first commit

.gitignore

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+*~
+*.swo
+*.swp
+rntviewer

Makefile

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+CXX = clang++
+
+d:
+	$(CXX) -g -O0 -o rntviewer rntviewer.cpp $(shell root-config --cflags --libs) -lROOTNTuple
+
+r:
+	$(CXX) -O2 -o rntviewer rntviewer.cpp $(shell root-config --cflags --libs) -lROOTNTuple

defer.hpp

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+#pragma once
+
+#include <functional>
+#include <utility>
+
+struct Defer_Guard {
+  template <typename F>
+  Defer_Guard(F && f) : _f(f) {}
+  ~Defer_Guard() { _f(); }
+  std::function<void()> _f;
+};
+struct Defer_Guard_Helper {
+	template <typename F>
+	Defer_Guard operator+(F&& f) { return Defer_Guard(std::forward<F>(f)); }
+};
+
+#define CONCAT_STR_INTERNAL(A, B) A##B
+#define CONCAT_STR(A, B) CONCAT_STR_INTERNAL(A, B)
+
+#define defer const auto CONCAT_STR(defer_, __LINE__) = Defer_Guard_Helper{} + [&]

rntviewer.cpp

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+#include "defer.hpp"
+
+#include <ROOT/RNTupleReader.hxx>
+
+#include <cstdio>
+
+#include <sys/mman.h>
+
+using namespace ROOT::Experimental;
+
+static size_t file_size(FILE *f)
+{
+  fseek(f, 0, SEEK_END);
+  size_t res = ftell(f);
+  fseek(f, 0, SEEK_SET);
+  return res;  
+}
+
+int main(int argc, char **argv)
+{
+  if (argc < 2) {
+    fprintf(stderr, "Usage: %s <ntuple_file.root>\n", argv[0]);
+    return 1;
+  }
+
+  // Open and map the file
+  const char *fname = argv[0];
+
+  FILE *file = fopen(fname, "rb");
+  defer { fclose(file); };
+
+  const int fd = fileno(file);
+  const size_t fsize = file_size(file);
+  void *fmem = mmap(0, fsize, PROT_READ, MAP_SHARED_VALIDATE, fd, 0);
+  if (!fmem) {
+    fprintf(stderr, "Failed to open file %s\n", fname);
+    return 1;
+  }
+  defer { munmap(fmem, fsize); };
+
+  // Get the RNTuple information
+
+  return 0;
+}