[ntuple] Fix writing of unaligned SoA class

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

@@ -63,9 +63,10 @@ class RSoAField : public RFieldBase {
    };
 
    TClass *fSoAClass = nullptr;
-   std::vector<std::size_t> fSoAMemberOffsets;    ///< The offset of the RVec members in the SoA type
-   std::vector<std::size_t> fRecordMemberIndexes; ///< Maps the SoA members to the members of the underlying record
    std::vector<RFieldBase *> fRecordMemberFields; ///< Direct access to the member fields of the underlying record
+   /// The offset of the RVec members in the SoA type in the order of subfields of the underlying record type.
+   /// In particular, the order is not necessarily the same then the order of RVec members in the SoA class.
+   std::vector<std::size_t> fSoAMemberOffsets;
    std::size_t fMaxAlignment = 1;
    ROOT::Internal::RColumnIndex fNWritten;
 

tree/ntuple/src/RFieldMeta.cxx

@@ -658,7 +658,6 @@ ROOT::Experimental::RSoAField::RSoAField(std::string_view fieldName, const RSoAF
    : ROOT::RFieldBase(fieldName, source.GetTypeName(), ROOT::ENTupleStructure::kCollection, false /* isSimple */),
      fSoAClass(source.fSoAClass),
      fSoAMemberOffsets(source.fSoAMemberOffsets),
-     fRecordMemberIndexes(source.fRecordMemberIndexes),
      fMaxAlignment(source.fMaxAlignment)
 {
    fTraits = source.GetTraits();
@@ -716,6 +715,8 @@ ROOT::Experimental::RSoAField::RSoAField(std::string_view fieldName, TClass *clS
       throw RException(R__FAIL("SoA fields with inheritance are currently unsupported"));
    }
 
+   fSoAMemberOffsets.resize(fRecordMemberFields.size());
+   unsigned int nMembers = 0;
    for (auto dataMember : ROOT::Detail::TRangeStaticCast<TDataMember>(*fSoAClass->GetListOfDataMembers())) {
       if ((dataMember->Property() & kIsStatic) || !dataMember->IsPersistent())
          continue;
@@ -752,13 +753,13 @@ ROOT::Experimental::RSoAField::RSoAField(std::string_view fieldName, TClass *clS
 
       fMaxAlignment = std::max(fMaxAlignment, vecField->GetAlignment());
 
-      fSoAMemberOffsets.emplace_back(dataMember->GetOffset());
-      fRecordMemberIndexes.emplace_back(itr->second);
+      assert(itr->second < fSoAMemberOffsets.size());
+      fSoAMemberOffsets[itr->second] = dataMember->GetOffset();
+      nMembers++;
    }
-   if (recordFieldNameToIdx.size() != fSoAMemberOffsets.size()) {
+   if (recordFieldNameToIdx.size() != nMembers) {
       throw RException(R__FAIL("missing SoA members"));
    }
-   assert(fRecordMemberFields.size() == fSoAMemberOffsets.size());
 
    std::string renormalizedAlias;
    if (ROOT::Internal::NeedsMetaNameAsAlias(fSoAClass->GetName(), renormalizedAlias))

tree/ntuple/test/SoAField.hxx

@@ -51,6 +51,12 @@ struct SoASimple {
    ClassDefNV(SoASimple, 2);
 };
 
+struct SoASimpleSwapped {
+   ROOT::RVec<float> fY;
+   ROOT::RVec<float> fX;
+   ClassDefNV(SoASimpleSwapped, 2);
+};
+
 struct SoASimpleBadArray {
    ROOT::RVec<float> fX;
    float fY[3];

tree/ntuple/test/SoAFieldLinkDef.h

@@ -13,6 +13,7 @@
 
 #pragma link C++ class RecordSimple+;
 #pragma link C++ options=rntupleSoARecord(RecordSimple) class SoASimple+;
+#pragma link C++ options=rntupleSoARecord(RecordSimple) class SoASimpleSwapped+;
 #pragma link C++ options=rntupleSoARecord(RecordSimple) class SoASimpleBadArray+;
 #pragma link C++ options=rntupleSoARecord(RecordSimple) class SoASimpleBadType+;
 #pragma link C++ options=rntupleSoARecord(RecordSimple) class SoASimpleUnexpectedMember+;

tree/ntuple/test/ntuple_soa.cxx

@@ -204,3 +204,25 @@ TEST(RNTuple, SoASimple)
    EXPECT_FLOAT_EQ(5.0, v(2).at(1).fX);
    EXPECT_FLOAT_EQ(6.0, v(2).at(1).fY);
 }
+
+TEST(RNTuple, SoASimpleSwapped)
+{
+   ROOT::TestSupport::FileRaii fileGuard("test_rntuple_soa_simple_swapped.root");
+
+   {
+      auto model = ROOT::RNTupleModel::Create();
+      model->AddField(std::make_unique<RSoAField>("f", "SoASimpleSwapped"));
+      auto writer = ROOT::RNTupleWriter::Recreate(std::move(model), "ntpl", fileGuard.GetPath());
+      auto soa = writer->GetModel().GetDefaultEntry().GetPtr<SoASimpleSwapped>("f");
+      soa->fX.push_back(1.0);
+      soa->fY.push_back(2.0);
+      writer->Fill();
+   }
+
+   auto reader = ROOT::RNTupleReader::Open("ntpl", fileGuard.GetPath());
+   EXPECT_EQ(1u, reader->GetNEntries());
+   auto v = reader->GetView<std::vector<RecordSimple>>("f");
+   EXPECT_EQ(1u, v(0).size());
+   EXPECT_FLOAT_EQ(1.0, v(0).at(0).fX);
+   EXPECT_FLOAT_EQ(2.0, v(0).at(0).fY);
+}