Add short document describing PIDHandler and its usage

doc/PIDHandler.md

@@ -0,0 +1,229 @@
+# PIDHandler introduction and usage
+
+This page contains some bigger picture introduction for the utilities that are
+availbale to work with `ParticleID`s and its related metadata. It also contains
+examples for the most common usage patterns.
+
+## PIDHandler basics
+The `PIDHandler` can be use to work with `ParticleIDCollection`s. The main
+features are
+- the retrieval meta information for a `ParticleIDCollection`, making it
+  possible to e.g. retrieve the indices of parameters from parameter names
+- the possibility to invert the relation to `ReconstructedParticle`s, which
+  allows to, e.g. get all `ParticleID`s that point to one
+  `ReconstructedParticle`.
+
+## ParticleIDMeta basics
+`ParticleIDMeta` is a simple struct that bundles all ParticleID meta information
+for one collection together. Whenever metadata is involved for ParticleIDs this
+will be the thing to use.
+
+## General considerations
+
+There is no strictly enforced coupling between the meta information that is set
+for a collection and the contents of that collection. In order for everything to
+work as expected the following assumptions have to be met
+- The collection name that is passed to the `setAlgoInfo` methods to set meta
+  information has to match the collection name used for putting collections into
+  events.
+- All elements of a `ParticleIDCollection` have the same value set for
+  `algorithmType`. Additionally, it is usually assumed that this is a unique
+  value.
+- The `ParticleIDMeta::paramNames` is assumed to match the parameters as they
+  are set in the `ParticleID` elements.
+
+Additionally there are a few usage considerations to be kept in mind
+- The `PIDHandler` can be used without any meta data about the contained
+  `ParticleID`s. However, in this case it will only be useful for inverting
+  relations and for getting `ParticleID`s that point to a given
+  `ReconstructedParticle`.
+- There is no guarantee that the meta information that is set in a `PIDHandler`
+  is consistent with the elements it knows about. Some constructors attempt to
+  construct a consistent handler, but it is always possible to set meta
+  information via `setMetaInfo`, which might be completely unrelated to the
+  actual contents of a handler.
+
+Given that there is no enforced consistency the interface and the utility
+functionality makes heavy use of
+[`std::optional`](https://en.cppreference.com/w/cpp/utility/optional) in its
+return values / types. This makes it possible to check whether the desired
+information was actually present, without having to reserve some special values
+to indicate that fact.
+
+## Usage examples
+
+**The following examples assume the following includes** (which will not be visible in the examples)
+
+```cpp
+#include <edm4hep/utils/ParticleIDUtils.h>
+#include <edm4hep/ParticleIDCollection.h>
+
+#include <podio/Frame.h>
+```
+
+**The following examples additionally assume, that there is a `metadata` and an
+`event` Frame present.** The `metadata` Frame can be obtained like this from
+standard EDM4hep files.
+
+```cpp
+#include <podio/ROOTReader.h>
+#include <podio/Frame.h>
+
+podio::Frame getMetadataFrame(const std::string& filename) {
+  podio::ROOTReader reader{};
+  reader.openFile(filename);
+  return podio::Frame(reader.readNextEntry(podio::Category::Metadata));
+}
+```
+
+Alternatively it can be created as an empty Frame and then just written using
+the `podio::Category::Metadata` category name.
+
+**Finally most of the examples assume that the desired values were found and
+simply get the `value` from the returned `std::optional` directly.** This means
+that most of the times you will see lines like this
+
+```cpp 
+const auto value = pidHandler.getAlgoType("someAlgo").value();
+```
+
+This will throw an exception if some information is not available. If you want /
+need to check first whether information is present you can do
+
+```cpp 
+const auto maybeValue = pidHandler.getAlgoType("someAlgo");
+if (maybeValue.has_value()) {
+  const auto value = maybeValue.value();
+}
+```
+
+### Creating a PIDHandler for a complete event
+
+If you want to work with a `PIDHandler` that has a somewhat global view of all
+`ParticleID`s that are present you can simply construct one from an event Frame
+
+```cpp
+const auto pidHandler = edm4hep::utils::PIDHandler::from(event);
+```
+
+You can also construct a `pidHandler` that populates some meta information
+internally by also passing in the `metadata` Frame to this constructor
+```cpp
+const auto pidHandler = edm4hep::utils::PIDHandler::from(event, metadata);
+```
+
+### Creating a PIDHandler from an arbitrary number of collections
+
+If you simply want to use a `PIDHandler` to invert the relations for a given set
+of `ParticleIDCollection`s you can do that via
+
+```cpp
+const auto& tofPID = event.get<edm4hep::ParticleIDCollection>("ToFPID");
+const auto& dNdXPID = event.get<edm4hep::ParticleIDCollection>("dNdXPID");
+const auto& mvaPID = event.get<edm4hep::ParticleIDCollection>("mvaPID");
+
+const auto pidHandler = edm4hep::utils::PIDHandler::from(tofPID, dNdXPID, mvaPID);
+```
+
+**This handler will not have any access to metadata!** If you need metadata as
+well, you can add that after the fact using the `addMetaInfo` method.
+
+### Using a PIDHandler to get the ParticleIDs for a reco particle
+
+If you have constructed a `PIDHandler` you can use it to get all or specific
+related `ParticleID` objects of a `ReconstructedParticle`
+
+```cpp
+const auto pidHandler = /* constructed as above somehow */;
+
+const auto recos = event.get<edm4hep::ReconstructedParticle>("recos");
+const auto reco = recos[0];
+
+// Get all related ParticleIDs
+const auto pids = pidHandler.getPIDs(reco);
+
+// Only get a specific ParticleID
+const auto tofAlgoType = pidHandler.getAlgoType("ToFPID").value();
+const auto tofPID = pidHandler.getPID(reco, tofAlgoType).value();
+```
+
+### Retrieving meta information for a collection
+
+If you simply want to get the meta information for a collection (called "ToFPID"
+in this example) do
+
+```cpp
+const auto algoInfo = edm4hep::utils::PIDHandler::getAlgoInfo(metadata, "ToFPID").value();
+```
+
+
+### Using meta information of a collection
+
+If you have retrieved the meta information you can directly use that to learn
+something about the `ParticleID`s it describes
+
+```cpp
+const auto& tofPIDs = event.get<edm4hep::ParticleIDCollection>("ToFPID");
+const auto algoInfo = edm4hep::utils::PIDHandler::getAlgoInfo(metadata, "ToFPID").value();
+const auto timeIndex = edm4hep::utils::getParamIndex(algoInfo, "time").value();
+
+for (const auto tof : tofPIDs) {
+  const auto time = tof.getParameters(timeIndex);
+  // ...
+}
+```
+
+If you have a `PIDHandler` with enough meta information set it is also possible
+to retrieve the `timeIndex` from that
+
+```cpp 
+const auto pidHandler = /* construct somehow see above */
+const auto tofAlgoType = pidHandler.getAlgoType("ToFPID").value();
+const auto timeIndex = pidHandler.getParamIndex("time").value();
+
+// ... as above
+```
+
+
+### Setting meta information for a collection
+
+If you have a `ParticleIDCollection` and want to persist some meta information
+for it the following snippet shows you how to do that
+
+```cpp
+// Create ParticleID meta information for an algorihtm
+const auto tofMeta = {"TimeOfFlight", 42, {"time", "time_error"}};
+
+auto tofPIDs = edm4hep::ParticleIDCollection{};
+// ... fill collection
+
+edm4hep::utils::PIDHandler::setAlgoInfo(metadata, tofPIDs, "ToFPID", tofMeta);
+event.put(std::move(tofPIDs), "ToFPID");
+```
+
+This will put the necessary metadata into the `metadata` Frame and also take
+care of setting the `algorithmType` field of all elements of the `tofPIDs`
+collection to `42` (the value it is set to in the `tofMeta` object).
+
+**Things to note:**
+- It is important to use the same names for putting the collection into the
+  `event` Frame and the one that is passed to `PIDHandler::setAlgoInfo`!
+
+### Setting meta information using a collection name 
+
+If you want to set the meta information for a `ParticleIDCollection` that you no
+longer have mutable access to, this can be done via
+
+```cpp
+// Create ParticleID meta information for an algorihtm
+const auto tofMeta = {"TimeOfFlight", 42, {"time", "time_error"}};
+
+edm4hep::utils::PIDHandler::setAlgoInfo(metadata, "ToFPID", tofMeta);
+```
+
+**Things to note:**
+- In this case it is user responsibility to set the `algoType` of the
+  `ParticleIDMeta` to the same value as the one that is used for the elements of
+  the collection.
+- It is important to use the same names for putting the collection into the
+  `event` Frame and the one that is passed to `PIDHandler::setAlgoInfo`!

utils/include/edm4hep/utils/ParticleIDUtils.h

@@ -25,6 +25,7 @@ struct ParticleIDMeta {
 std::optional<int> getParamIndex(const ParticleIDMeta& pidMetaInfo, const std::string& param);
 
 /// Utility class to invert the ParticleID to ReconstructedParticle relation
+/// See [this page](@ref md_doc_2_p_i_d_handler) for example usage and more information.
 class PIDHandler {
 
   using RecoPidMapT = std::multimap<edm4hep::ReconstructedParticle, edm4hep::ParticleID>;