Add test for projection of nested fields

Closes #28

Author: hahnjo

projections/README.md

@@ -3,3 +3,4 @@
  * [`cardinality`](cardinality): `ROOT::RNTupleCardinality`
  * [`collection`](collection): of collection fields
  * [`leaf`](leaf): of leaf fields
+ * [`nested`](nested): projection of nested fields

projections/nested/README.md

@@ -0,0 +1,11 @@
+# Projections of Nested Fields
+
+## Fields
+
+ * `VectorPair` of type `std::vector<std::pair<std::int32_t, float>>`
+ * `VectorInt` and `VectorFloat` projected fields of types `std::vector<std::int32_t>` and `std::vector<float>`
+
+## Entries
+
+1. Ascending values
+2. Empty collection

projections/nested/read.C

@@ -0,0 +1,93 @@
+#include <ROOT/REntry.hxx>
+#include <ROOT/RNTupleReader.hxx>
+#include <ROOT/RVec.hxx>
+
+using ROOT::Experimental::REntry;
+using ROOT::Experimental::RNTupleReader;
+
+#include <cstdint>
+#include <fstream>
+#include <ostream>
+#include <string>
+#include <string_view>
+#include <utility>
+#include <vector>
+
+template <typename T> static void PrintValue(const T &value, std::ostream &os);
+
+template <> void PrintValue(const std::int32_t &value, std::ostream &os) {
+  os << value;
+}
+
+template <> void PrintValue(const float &value, std::ostream &os) {
+  os << "\"" << value << "\"";
+}
+
+template <>
+void PrintValue(const std::pair<std::int32_t, float> &value, std::ostream &os) {
+  os << "[\n";
+  os << "        ";
+  PrintValue(value.first, os);
+  os << ",\n";
+  os << "        ";
+  PrintValue(value.second, os);
+  os << "\n";
+  os << "      ]";
+}
+
+template <typename T>
+static void PrintVectorValue(const REntry &entry, std::string_view name,
+                             std::ostream &os, bool last = false) {
+  auto &value = *entry.GetPtr<std::vector<T>>(name);
+  os << "    \"" << name << "\": [";
+  bool first = true;
+  for (auto element : value) {
+    if (first) {
+      first = false;
+    } else {
+      os << ",";
+    }
+    os << "\n      ";
+    PrintValue(element, os);
+  }
+  if (!value.empty()) {
+    os << "\n    ";
+  }
+  os << "]";
+  if (!last) {
+    os << ",";
+  }
+  os << "\n";
+}
+
+void read(std::string_view input = "projections.nested.root",
+          std::string_view output = "projections.nested.json") {
+  std::ofstream os(std::string{output});
+  // Print floating-point numbers as hexadecimal literals.
+  os << std::hexfloat;
+  os << "[\n";
+
+  auto reader = RNTupleReader::Open("ntpl", input);
+  auto &entry = reader->GetModel().GetDefaultEntry();
+  bool first = true;
+  for (auto index : *reader) {
+    reader->LoadEntry(index);
+
+    if (first) {
+      first = false;
+    } else {
+      os << ",\n";
+    }
+    os << "  {\n";
+
+    PrintVectorValue<std::pair<std::int32_t, float>>(entry, "VectorPair", os);
+    PrintVectorValue<std::int32_t>(entry, "VectorInt", os);
+    PrintVectorValue<float>(entry, "VectorFloat", os, /*last=*/true);
+
+    os << "  }";
+    // Newline is intentionally missing, may need to print a comma before the
+    // next entry.
+  }
+  os << "\n";
+  os << "]\n";
+}

projections/nested/write.C

@@ -0,0 +1,55 @@
+#include <ROOT/RField.hxx>
+#include <ROOT/RNTupleModel.hxx>
+#include <ROOT/RNTupleWriteOptions.hxx>
+#include <ROOT/RNTupleWriter.hxx>
+#include <ROOT/RVec.hxx>
+
+using ROOT::Experimental::RField;
+using ROOT::Experimental::RNTupleModel;
+using ROOT::Experimental::RNTupleWriteOptions;
+using ROOT::Experimental::RNTupleWriter;
+
+#include <cstdint>
+#include <memory>
+#include <string>
+#include <string_view>
+#include <utility>
+#include <vector>
+
+template <typename T>
+static void AddProjectedField(RNTupleModel &model, std::string_view name,
+                              std::string_view source) {
+  auto field = std::make_unique<RField<std::vector<T>>>(name);
+  model.AddProjectedField(std::move(field),
+                          [&name, &source](const std::string &fieldName) {
+                            if (fieldName == name) {
+                              return std::string("VectorPair");
+                            } else {
+                              return "VectorPair._0." + std::string(source);
+                            }
+                          });
+}
+
+void write(std::string_view filename = "projections.nested.root") {
+  auto model = RNTupleModel::Create();
+
+  auto VectorPair =
+      model->MakeField<std::vector<std::pair<std::int32_t, float>>>(
+          "VectorPair");
+  AddProjectedField<std::int32_t>(*model, "VectorInt", "_0");
+  AddProjectedField<float>(*model, "VectorFloat", "_1");
+
+  RNTupleWriteOptions options;
+  options.SetCompression(0);
+  auto writer =
+      RNTupleWriter::Recreate(std::move(model), "ntpl", filename, options);
+
+  // First entry: ascending values
+  VectorPair->emplace_back(1, 2.0);
+  VectorPair->emplace_back(3, 4.0);
+  writer->Fill();
+
+  // Second entry: empty collection
+  VectorPair->clear();
+  writer->Fill();
+}