Add a tutorial for TBufferMerger

tutorials/multicore/mt103_fillNtuples.C

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+/// \file
+/// \ingroup tutorial_multicore
+///
+/// Fill an Ntuple in distinct workers, and write the output to a file.
+/// This tutorial illustrates the basics of how it's possible with ROOT
+/// to write simultaneously to a single output file using TBufferMerger.
+///
+/// \macro_code
+///
+/// \author Guilherme Amadio
+/// \date May 2017
+
+#include <ROOT/TBufferMerger.hxx>
+
+using ROOT::Experimental::TBufferMerger;
+using ROOT::Experimental::TBufferMergerFile;
+
+#include <random>
+#include <thread>
+
+// A simple function to fill the ntuple with random values
+void fill(TNtuple &ntuple, size_t n)
+{
+   std::random_device rd;
+   std::default_random_engine rng(rd());
+   std::normal_distribution<double> dist(0.0, 1.0);
+
+   auto gaussian_random = [&]() { return dist(rng); };
+
+   for (auto i : ROOT::TSeqI(n)) ntuple.Fill(gaussian_random());
+}
+
+void mt103_fillNtuples()
+{
+   // Avoid unnecessary output
+   gROOT->SetBatch();
+
+   // Make ROOT thread-safe
+   ROOT::EnableThreadSafety();
+
+   // Total number of events
+   const size_t nEvents = 65535;
+
+   // Match number of threads to what the hardware can do
+   const size_t nWorkers = std::thread::hardware_concurrency();
+
+   // Split work in equal parts
+   const size_t nEventsPerWorker = nEvents / nWorkers;
+
+   // Create the TBufferMerger
+   TBufferMerger merger("mp103_fillNtuple.root");
+
+   // Define what each worker will do
+   auto work_function = [&]() {
+      auto f = merger.GetFile();
+      TNtuple ntrand("ntrand", "Random Numbers", "r");
+      fill(ntrand, nEventsPerWorker);
+      ntrand.Write();
+      f->Write();
+   };
+
+   // Create worker threads
+   std::vector<std::thread> workers;
+
+   for (auto i : ROOT::TSeqI(nWorkers)) workers.emplace_back(work_function);
+
+   // Make sure workers are done
+   for (auto &&worker : workers) worker.join();
+}