Add tool to transform between TTrees and RNTuples (#541)

tools/CMakeLists.txt

@@ -1,5 +1,8 @@
 install(PROGRAMS ${CMAKE_CURRENT_LIST_DIR}/podio-dump DESTINATION ${CMAKE_INSTALL_BINDIR})
 install(PROGRAMS ${CMAKE_CURRENT_LIST_DIR}/podio-vis DESTINATION ${CMAKE_INSTALL_BINDIR})
+if(ENABLE_RNTUPLE)
+  install(PROGRAMS ${CMAKE_CURRENT_LIST_DIR}/podio-ttree-to-rntuple DESTINATION ${CMAKE_INSTALL_BINDIR})
+endif()
 
 # Add a very basic tests here to make sure that podio-dump at least runs
 if(BUILD_TESTING)

tools/podio-ttree-to-rntuple

@@ -0,0 +1,24 @@
+#!/usr/bin/env python3
+"""podio-ttree-to-rntuple tool to create an rntuple file from a ttree file or viceversa"""
+
+import argparse
+import podio.root_io
+
+parser = argparse.ArgumentParser(description='podio-ttree-to-rntuple tool to create'
+                                 'an rntuple file from a ttree file or viceversa')
+parser.add_argument('input_file', help='input file')
+parser.add_argument('output_file', help='output file')
+parser.add_argument('-r', '--reverse', action='store_true',
+                    help='reverse the conversion (from RNTuple to TTree)')
+args = parser.parse_args()
+
+if not args.reverse:
+  reader = podio.root_io.Reader(args.input_file)
+  writer = podio.root_io.RNTupleWriter(args.output_file)
+else:
+  reader = podio.root_io.RNTupleReader(args.input_file)
+  writer = podio.root_io.Writer(args.output_file)
+
+for category in reader.categories:
+  for frame in reader.get(category):
+    writer.write_frame(frame, category)