[ntuple] add streaming vector tutorial

Author: jblomer

tutorials/v7/ntuple/ntpl015_streaming_vector.C

@@ -0,0 +1,148 @@
+/// \file
+/// \ingroup tutorial_ntuple
+///
+/// Example of a streaming vector: a special purpose container that reads large vectors piece-wise.
+///
+/// \macro_code
+///
+/// \date November 2024
+/// \author Peter van Gemmeren, the ROOT Team
+
+// NOTE: The RNTuple classes are experimental at this point.
+// Functionality and interface are still subject to changes.
+
+#include <ROOT/RNTupleModel.hxx>
+#include <ROOT/RNTupleReader.hxx>
+#include <ROOT/RNTupleUtil.hxx>
+#include <ROOT/RNTupleView.hxx>
+#include <ROOT/RNTupleWriter.hxx>
+
+#include <TRandom3.h>
+
+#include <cstdint>
+#include <iostream>
+
+#include <vector>
+
+using namespace ROOT::Experimental;
+
+constexpr char const* kFileName = "ntpl015_streaming_vector.root";
+constexpr char const* kNTupleName = "ntpl";
+constexpr char const* kFieldName = "LargeVector";
+constexpr unsigned int kNEvents = 10;
+constexpr unsigned int kVectorSize = 1'000'000;
+
+void CreateRNTuple() {
+   std::unique_ptr<RNTupleModel> model = RNTupleModel::Create();
+   auto ptrLargeVector = model->MakeField<std::vector<std::uint32_t>>(kFieldName);
+   auto writer = RNTupleWriter::Recreate(std::move(model), kNTupleName, kFileName);
+
+   auto prng = std::make_unique<TRandom3>();
+   prng->SetSeed();
+
+   for (NTupleSize_t i = 0; i < kNEvents; i++) {
+      ptrLargeVector->clear();
+      for (std::size_t j = 0; j < kVectorSize; j++)
+         ptrLargeVector->emplace_back(prng->Integer(-1));
+      writer->Fill();
+   }
+   std::cout << "RNTuple written" << std::endl;
+}
+
+/*
+ * ==================================================================================================
+ */
+
+void ReadRNTupleSimple()
+{
+   auto reader = RNTupleReader::Open(kNTupleName, kFileName);
+
+   const auto nEntries = reader->GetNEntries();
+   std::cout << "Simple reading, found " << nEntries << " entries" << std::endl;
+
+   auto ptrLargeVector = reader->GetModel().GetDefaultEntry().GetPtr<std::vector<std::uint32_t>>(kFieldName);
+   for (NTupleSize_t i = 0; i < nEntries; i++) {
+      reader->LoadEntry(i);
+
+      const auto vectorSize = ptrLargeVector->size();
+      uint64_t sum = 0;
+      for (auto val : *ptrLargeVector)
+         sum += val;
+
+      std::cout << "Size and sum of vector: " << vectorSize << " " << sum << std::endl;
+    }
+    std::cout << "RNTuple simple read" << std::endl;
+}
+
+/*
+ * ==================================================================================================
+ */
+
+template <class T>
+class RStreamingVector {
+   RNTupleCollectionView fVectorView;
+   RNTupleView<T> fItemView;
+   std::unique_ptr<RNTupleClusterRange> fRange;
+   NTupleSize_t fEntry{0};
+   NTupleSize_t fSize{0};
+
+public:
+   class iterator {
+      RNTupleClusterRange::RIterator fIndex;
+      RNTupleView<T> &fView;
+
+   public:
+      iterator(RNTupleClusterRange::RIterator index, RNTupleView<T> &view) : fIndex(index), fView(view) {}
+      ~iterator() = default;
+
+      iterator  operator++(int) /* postfix */        { auto r = *this; ++(*this); return r; }
+      iterator& operator++()    /* prefix */         { ++fIndex; return *this; }
+      const T& operator* ()                          { return fView.operator()(*fIndex); }
+      const T* operator->()                          { return &fView.operator()(*fIndex); }
+      bool      operator==(const iterator& rh) const { return fIndex == rh.fIndex; }
+      bool      operator!=(const iterator& rh) const { return fIndex != rh.fIndex; }
+   };
+
+   RStreamingVector(RNTupleReader &reader, const std::string &fieldName)
+      : fVectorView(reader.GetCollectionView(fieldName)), fItemView(fVectorView.GetView<T>("_0")) {}
+   ~RStreamingVector() = default;
+
+   NTupleSize_t size() const { return fSize; }
+
+   iterator begin() { return iterator(fRange->begin(), fItemView); }
+   iterator end()   { return iterator(fRange->end(), fItemView); }
+
+   void LoadEntry(NTupleSize_t entry) {
+      fEntry = entry;
+      fRange = std::make_unique<RNTupleClusterRange>(fVectorView.GetCollectionRange(fEntry));
+      fSize = fVectorView.operator()(fEntry);
+   }
+};
+
+void ReadRNTupleStreamingVector()
+{
+   auto reader = RNTupleReader::Open(kNTupleName, kFileName);
+
+   const auto nEntries = reader->GetNEntries();
+   std::cout << "Streamed reading, found " << nEntries << " entries" << std::endl;
+
+   RStreamingVector<std::uint32_t> streamingVector(*reader, kFieldName);
+   for (NTupleSize_t i = 0; i < nEntries; i++) {
+      streamingVector.LoadEntry(i);
+
+      const auto vectorSize = streamingVector.size();
+      uint64_t sum = 0;
+      for (auto val : streamingVector)
+         sum += val;
+
+      std::cout << "Size and sum of vector: " << vectorSize << " " << sum << std::endl;
+   }
+   std::cout << "RNTuple streaming read" << std::endl;
+}
+
+void ntpl015_streaming_vector()
+{
+   CreateRNTuple();
+   ReadRNTupleSimple();
+   ReadRNTupleStreamingVector();
+}