[ntuple] add RPagePersistentSink::AddAliasColumn

Author: silverweed

tree/ntuple/inc/ROOT/RPageStorage.hxx

@@ -547,6 +547,10 @@ public:
    void
    AddColumnRepresentation(const ROOT::RFieldDescriptor &field, std::span<const ENTupleColumnType> newRepresentation);
 
+   /// Adds a new alias column pointing to an existing column with the given physical id to the given field.
+   void AddAliasColumn(const ROOT::RNTupleDescriptor &desc, const ROOT::RFieldDescriptor &field,
+                       ROOT::DescriptorId_t physicalId);
+
    void CommitSuppressedColumn(ColumnHandle_t columnHandle) final;
    void CommitPage(ColumnHandle_t columnHandle, const ROOT::Internal::RPage &page) final;
    void CommitSealedPage(ROOT::DescriptorId_t physicalColumnId, const RPageStorage::RSealedPage &sealedPage) final;

tree/ntuple/src/RPageStorage.cxx

@@ -1114,6 +1114,27 @@ void ROOT::Internal::RPagePersistentSink::AddColumnRepresentation(const ROOT::RF
    }
 }
 
+void ROOT::Internal::RPagePersistentSink::AddAliasColumn(const ROOT::RNTupleDescriptor &desc,
+                                                         const ROOT::RFieldDescriptor &field,
+                                                         ROOT::DescriptorId_t physicalId)
+{
+   const auto &pointedColumn = desc.GetColumnDescriptor(physicalId);
+   assert(!pointedColumn.IsAliasColumn());
+
+   const auto columnId = fDescriptorBuilder.GetDescriptor().GetNLogicalColumns();
+   RColumnDescriptorBuilder columnBuilder;
+   columnBuilder.LogicalColumnId(columnId)
+      .PhysicalColumnId(physicalId)
+      .FieldId(field.GetId())
+      .Type(pointedColumn.GetType())
+      .Index(pointedColumn.GetIndex())
+      .BitsOnStorage(pointedColumn.GetBitsOnStorage())
+      .ValueRange(pointedColumn.GetValueRange())
+      .FirstElementIndex(pointedColumn.GetFirstElementIndex())
+      .RepresentationIndex(pointedColumn.GetRepresentationIndex());
+   fDescriptorBuilder.AddColumn(columnBuilder.MakeDescriptor().Unwrap());
+}
+
 void ROOT::Internal::RPagePersistentSink::CommitSuppressedColumn(ColumnHandle_t columnHandle)
 {
    fOpenColumnRanges.at(columnHandle.fPhysicalId).SetIsSuppressed(true);