[ntuple] Support creating ntuples within directories in RNTupleImporter

tree/ntupleutil/v7/inc/ROOT/RNTupleImporter.hxx

@@ -56,6 +56,7 @@ importer->Import();
 ~~~
 
 The output file is created if it does not exist, otherwise the ntuple is added to the existing file.
+Directories in the output file are created as necessary, allowing ntuples to be stored in a nested structure (e.g. DirName/TreeName).
 Note that input file and output file can be identical if the ntuple is stored under a different name than the tree
 (use `SetNTupleName()`).
 

tree/ntupleutil/v7/src/RNTupleImporter.cxx

@@ -371,13 +371,28 @@ ROOT::RResult<void> ROOT::Experimental::RNTupleImporter::PrepareSchema()
 
 void ROOT::Experimental::RNTupleImporter::Import()
 {
-   if (fDestFile->FindKey(fNTupleName.c_str()) != nullptr)
+   TDirectory *targetDir = fDestFile.get();
+   // Extract the ntuple name and directory name
+   auto lastSlash = fNTupleName.find_last_of('/');
+   std::string ntupleName = fNTupleName;
+   if (lastSlash != std::string::npos) {
+      // Create the directory in the output file if it does not exist
+      auto dirName = fNTupleName.substr(0, lastSlash);
+      ntupleName = fNTupleName.substr(lastSlash + 1);
+      targetDir = fDestFile->mkdir(dirName.c_str(), "", true);
+      if (!targetDir) {
+         throw RException(R__FAIL("Failed to create directory " + dirName + " in file " + fDestFileName));
+      }
+   }
+
+   if (targetDir->FindKey(ntupleName.c_str()) != nullptr) {
       throw RException(R__FAIL("Key '" + fNTupleName + "' already exists in file " + fDestFileName));
+   }
 
    PrepareSchema();
 
    std::unique_ptr<ROOT::Internal::RPageSink> sink =
-      std::make_unique<ROOT::Internal::RPageSinkFile>(fNTupleName, *fDestFile, fWriteOptions);
+      std::make_unique<ROOT::Internal::RPageSinkFile>(ntupleName, *targetDir, fWriteOptions);
    sink->GetMetrics().Enable();
    auto ctrZippedBytes = sink->GetMetrics().GetCounter("RPageSinkFile.szWritePayload");
 

tree/ntupleutil/v7/test/ntuple_importer.cxx

@@ -35,6 +35,28 @@ TEST(RNTupleImporter, Empty)
    EXPECT_THROW(importer->Import(), ROOT::RException);
 }
 
+TEST(RNTupleImporter, CreateInDirectory)
+{
+   FileRaii fileGuard("test_ntuple_importer_create_in_directory.root");
+   {
+      std::unique_ptr<TFile> file(TFile::Open(fileGuard.GetPath().c_str(), "RECREATE"));
+      TDirectory *dir = file->mkdir("dir1");
+      ASSERT_NE(dir, nullptr);
+      file->cd("dir1");
+      auto tree = std::make_unique<TTree>("tree", "");
+      tree->Write();
+   }
+
+   auto importer = RNTupleImporter::Create(fileGuard.GetPath(), "dir1/tree", fileGuard.GetPath());
+   importer->SetIsQuiet(true);
+   EXPECT_THROW(importer->Import(), ROOT::RException);
+   importer->SetNTupleName("dir2/ntuple");
+   importer->Import();
+   auto reader = RNTupleReader::Open("dir2/ntuple", fileGuard.GetPath());
+   EXPECT_EQ(0U, reader->GetNEntries());
+   EXPECT_THROW(importer->Import(), ROOT::RException);
+}
+
 TEST(RNTupleImporter, CreateFromTree)
 {
    FileRaii fileGuard("test_ntuple_importer_empty.root");