Add a first version of a PIDHandler

Rename the EDM4HEP::utils target to not have a library that is called
libutils.so

cmake/EDM4HEPConfig.cmake.in

@@ -21,4 +21,8 @@ include("${CMAKE_CURRENT_LIST_DIR}/EDM4HEPTargets.cmake")
 # print the default "Found:" message and check library location
 include(FindPackageHandleStandardArgs)
 get_property(TEST_EDM4HEP_LIBRARY TARGET EDM4HEP::edm4hep PROPERTY LOCATION)
+
+# For backwards compatibility at least for some time
+add_library(EDM4HEP::utils ALIAS EDM4HEP::edm4hepUtils)
+
 find_package_handle_standard_args(EDM4HEP DEFAULT_MSG CMAKE_CURRENT_LIST_FILE TEST_EDM4HEP_LIBRARY)

test/downstream-project-cmake-test/CMakeLists.txt

@@ -37,3 +37,4 @@ endif()
 
 add_executable(appUsingEDM4hep main.cxx)
 target_link_libraries(appUsingEDM4hep EDM4HEP::edm4hep EDM4HEP::utils edm4dis)
+# target_link_libraries(appUsingEDM4hep EDM4HEP::edm4hep EDM4HEP::edm4hepUtils edm4dis)

test/utils/CMakeLists.txt

@@ -17,7 +17,7 @@ include(Catch)
 
 add_executable(unittests_edm4hep
   test_kinematics.cpp test_vector_utils.cpp)
-target_link_libraries(unittests_edm4hep edm4hep EDM4HEP::utils Catch2::Catch2 Catch2::Catch2WithMain)
+target_link_libraries(unittests_edm4hep edm4hep EDM4HEP::edm4hepUtils Catch2::Catch2 Catch2::Catch2WithMain)
 
 option(SKIP_CATCH_DISCOVERY "Skip the Catch2 test discovery" OFF)
 

utils/CMakeLists.txt

@@ -8,9 +8,17 @@ target_include_directories(kinematics
 target_link_libraries(kinematics PUBLIC INTERFACE ROOT::Core)
 target_compile_features(kinematics INTERFACE cxx_std_17)
 
-add_library(utils INTERFACE)
-add_library(EDM4HEP::utils ALIAS utils)
-target_link_libraries(utils INTERFACE kinematics)
+set(utils_sources
+  src/ParticleIDUtils.cc
+)
+
+add_library(edm4hepUtils SHARED ${utils_sources})
+add_library(EDM4HEP::edm4hepUtils ALIAS edm4hepUtils)
+target_include_directories(edm4hepUtils
+  PUBLIC $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/include>
+         $<INSTALL_INTERFACE:${CMAKE_INSTALL_INCLUDEDIR}>)
+target_link_libraries(edm4hepUtils INTERFACE kinematics)
+target_link_libraries(edm4hepUtils PUBLIC EDM4HEP::edm4hep kinematics)
 
 
 set(sources src/dataframe.cc)
@@ -26,7 +34,7 @@ target_link_libraries(edm4hepRDF PUBLIC edm4hep ROOT::Physics ROOT::ROOTVecOps)
 install(DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}/include/ DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}
   PATTERN "CMakeLists.txt" EXCLUDE)
 
-install(TARGETS utils kinematics edm4hepRDF
+install(TARGETS edm4hepUtils kinematics edm4hepRDF
   EXPORT EDM4HEPTargets
   RUNTIME DESTINATION "${CMAKE_INSTALL_BINDIR}" COMPONENT bin
   RUNTIME DESTINATION "${CMAKE_INSTALL_LIBDIR}" COMPONENT shlib

utils/include/edm4hep/utils/ParticleIDUtils.h

@@ -0,0 +1,48 @@
+#ifndef EDM4HEP_UTILS_PARTICLEIDUTILS_H
+#define EDM4HEP_UTILS_PARTICLEIDUTILS_H
+
+#include <edm4hep/ParticleIDCollection.h>
+#include <edm4hep/ReconstructedParticle.h>
+
+#include <map>
+#include <type_traits>
+#include <vector>
+
+namespace edm4hep::utils {
+
+/// Utility class to invert the ParticleID to ReconstructedParticle relation
+class PIDHandler {
+
+  using MapType = std::multimap<edm4hep::ReconstructedParticle, edm4hep::ParticleID>;
+
+  MapType m_recoPidMap{}; ///< The internal map from recos to pids
+
+public:
+  PIDHandler() = default;
+  ~PIDHandler() = default;
+  PIDHandler(const PIDHandler&) = default;
+  PIDHandler& operator=(const PIDHandler&) = default;
+  PIDHandler(PIDHandler&&) = default;
+  PIDHandler& operator=(PIDHandler&&) = default;
+
+  /// Construct a PIDHandler from an arbitrary number of ParticleIDCollections
+  template <typename... PIDColls>
+  static PIDHandler from(const ParticleIDCollection& coll, const PIDColls&... pidColls) {
+    static_assert((std::is_same_v<PIDColls, edm4hep::ParticleIDCollection> && ...),
+                  "PIDHandler can only be constructed from ParticleIDCollections");
+    PIDHandler handler{};
+    handler.addColl(coll);
+    (handler.addColl(pidColls), ...);
+    return handler;
+  }
+
+  /// Add the information from one ParticleIDCollection to the handler
+  void addColl(const edm4hep::ParticleIDCollection& coll);
+
+  /// Retrieve all ParticleIDs that are related to the passed
+  /// RecontstructedParticle
+  std::vector<edm4hep::ParticleID> getPIDs(const edm4hep::ReconstructedParticle& reco) const;
+};
+} // namespace edm4hep::utils
+
+#endif // EDM4HEP_UTILS_PARTICLEIDUTILS_H

utils/src/ParticleIDUtils.cc

@@ -0,0 +1,26 @@
+#include "edm4hep/ReconstructedParticle.h"
+#include <edm4hep/utils/ParticleIDUtils.h>
+
+#include <iterator>
+
+namespace edm4hep::utils {
+
+void PIDHandler::addColl(const edm4hep::ParticleIDCollection& coll) {
+  for (const auto pid : coll) {
+    m_recoPidMap.emplace(pid.getParticle(), pid);
+  }
+}
+
+std::vector<edm4hep::ParticleID> PIDHandler::getPIDs(const edm4hep::ReconstructedParticle& reco) const {
+  std::vector<edm4hep::ParticleID> pids;
+  const auto& [begin, end] = m_recoPidMap.equal_range(reco);
+  pids.reserve(std::distance(begin, end));
+
+  for (auto it = begin; it != end; ++it) {
+    pids.emplace_back(it->second);
+  }
+
+  return pids;
+}
+
+} // namespace edm4hep::utils