Draft: Adding skeleton helper library.

new file: source/CMakeLists.txt new file: source/include/eicd/helpers.h new file: source/src/helpers.cxx

source/include/eicd/helpers.h

+#ifndef EICD_HELPERS_HH
+#define EICD_HELPERS_HH
+
+
+#include <algorithm>
+#include <cmath>
+#include <exception>
+#include <limits>
+#include <string>
+#include <vector>
+
+#include <Math/Vector4D.h>
+
+#include "eicd/TrackParametersCollection.h"
+#include "eicd/ReconstructedParticleCollection.h"
+#include "eicd/ReconstructedParticleData.h"
+
+namespace eicd::helpers {
+
+  /** Four momentum from track and mass.
+   * Get a vector of 4-momenta from raw tracking info, using an externally
+   * provided particle mass assumption. 
+   */
+  inline auto momenta_from_tracking(const std::vector<eic::TrackParametersData>& tracks,
+                                    const double                                 mass)
+  {
+    std::vector<ROOT::Math::PxPyPzMVector> momenta{tracks.size()};
+    // transform our raw tracker info into proper 4-momenta
+    std::transform(tracks.begin(), tracks.end(), momenta.begin(), [mass](const auto& track) {
+      // make sure we don't divide by zero
+      if (fabs(track.qOverP) < 1e-9) {
+        return ROOT::Math::PxPyPzMVector{};
+      }
+      const double p  = fabs(1. / track.qOverP);
+      const double px = p * cos(track.phi) * sin(track.theta);
+      const double py = p * sin(track.phi) * sin(track.theta);
+      const double pz = p * cos(track.theta);
+      return ROOT::Math::PxPyPzMVector{px, py, pz, mass};
+    });
+    return momenta;
+  }
+} // namespace eicd::helpers
+#endif