Add the possibility of starting from the Nth event instead of the first one

k4FWCore/components/IOSvc.cpp

@@ -61,10 +61,16 @@ StatusCode IOSvc::initialize() {
     m_entries = m_reader->getEntries(podio::Category::Event);
   }
 
+  if ((m_entries && m_firstEventEntry >= m_entries) || m_firstEventEntry < 0) {
+    error() << "First event entry is larger than the number of entries in the file or negative" << endmsg;
+    return StatusCode::FAILURE;
+  }
+
   if (!m_writingFileNameDeprecated.empty()) {
     warning() << ".output is deprecated, use .Output instead in the steering file" << endmsg;
     m_writingFileName = m_writingFileNameDeprecated;
   }
+  m_nextEntry = m_firstEventEntry;
 
   m_switch = KeepDropSwitch(m_outputCommands);
 

k4FWCore/components/IOSvc.h

@@ -67,6 +67,7 @@ class IOSvc : public extends<Service, IIOSvc, IIncidentListener> {
   Gaudi::Property<bool> m_importedFromk4FWCore{
       this, "ImportedFromk4FWCore", false,
       "This is set to true when IOSvc is imported from k4FWCore instead of Configurables in python"};
+  Gaudi::Property<int> m_firstEventEntry{this, "FirstEventEntry", 0, "First event entry to read"};
 
   std::mutex m_changeBufferLock;
 

test/k4FWCoreTest/CMakeLists.txt

@@ -141,9 +141,10 @@ add_test_with_env(createEventHeaderConcurrent options/createEventHeaderConcurren
 
 add_test_with_env(FunctionalWrongImport options/ExampleFunctionalWrongImport.py)
 set_tests_properties(FunctionalWrongImport PROPERTIES PASS_REGULAR_EXPRESSION "ImportError: Importing ApplicationMgr or IOSvc from Configurables is not allowed.")
+add_test_with_env(FunctionalReadNthEvent options/ExampleFunctionalReadNthEvent.py)
 
 add_test(NAME FunctionalCheckFiles COMMAND python3 ${CMAKE_CURRENT_LIST_DIR}/options/CheckOutputFiles.py)
-set_tests_properties(FunctionalCheckFiles PROPERTIES DEPENDS "FunctionalFile;FunctionalMTFile;FunctionalMultipleFile;FunctionalOutputCommands;FunctionalProducerAbsolutePath;FunctionalTransformerRuntimeEmpty;FunctionalMix;FunctionalMixIOSvc;FunctionalTransformerHist;FunctionalCollectionMerger;FunctionalFilterFile;FunctionalMetadata;FunctionalMetadataOldAlgorithm;createEventHeaderConcurrent")
+set_tests_properties(FunctionalCheckFiles PROPERTIES DEPENDS "FunctionalFile;FunctionalMTFile;FunctionalMultipleFile;FunctionalOutputCommands;FunctionalProducerAbsolutePath;FunctionalTransformerRuntimeEmpty;FunctionalMix;FunctionalMixIOSvc;FunctionalTransformerHist;FunctionalCollectionMerger;FunctionalFilterFile;FunctionalMetadata;FunctionalMetadataOldAlgorithm;createEventHeaderConcurrent;FunctionalReadNthEvent")
 
 # Do this after checking the files not to overwrite them
 add_test_with_env(FunctionalFile_toolong options/ExampleFunctionalFile.py -n 999 PROPERTIES DEPENDS FunctionalCheckFiles PASS_REGULAR_EXPRESSION

test/k4FWCoreTest/options/CheckOutputFiles.py

@@ -209,3 +209,8 @@ def check_events(filename, number):
     if event_number in seen_event_numbers:
         raise RuntimeError(f"Event number {event_number} is duplicated")
     seen_event_numbers.add(event_number)
+
+check_events(
+    "functional_nth_event.root",
+    3,
+)

test/k4FWCoreTest/options/ExampleFunctionalReadNthEvent.py

@@ -0,0 +1,41 @@
+#
+# Copyright (c) 2014-2024 Key4hep-Project.
+#
+# This file is part of Key4hep.
+# See https://key4hep.github.io/key4hep-doc/ for further info.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#
+
+# This is an example for reading from a file starting from a certain event and
+# tranforming so that later we can check the number of events in the output file
+
+from Gaudi.Configuration import INFO
+from Configurables import ExampleFunctionalTransformer
+from Configurables import EventDataSvc
+from k4FWCore import ApplicationMgr, IOSvc
+
+svc = IOSvc("IOSvc")
+svc.Input = "functional_producer.root"
+svc.Output = "functional_nth_event.root"
+svc.FirstEventEntry = 7
+
+consumer = ExampleFunctionalTransformer(Offset=0)
+
+mgr = ApplicationMgr(
+    TopAlg=[consumer],
+    EvtSel="NONE",
+    EvtMax=-1,
+    ExtSvc=[EventDataSvc("EventDataSvc")],
+    OutputLevel=INFO,
+)