From 2b0e08f34d35751e2fb5e64b37b0f89131a25032 Mon Sep 17 00:00:00 2001
From: Jonas Hahnfeld <jonas.hahnfeld@cern.ch>
Date: Thu, 15 Jan 2026 13:57:50 +0100
Subject: [PATCH] Add tests for projections of leaf fields

Closes #26
---
 projections/README.md      |  1 +
 projections/leaf/README.md | 10 +++++
 projections/leaf/read.C    | 85 ++++++++++++++++++++++++++++++++++++++
 projections/leaf/write.C   | 41 ++++++++++++++++++
 4 files changed, 137 insertions(+)
 create mode 100644 projections/leaf/README.md
 create mode 100644 projections/leaf/read.C
 create mode 100644 projections/leaf/write.C

diff --git a/projections/README.md b/projections/README.md
index b1003d7..172f4af 100644
--- a/projections/README.md
+++ b/projections/README.md
@@ -1,3 +1,4 @@
 # Projections
 
  * [`cardinality`](cardinality): `ROOT::RNTupleCardinality`
+ * [`leaf`](leaf): of leaf fields
diff --git a/projections/leaf/README.md b/projections/leaf/README.md
new file mode 100644
index 0000000..b34a7d5
--- /dev/null
+++ b/projections/leaf/README.md
@@ -0,0 +1,10 @@
+# Projections of Leaf Fields
+
+## Fields
+
+ * `Pair` of type `std::pair<std::int32_t, float>`
+ * `Int32` and `Float` projected fields of types `std::int32_t` and `float`
+
+## Entries
+
+1. Ascending values
diff --git a/projections/leaf/read.C b/projections/leaf/read.C
new file mode 100644
index 0000000..6abb391
--- /dev/null
+++ b/projections/leaf/read.C
@@ -0,0 +1,85 @@
+#include <ROOT/REntry.hxx>
+#include <ROOT/RNTupleReader.hxx>
+#include <ROOT/RNTupleUtil.hxx>
+
+using ROOT::Experimental::REntry;
+using ROOT::Experimental::RNTupleReader;
+
+#include <cstdint>
+#include <fstream>
+#include <ostream>
+#include <string>
+#include <string_view>
+#include <utility>
+
+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 <typename T, typename U>
+static void PrintPairValue(const REntry &entry, std::string_view name,
+                           std::ostream &os, bool last = false) {
+  auto &value = *entry.GetPtr<std::pair<T, U>>(name);
+  os << "    \"" << name << "\": [\n";
+  os << "      ";
+  PrintValue(value.first, os);
+  os << ",\n";
+  os << "      ";
+  PrintValue(value.second, os);
+  os << "\n";
+  os << "    ]";
+  if (!last) {
+    os << ",";
+  }
+  os << "\n";
+}
+
+template <typename T>
+static void PrintValue(const REntry &entry, std::string_view name,
+                       std::ostream &os, bool last = false) {
+  T value = *entry.GetPtr<T>(name);
+  os << "    \"" << name << "\": ";
+  PrintValue(value, os);
+  if (!last) {
+    os << ",";
+  }
+  os << "\n";
+}
+
+void read(std::string_view input = "projections.leaf.root",
+          std::string_view output = "projections.leaf.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";
+
+    PrintPairValue<std::int32_t, float>(entry, "Pair", os);
+    PrintValue<std::int32_t>(entry, "Int32", os);
+    PrintValue<float>(entry, "Float", os, /*last=*/true);
+
+    os << "  }";
+    // Newline is intentionally missing, may need to print a comma before the
+    // next entry.
+  }
+  os << "\n";
+  os << "]\n";
+}
diff --git a/projections/leaf/write.C b/projections/leaf/write.C
new file mode 100644
index 0000000..905e4f2
--- /dev/null
+++ b/projections/leaf/write.C
@@ -0,0 +1,41 @@
+#include <ROOT/RField.hxx>
+#include <ROOT/RNTupleModel.hxx>
+#include <ROOT/RNTupleWriteOptions.hxx>
+#include <ROOT/RNTupleWriter.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>
+
+template <typename T>
+static void AddProjectedField(RNTupleModel &model, std::string_view name,
+                              std::string_view source) {
+  auto field = std::make_unique<RField<T>>(name);
+  model.AddProjectedField(std::move(field), [&source](const std::string &) {
+    return std::string{source};
+  });
+}
+
+void write(std::string_view filename = "projections.leaf.root") {
+  auto model = RNTupleModel::Create();
+
+  auto Pair = model->MakeField<std::pair<std::int32_t, float>>("Pair");
+  AddProjectedField<std::int32_t>(*model, "Int32", "Pair._0");
+  AddProjectedField<float>(*model, "Float", "Pair._1");
+
+  RNTupleWriteOptions options;
+  options.SetCompression(0);
+  auto writer =
+      RNTupleWriter::Recreate(std::move(model), "ntpl", filename, options);
+
+  // First entry: ascending values
+  *Pair = {1, 2.0};
+  writer->Fill();
+}