[ntuple] automatically evolve fixed size array into RVec

Author: jblomer

tree/ntuple/inc/ROOT/RField/RFieldSequenceContainer.hxx

@@ -154,6 +154,7 @@ protected:
    void ReadGlobalImpl(ROOT::NTupleSize_t globalIndex, void *to) final;
    std::size_t ReadBulkImpl(const RBulkSpec &bulkSpec) final;
 
+   std::unique_ptr<RFieldBase> BeforeConnectPageSource(ROOT::Internal::RPageSource &pageSource) final;
    void ReconcileOnDiskField(const RNTupleDescriptor &desc) final;
 
    void CommitClusterImpl() final { fNWritten = 0; }

tree/ntuple/src/RFieldSequenceContainer.cxx

@@ -424,6 +424,22 @@ void ROOT::RRVecField::GenerateColumns(const ROOT::RNTupleDescriptor &desc)
    GenerateColumnsImpl<ROOT::Internal::RColumnIndex>(desc);
 }
 
+std::unique_ptr<ROOT::RFieldBase> ROOT::RRVecField::BeforeConnectPageSource(Internal::RPageSource &pageSource)
+{
+   if (GetOnDiskId() == kInvalidDescriptorId)
+      return nullptr;
+
+   const auto descGuard = pageSource.GetSharedDescriptorGuard();
+   const auto &fieldDesc = descGuard->GetFieldDescriptor(GetOnDiskId());
+   if (fieldDesc.GetTypeName().rfind("std::array<", 0) == 0) {
+      auto substitute = std::make_unique<RArrayAsRVecField>(
+         GetFieldName(), fSubfields[0]->Clone(fSubfields[0]->GetFieldName()), fieldDesc.GetNRepetitions());
+      substitute->SetOnDiskId(GetOnDiskId());
+      return substitute;
+   }
+   return nullptr;
+}
+
 void ROOT::RRVecField::ReconcileOnDiskField(const RNTupleDescriptor &desc)
 {
    EnsureMatchingOnDiskField(desc.GetFieldDescriptor(GetOnDiskId()), kDiffTypeName);

tree/ntuple/test/ntuple_evolution_shape.cxx

@@ -956,6 +956,50 @@ struct RenamedIntermediateDerived : public RenamedIntermediate2 {
    }
 }
 
+TEST(RNTupleEvolution, ArrayAsVector)
+{
+   FileRaii fileGuard("test_ntuple_evolution_array_as_vector.root");
+
+   ExecInFork([&] {
+      // The child process writes the file and exits, but the file must be preserved to be read by the parent.
+      fileGuard.PreserveFile();
+
+      ASSERT_TRUE(gInterpreter->Declare(R"(
+struct ArrayAsVector {
+   std::array<int, 2> fArr = {1, 2};
+   int x = 3;
+};
+)"));
+
+      auto model = RNTupleModel::Create();
+      model->AddField(RFieldBase::Create("f", "ArrayAsVector").Unwrap());
+
+      auto writer = RNTupleWriter::Recreate(std::move(model), "ntpl", fileGuard.GetPath());
+      writer->Fill();
+
+      writer.reset();
+   });
+
+   ASSERT_TRUE(gInterpreter->Declare(R"(
+struct ArrayAsVector {
+   ROOT::RVec<short> fArr;
+   int x;
+};
+)"));
+
+   auto reader = RNTupleReader::Open("ntpl", fileGuard.GetPath());
+   ASSERT_EQ(1, reader->GetNEntries());
+
+   void *ptr = reader->GetModel().GetDefaultEntry().GetPtr<void>("f").get();
+   DeclarePointer("ArrayAsVector", "ptrArrayAsVector", ptr);
+
+   reader->LoadEntry(0);
+   EXPECT_EVALUATE_EQ("ptrArrayAsVector->x", 3);
+   EXPECT_EVALUATE_EQ("ptrArrayAsVector->fArr.size()", 2);
+   EXPECT_EVALUATE_EQ("ptrArrayAsVector->fArr[0]", 1);
+   EXPECT_EVALUATE_EQ("ptrArrayAsVector->fArr[1]", 2);
+}
+
 TEST(RNTupleEvolution, StreamerField)
 {
    FileRaii fileGuard("test_ntuple_evolution_streamer_field.root");

tree/ntuple/test/ntuple_evolution_type.cxx

@@ -229,3 +229,25 @@ TEST(RNTupleEvolution, Enum)
    EXPECT_EQ(42, ve1(0));
    EXPECT_EQ(137, ve2(0));
 }
+
+TEST(RNTupleEvolution, ArrayAsRVec)
+{
+   FileRaii fileGuard("test_ntuple_evolution_array_as_rvec.root");
+   {
+      auto model = ROOT::RNTupleModel::Create();
+      auto a = model->MakeField<std::array<int, 2>>("a");
+      auto writer = ROOT::RNTupleWriter::Recreate(std::move(model), "ntpl", fileGuard.GetPath());
+
+      *a = {1, 2};
+      writer->Fill();
+   }
+
+   auto reader = RNTupleReader::Open("ntpl", fileGuard.GetPath());
+
+   auto a = reader->GetView<ROOT::RVec<short>>("a");
+   const auto &f = a.GetField(); // necessary to silence clang warning
+   EXPECT_EQ(typeid(f), typeid(ROOT::RArrayAsRVecField));
+   EXPECT_EQ(2u, a(0).size());
+   EXPECT_EQ(1, a(0)[0]);
+   EXPECT_EQ(2, a(0)[1]);
+}