From 535f5439dfa83c6292fa1d61f4c662c7978d3cf1 Mon Sep 17 00:00:00 2001
From: Jakob Blomer <jblomer@cern.ch>
Date: Wed, 6 Nov 2024 16:02:29 +0100
Subject: [PATCH] [ntuple] allow for writing into TFile subdir (WIP)

---
 tree/ntuple/v7/inc/ROOT/RMiniFile.hxx |  3 +++
 tree/ntuple/v7/src/RMiniFile.cxx      | 21 +++++++++++++++++++--
 2 files changed, 22 insertions(+), 2 deletions(-)

diff --git a/tree/ntuple/v7/inc/ROOT/RMiniFile.hxx b/tree/ntuple/v7/inc/ROOT/RMiniFile.hxx
index 412294ece0a38..9bd330c0d13c9 100644
--- a/tree/ntuple/v7/inc/ROOT/RMiniFile.hxx
+++ b/tree/ntuple/v7/inc/ROOT/RMiniFile.hxx
@@ -104,6 +104,7 @@ class RNTupleFileWriter {
 private:
    struct RFileProper {
       TFile *fFile = nullptr;
+      std::string fDirectory;
       /// Low-level writing using a TFile
       void Write(const void *buffer, size_t nbytes, std::int64_t offset);
       /// Writes an RBlob opaque key with the provided buffer as data record and returns the offset of the record
@@ -178,6 +179,8 @@ private:
 
    explicit RNTupleFileWriter(std::string_view name, std::uint64_t maxKeySize);
 
+   static std::pair<std::string_view, std::string_view> SplitNTupleName(std::string_view nameAndDir);
+
    /// For a TFile container written by a C file stream, write the header and TFile object
    void WriteTFileSkeleton(int defaultCompression);
    /// The only key that will be visible in file->ls()
diff --git a/tree/ntuple/v7/src/RMiniFile.cxx b/tree/ntuple/v7/src/RMiniFile.cxx
index f5de3a53bf854..7881f187d09c6 100644
--- a/tree/ntuple/v7/src/RMiniFile.cxx
+++ b/tree/ntuple/v7/src/RMiniFile.cxx
@@ -1105,12 +1105,27 @@ ROOT::Experimental::Internal::RNTupleFileWriter::Recreate(std::string_view ntupl
    return writer;
 }
 
+std::pair<std::string_view, std::string_view> ROOT::Experimental::Internal::RNTupleFileWriter::SplitNTupleName(
+   std::string_view nameAndDir)
+{
+   const auto posLastSlash = nameAndDir.find_last_of('/');
+   if (posLastSlash == std::string::npos)
+      return std::make_pair("", nameAndDir);
+
+   if (posLastSlash == nameAndDir.size() - 1)
+      throw RException(R__FAIL("empty RNTuple name"));
+
+   return std::make_pair(nameAndDir.substr(0, posLastSlash), nameAndDir.substr(posLastSlash + 1));
+}
+
 std::unique_ptr<ROOT::Experimental::Internal::RNTupleFileWriter>
 ROOT::Experimental::Internal::RNTupleFileWriter::Append(std::string_view ntupleName, TFile &file,
                                                         std::uint64_t maxKeySize)
 {
-   auto writer = std::unique_ptr<RNTupleFileWriter>(new RNTupleFileWriter(ntupleName, maxKeySize));
+   auto [dir, name] = SplitNTupleName(ntupleName);
+   auto writer = std::unique_ptr<RNTupleFileWriter>(new RNTupleFileWriter(name, maxKeySize));
    writer->fFileProper.fFile = &file;
+   writer->fFileProper.fDirectory = dir;
    return writer;
 }
 
@@ -1124,7 +1139,9 @@ void ROOT::Experimental::Internal::RNTupleFileWriter::Commit()
 {
    if (fFileProper) {
       // Easy case, the ROOT file header and the RNTuple streaming is taken care of by TFile
-      fFileProper.fFile->WriteObject(&fNTupleAnchor, fNTupleName.c_str());
+      auto dir = fFileProper.fFile->GetDirectory(fFileProper.fDirectory.c_str());
+      dir->cd();
+      dir->WriteObject(&fNTupleAnchor, fNTupleName.c_str());
 
       // Make sure the streamer info records used in the RNTuple are written to the file
       TBufferFile buf(TBuffer::kWrite);