[ntuple] Add RMiniFile::GetNTupleProperAtOffset

Author: silverweed

tree/ntuple/inc/ROOT/RMiniFile.hxx

@@ -68,6 +68,8 @@ private:
    /// Used when the file turns out to be a TFile container. The ntuplePath variable is either the ntuple name
    /// or an ntuple name preceded by a directory (`myNtuple` or `foo/bar/myNtuple` or `/foo/bar/myNtuple`)
    RResult<RNTuple> GetNTupleProper(std::string_view ntuplePath);
+   /// Loads an RNTuple anchor from a TFile at the given file offset (unzipping it if necessary).
+   RResult<RNTuple> GetNTupleProperAtOffset(std::uint64_t keyOffset);
 
    /// Searches for a key with the given name and type in the key index of the directory starting at offsetDir.
    /// The offset points to the start of the TDirectory DATA section, without the key and without the name and title

tree/ntuple/src/RMiniFile.cxx

@@ -754,6 +754,14 @@ ROOT::RResult<ROOT::RNTuple> ROOT::Internal::RMiniFileReader::GetNTupleProper(st
       return R__FAIL("no RNTuple named '" + std::string(ntupleName) + "' in file '" + fRawFile->GetUrl() + "'");
    }
 
+   auto res = GetNTupleProperAtOffset(offset);
+   return res;
+}
+
+ROOT::RResult<ROOT::RNTuple> ROOT::Internal::RMiniFileReader::GetNTupleProperAtOffset(std::uint64_t keyOffset)
+{
+   auto offset = keyOffset;
+   RTFKey key;
    ReadBuffer(&key, sizeof(key), offset);
    offset = key.GetSeekKey() + key.fKeyLen;