write.C

#include <ROOT/RField.hxx>
#include <ROOT/RNTupleModel.hxx>
#include <ROOT/RNTupleUtil.hxx>
#include <ROOT/RNTupleWriteOptions.hxx>
#include <ROOT/RNTupleWriter.hxx>

using ROOT::Experimental::EColumnType;
using ROOT::Experimental::RField;
using ROOT::Experimental::RNTupleModel;
using ROOT::Experimental::RNTupleWriteOptions;
using ROOT::Experimental::RNTupleWriter;

#include <TSystem.h>

#include <cstdint>
#include <filesystem>
#include <map>
#include <memory>
#include <string_view>

using Multimap =
    std::multimap<std::string, std::multimap<std::string, std::int32_t>>;

static std::shared_ptr<Multimap> MakeMultimapField(RNTupleModel &model,
                                                   std::string_view name,
                                                   EColumnType indexType) {
  auto field = std::make_unique<RField<Multimap>>(name);
  field->SetColumnRepresentatives({{indexType}});
  field->GetSubFields()[0]->GetSubFields()[1]->SetColumnRepresentatives(
      {{indexType}});
  model.AddField(std::move(field));
  return model.GetDefaultEntry().GetPtr<Multimap>(name);
}

void write(std::string_view filename = "types.multimap.nested.root") {
  if (gSystem->Load("libNestedMultimap") == -1)
    throw std::runtime_error("could not find the required ROOT dictionaries, "
                             "please make sure to run `make` first");

  auto model = RNTupleModel::Create();

  // Non-split index encoding
  auto Index32 = MakeMultimapField(*model, "Index32", EColumnType::kIndex32);
  auto Index64 = MakeMultimapField(*model, "Index64", EColumnType::kIndex64);

  // Split index encoding
  auto SplitIndex32 =
      MakeMultimapField(*model, "SplitIndex32", EColumnType::kSplitIndex32);
  auto SplitIndex64 =
      MakeMultimapField(*model, "SplitIndex64", EColumnType::kSplitIndex64);

  RNTupleWriteOptions options;
  options.SetCompression(0);
  auto writer =
      RNTupleWriter::Recreate(std::move(model), "ntpl", filename, options);

  // First entry: single-element maps, with ascending values
  *Index32 = {{"a", {{"aa", 1}}}};
  *Index64 = {{"b", {{"bb", 2}}}};
  *SplitIndex32 = {{"c", {{"cc", 3}}}};
  *SplitIndex64 = {{"d", {{"dd", 4}}}};
  writer->Fill();

  // Second entry: empty maps
  *Index32 = {};
  *Index64 = {};
  *SplitIndex32 = {};
  *SplitIndex64 = {};
  writer->Fill();

  // Third entry: increasing number of elements in the outer map
  *Index32 = {{"a", {{"aa", 1}}}};
  *Index64 = {{"b", {{"ba", 2}}}, {"c", {{"ca", 3}}}};
  *SplitIndex32 = {
      {"d", {{"da", 4}}}, {"e", {{"ea", 5}, {"eb", 6}}}, {"f", {}}};
  *SplitIndex64 = {{"g", {{"ga", 7}, {"gb", 8}}},
                   {"h", {}},
                   {"i", {{"ia", 9}}},
                   {"j", {{"ja", 10}, {"jb", 11}, {"jc", 12}}}};
  writer->Fill();
}