Add test for projections of collection fields

Closes #27

Author: hahnjo

projections/README.md

@@ -1,4 +1,5 @@
 # Projections
 
  * [`cardinality`](cardinality): `ROOT::RNTupleCardinality`
+ * [`collection`](collection): of collection fields
  * [`leaf`](leaf): of leaf fields

projections/collection/README.md

@@ -0,0 +1,11 @@
+# Projections of Collection Fields
+
+## Fields
+
+ * `Vector` of type `std::vector<std::int32_t>`
+ * A projected field `Projected` of types `ROOT::RVec<std::int32_t>`
+
+## Entries
+
+1. Ascending values
+2. Empty collection

projections/collection/read.C

@@ -0,0 +1,67 @@
+#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 <vector>
+
+template <typename C>
+static void PrintCollectionValue(const REntry &entry, std::string_view name,
+                                 std::ostream &os, bool last = false) {
+  C &value = *entry.GetPtr<C>(name);
+  os << "    \"" << name << "\": [";
+  bool first = true;
+  for (auto element : value) {
+    if (first) {
+      first = false;
+    } else {
+      os << ",";
+    }
+    os << "\n      " << element;
+  }
+  if (!value.empty()) {
+    os << "\n    ";
+  }
+  os << "]";
+  if (!last) {
+    os << ",";
+  }
+  os << "\n";
+}
+
+void read(std::string_view input = "projections.collection.root",
+          std::string_view output = "projections.collection.json") {
+  std::ofstream os(std::string{output});
+  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";
+
+    PrintCollectionValue<std::vector<std::int32_t>>(entry, "Vector", os);
+    PrintCollectionValue<ROOT::RVec<std::int32_t>>(entry, "Projected", os,
+                                                   /*last=*/true);
+
+    os << "  }";
+    // Newline is intentionally missing, may need to print a comma before the
+    // next entry.
+  }
+  os << "\n";
+  os << "]\n";
+}

projections/collection/write.C

@@ -0,0 +1,45 @@
+#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 <vector>
+
+void write(std::string_view filename = "projections.collection.root") {
+  auto model = RNTupleModel::Create();
+
+  auto Vector = model->MakeField<std::vector<std::int32_t>>("Vector");
+  auto Projected =
+      std::make_unique<RField<ROOT::RVec<std::int32_t>>>("Projected");
+  model->AddProjectedField(std::move(Projected),
+                           [](const std::string &fieldName) {
+                             if (fieldName == "Projected") {
+                               return "Vector";
+                             } else {
+                               return "Vector._0";
+                             }
+                           });
+
+  RNTupleWriteOptions options;
+  options.SetCompression(0);
+  auto writer =
+      RNTupleWriter::Recreate(std::move(model), "ntpl", filename, options);
+
+  // First entry: ascending values
+  *Vector = {1, 2};
+  writer->Fill();
+
+  // Second entry: empty collection
+  Vector->clear();
+  writer->Fill();
+}