}
   /// #include <ROOT/RNTupleReader.hxx>
   /// using ROOT::Experimental::ENTupleInfo;
   /// using ROOT::Experimental::RNTupleReader;
   ///
   /// #include <iostream>
   ///
   /// auto ntuple = RNTupleReader::Open("myNTuple", "some/file.root");
   /// // metrics must be turned on beforehand
   /// ntuple->EnableMetrics();
   ///
   /// for (auto i : ntuple->GetEntryRange()) {
   ///    ntuple->LoadEntry(i);
   /// }
   /// ntuple->PrintInfo(ENTupleInfo::kMetrics);
   /// ~~~
   void EnableMetrics() { fMetrics.Enable(); }
   const Detail::RNTupleMetrics &GetMetrics() const { return fMetrics; }
}; // class RNTupleReader
 
} // namespace Experimental
} // namespace ROOT
 
#endif // ROOT7_RNTupleReader