Add tests for structure of RNTuple

... with clusters and cluster groups.

Closes #29

Author: hahnjo

structure/README.md

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+# Structure
+
+For the RNTuple Validation Suite, we assume that the structure is orthogonal to the supported types and serialized data.
+Therefore all tests in this category write a single `Int32` field with type `std::int32_t` the entries have ascending values.
+
+ * [`clusters`](clusters): multiple clusters
+ * [`cluster_groups`](cluster_groups): multiple cluster groups

structure/cluster_groups/README.md

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+# Cluster Groups
+
+Two cluster groups with two clusters each and a total of four entries.

structure/cluster_groups/read.C

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+#include "../read_structure.hxx"
+
+void read(std::string_view input = "structure.cluster_groups.root",
+          std::string_view output = "structure.cluster_groups.json") {
+  read_structure(input, output);
+}

structure/cluster_groups/write.C

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+#include <ROOT/RNTupleModel.hxx>
+#include <ROOT/RNTupleWriteOptions.hxx>
+#include <ROOT/RNTupleWriter.hxx>
+
+using ROOT::Experimental::RNTupleModel;
+using ROOT::Experimental::RNTupleWriteOptions;
+using ROOT::Experimental::RNTupleWriter;
+
+#include <cstdint>
+#include <string_view>
+
+void write(std::string_view filename = "structure.cluster_groups.root") {
+  auto model = RNTupleModel::Create();
+
+  auto Int32 = model->MakeField<std::int32_t>("Int32");
+
+  RNTupleWriteOptions options;
+  options.SetCompression(0);
+  auto writer =
+      RNTupleWriter::Recreate(std::move(model), "ntpl", filename, options);
+
+  *Int32 = 1;
+  writer->Fill();
+  writer->CommitCluster();
+
+  *Int32 = 2;
+  writer->Fill();
+  writer->CommitCluster(/*commitClusterGroup=*/true);
+
+  *Int32 = 3;
+  writer->Fill();
+  writer->CommitCluster();
+
+  *Int32 = 4;
+  writer->Fill();
+  writer->CommitCluster(/*commitClusterGroup=*/true);
+}

structure/clusters/README.md

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+# Clusters
+
+Two clusters with one entry each.

structure/clusters/read.C

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+#include "../read_structure.hxx"
+
+void read(std::string_view input = "structure.clusters.root",
+          std::string_view output = "structure.clusters.json") {
+  read_structure(input, output);
+}

structure/clusters/write.C

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+#include <ROOT/RNTupleModel.hxx>
+#include <ROOT/RNTupleWriteOptions.hxx>
+#include <ROOT/RNTupleWriter.hxx>
+
+using ROOT::Experimental::RNTupleModel;
+using ROOT::Experimental::RNTupleWriteOptions;
+using ROOT::Experimental::RNTupleWriter;
+
+#include <cstdint>
+#include <string_view>
+
+void write(std::string_view filename = "structure.clusters.root") {
+  auto model = RNTupleModel::Create();
+
+  auto Int32 = model->MakeField<std::int32_t>("Int32");
+
+  RNTupleWriteOptions options;
+  options.SetCompression(0);
+  auto writer =
+      RNTupleWriter::Recreate(std::move(model), "ntpl", filename, options);
+
+  *Int32 = 1;
+  writer->Fill();
+  writer->CommitCluster();
+
+  *Int32 = 2;
+  writer->Fill();
+  writer->CommitCluster();
+}

structure/read_structure.hxx

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+#include <ROOT/REntry.hxx>
+#include <ROOT/RNTupleReader.hxx>
+
+using ROOT::Experimental::RNTupleReader;
+
+#include <cstdint>
+#include <fstream>
+#include <ostream>
+#include <string>
+#include <string_view>
+
+void read_structure(std::string_view input, std::string_view output) {
+  std::ofstream os(std::string{output});
+  os << "[\n";
+
+  auto reader = RNTupleReader::Open("ntpl", input);
+  auto Int32 =
+      reader->GetModel().GetDefaultEntry().GetPtr<std::int32_t>("Int32");
+  bool first = true;
+  for (auto index : *reader) {
+    reader->LoadEntry(index);
+
+    if (first) {
+      first = false;
+    } else {
+      os << ",\n";
+    }
+    os << "  {\n";
+    os << "    \"Int32\": " << *Int32 << "\n";
+    os << "  }";
+    // Newline is intentionally missing, may need to print a comma before the
+    // next entry.
+  }
+  os << "\n";
+  os << "]\n";
+}