add example loading an entire RNTuple into a LLAMA view

CMakeLists.txt

@@ -69,6 +69,12 @@ if (LLAMA_BUILD_EXAMPLES)
 		message(WARNING "Could not find alpaka. Alpaka examples are disabled.")
 	endif()
 
+	# ROOT examples
+	find_package(ROOT QUIET)
+	if (ROOT_FOUND)
+		add_subdirectory("examples/hep_rntuple")
+	endif()
+
 	# CUDA examples
 	include(CheckLanguage)
 	check_language(CUDA)

examples/hep_rntuple/CMakeLists.txt

@@ -0,0 +1,11 @@
+cmake_minimum_required (VERSION 3.15)
+project(llama-hep_rntuple)
+
+set(CMAKE_CXX_STANDARD 17)
+
+find_package(ROOT REQUIRED)
+if (NOT TARGET llama::llama)
+	find_package(llama REQUIRED)
+endif()
+add_executable(${PROJECT_NAME} hep_rntuple.cpp)
+target_link_libraries(${PROJECT_NAME} PRIVATE ROOT::Hist ROOT::Graf ROOT::Gpad ROOT::ROOTNTuple llama::llama)

examples/hep_rntuple/hep_rntuple.cpp

@@ -0,0 +1,53 @@
+// This example uses a non-public CMS NanoAOD file called: ttjet_13tev_june2019_lzma.
+// Please ask contact us if you need it.
+
+#include "../common/ttjet_13tev_june2019.hpp"
+
+#include <RConfigure.h>
+#define R__HAS_STD_STRING_VIEW
+#include <ROOT/RNTuple.hxx>
+#include <ROOT/RNTupleDS.hxx>
+#include <ROOT/RNTupleModel.hxx>
+#include <ROOT/RNTupleOptions.hxx>
+#include <ROOT/RNTupleView.hxx>
+#include <chrono>
+#include <llama/DumpMapping.hpp>
+#include <llama/llama.hpp>
+
+int main(int argc, const char* argv[])
+{
+    if (argc != 2)
+    {
+        fmt::print("Please specify input file!\n");
+        return 1;
+    }
+
+    using namespace std::chrono;
+    using namespace ROOT::Experimental;
+
+    auto ntuple = RNTupleReader::Open(RNTupleModel::Create(), "NTuple", argv[1]);
+    const auto n = ntuple->GetNEntries();
+
+    auto start = steady_clock::now();
+    auto view = llama::allocView(llama::mapping::SoA<llama::ArrayDims<1>, Event, true>{llama::ArrayDims{n}});
+    fmt::print("Alloc LLAMA view: {}ms\n", duration_cast<milliseconds>(steady_clock::now() - start).count());
+
+    std::size_t totalSize = 0;
+    for (auto i = 0u; i < view.mapping.blobCount; i++)
+        totalSize += view.mapping.blobSize(i);
+    fmt::print("Total LLAMA view memory: {}MiB in {} blobs\n", totalSize / 1024 / 1024, view.mapping.blobCount);
+
+    start = steady_clock::now();
+    llama::forEachLeaf<Event>(
+        [&](auto coord)
+        {
+            using Name = llama::GetTag<Event, decltype(coord)>;
+            using Type = llama::GetType<Event, decltype(coord)>;
+            auto column = ntuple->GetView<Type>(llama::structName<Name>());
+            for (std::size_t i = 0; i < n; i++)
+                view(i)(coord) = column(i);
+        });
+    fmt::print("Copy RNTuple -> LLAMA view: {}ms\n", duration_cast<milliseconds>(steady_clock::now() - start).count());
+
+    start = steady_clock::now();
+}