Add a LinkNavigator utility

include/podio/LinkNavigator.h

@@ -0,0 +1,106 @@
+#ifndef PODIO_LINKNAVIGATOR_H
+#define PODIO_LINKNAVIGATOR_H
+
+#include "podio/detail/LinkFwd.h"
+
+#include <map>
+#include <tuple>
+#include <utility>
+#include <vector>
+
+namespace podio {
+
+namespace detail::associations {
+  /// A small struct that simply bundles an object and its weight for a more
+  /// convenient return value for the LinkNavigator
+  ///
+  /// @note In most uses the names of the members should not really matter as it
+  /// is possible to us this via structured bindings
+  template <typename T>
+  struct WeightedObject {
+    WeightedObject(T obj, float w) : o(obj), weight(w) {
+    }
+    T o;          ///< The object
+    float weight; ///< The weight in the association
+  };
+} // namespace detail::associations
+
+/// A helper class to more easily handle one-to-many associations.
+///
+/// Internally simply populates two maps in its constructor and then queries
+/// them to retrieve objects that are associated with another.
+///
+/// @note There are no guarantees on the order of the objects in these maps.
+/// Hence, there are also no guarantees on the order of the returned objects,
+/// even if there inherintly is an order to them in the underlying associations
+/// collection.
+template <typename LinkCollT>
+class LinkNavigator {
+  using FromT = LinkCollT::from_type;
+  using ToT = LinkCollT::to_type;
+
+  template <typename T>
+  using WeightedObject = detail::associations::WeightedObject<T>;
+
+public:
+  /// Construct a navigator from an association collection
+  LinkNavigator(const LinkCollT& associations);
+
+  /// We do only construct from a collection
+  LinkNavigator() = delete;
+  LinkNavigator(const LinkNavigator&) = default;
+  LinkNavigator& operator=(const LinkNavigator&) = default;
+  LinkNavigator(LinkNavigator&&) = default;
+  LinkNavigator& operator=(LinkNavigator&&) = default;
+  ~LinkNavigator() = default;
+
+  /// Get all the objects and weights that are associated to the passed object
+  ///
+  /// @param object The object that is labeled *to* in the association
+  ///
+  /// @returns A vector of all objects and their weights that are associated to
+  ///          the passed object
+  std::vector<WeightedObject<FromT>> getAssociated(const ToT& object) const {
+    const auto& [begin, end] = m_to2from.equal_range(object);
+    std::vector<WeightedObject<FromT>> result;
+    result.reserve(std::distance(begin, end));
+
+    for (auto it = begin; it != end; ++it) {
+      result.emplace_back(it->second);
+    }
+    return result;
+  }
+
+  /// Get all the objects and weights that are associated to the passed object
+  ///
+  /// @param object The object that is labeled *from* in the association
+  ///
+  /// @returns A vector of all objects and their weights that are associated to
+  ///          the passed object
+  std::vector<WeightedObject<ToT>> getAssociated(const FromT& object) const {
+    const auto& [begin, end] = m_from2to.equal_range(object);
+    std::vector<WeightedObject<ToT>> result;
+    result.reserve(std::distance(begin, end));
+
+    for (auto it = begin; it != end; ++it) {
+      result.emplace_back(it->second);
+    }
+    return result;
+  }
+
+private:
+  std::multimap<FromT, WeightedObject<ToT>> m_from2to{}; ///< Map the from to the to objects
+  std::multimap<ToT, WeightedObject<FromT>> m_to2from{}; ///< Map the to to the from objects
+};
+
+template <typename LinkCollT>
+LinkNavigator<LinkCollT>::LinkNavigator(const LinkCollT& associations) {
+  for (const auto& [from, to, weight] : associations) {
+    m_from2to.emplace(std::piecewise_construct, std::forward_as_tuple(from), std::forward_as_tuple(to, weight));
+    m_to2from.emplace(std::piecewise_construct, std::forward_as_tuple(to), std::forward_as_tuple(from, weight));
+  }
+}
+
+} // namespace podio
+
+#endif // PODIO_LINKNAVIGATOR_H

include/podio/detail/LinkCollectionImpl.h

@@ -40,6 +40,8 @@ class LinkCollection : public podio::CollectionBase {
   using CollectionDataT = podio::LinkCollectionData<FromT, ToT>;
 
 public:
+  using from_type = FromT;
+  using to_type = ToT;
   using value_type = Link<FromT, ToT>;
   using mutable_type = MutableLink<FromT, ToT>;
   using const_iterator = LinkCollectionIterator<FromT, ToT>;

tests/unittests/links.cpp

@@ -1,6 +1,7 @@
 #include "catch2/catch_test_macros.hpp"
 
 #include "podio/LinkCollection.h"
+#include "podio/LinkNavigator.h"
 
 #include "datamodel/ExampleClusterCollection.h"
 #include "datamodel/ExampleHitCollection.h"
@@ -473,3 +474,51 @@ TEST_CASE("Link JSON conversion", "[links][json]") {
 }
 
 #endif
+
+TEST_CASE("LinkNavigator basics", "[links]") {
+  TestLColl coll{};
+  std::vector<ExampleHit> hits(11);
+  std::vector<ExampleCluster> clusters(3);
+
+  for (size_t i = 0; i < 10; ++i) {
+    auto a = coll.create();
+    a.set(hits[i]);
+    a.set(clusters[i % 3]);
+    a.setWeight(i * 0.1f);
+  }
+
+  auto a = coll.create();
+  a.set(hits[10]);
+
+  podio::LinkNavigator nav{coll};
+
+  for (size_t i = 0; i < 10; ++i) {
+    const auto& hit = hits[i];
+    const auto assocClusters = nav.getAssociated(hit);
+    REQUIRE(assocClusters.size() == 1);
+    const auto& [cluster, weight] = assocClusters[0];
+    REQUIRE(cluster == clusters[i % 3]);
+    REQUIRE(weight == i * 0.1f);
+  }
+
+  const auto& cluster1 = clusters[0];
+  auto assocHits = nav.getAssociated(cluster1);
+  REQUIRE(assocHits.size() == 4);
+  for (size_t i = 0; i < 4; ++i) {
+    const auto& [hit, weight] = assocHits[i];
+    REQUIRE(hit == hits[i * 3]);
+    REQUIRE(weight == i * 3 * 0.1f);
+  }
+
+  const auto& cluster2 = clusters[1];
+  assocHits = nav.getAssociated(cluster2);
+  REQUIRE(assocHits.size() == 3);
+  for (size_t i = 0; i < 3; ++i) {
+    const auto& [hit, weight] = assocHits[i];
+    REQUIRE(hit == hits[i * 3 + 1]);
+    REQUIRE(weight == (i * 3 + 1) * 0.1f);
+  }
+
+  const auto [noCluster, noWeight] = nav.getAssociated(hits[10])[0];
+  REQUIRE_FALSE(noCluster.isAvailable());
+}