[math] add DeltaR to GenVector

math/genvector/inc/Math/GenVector/LorentzVector.h

@@ -24,6 +24,8 @@
 
 #include "Math/GenVector/GenVectorIO.h"
 
+#include "TMath.h"
+
 #include <cmath>
 #include <string>
 
@@ -364,6 +366,29 @@ ROOT provides specialisations and aliases to them of the ROOT::Math::LorentzVect
        */
        Scalar Eta() const { return fCoordinates.Eta(); }
 
+       /**
+          deltaRapidity between this and vector v
+          \f[ \Delta R = \sqrt { \Delta \eta ^2 + \Delta \phi ^2 } \f]
+          \param useRapidity true to use Rapidity(), false to use Eta()
+       */
+       template <class OtherLorentzVector>
+       Scalar DeltaR(const OtherLorentzVector &v, const bool useRapidity = false) const
+       {
+          const double delta = useRapidity ? Rapidity() - v.Rapidity() : Eta() - v.Eta();
+          double dphi = Phi() - v.Phi();
+          // convert dphi angle to the interval (-PI,PI]
+          if (dphi > TMath::Pi() || dphi <= -TMath::Pi()) {
+             if (dphi > 0) {
+                int n = static_cast<int>(dphi / TMath::TwoPi() + 0.5);
+                dphi -= TMath::TwoPi() * n;
+             } else {
+                int n = static_cast<int>(0.5 - dphi / TMath::TwoPi());
+                dphi += TMath::TwoPi() * n;
+             }
+          }
+          return std::sqrt(delta * delta + dphi * dphi);
+       }
+
        /**
           get the spatial components of the Vector in a
           DisplacementVector based on Cartesian Coordinates

math/genvector/test/testGenVector.cxx

@@ -19,6 +19,8 @@
 #include "Math/RotationZYX.h"
 
 #include "Math/LorentzRotation.h"
+#include "Math/PtEtaPhiM4D.h"
+#include "Math/LorentzVector.h"
 
 #include "Math/VectorUtil.h"
 #ifndef NO_SMATRIX
@@ -794,6 +796,24 @@ int testVectorUtil() {
 
 }
 
+int testLorentzVector()
+{
+   std::cout << "testing LorentzVector  \t:\t";
+   int iret = 0;
+   LorentzVector<PtEtaPhiM4D<float>> v1(1,2,3,4);
+   LorentzVector<PtEtaPhiM4D<float>> v2(5,6,7,8);
+   iret |= compare(v1.DeltaR(v2), 4.60575f);
+   // Result cross-validated using:
+   // TLorentzVector t1, t2;
+   // t1.SetPtEtaPhiE(1,2,3,4); t2.SetPtEtaPhiE(5,6,7,8);
+   // t1.DeltaR(t2)
+   if (iret == 0)
+      std::cout << "\t\t\tOK\n";
+   else
+      std::cout << "\t\t\t\t\t\tFAILED\n";
+   return iret;
+}
+
 int testGenVector() {
 
   int iret = 0;
@@ -808,7 +828,7 @@ int testGenVector() {
   iret |= testTransform3D();
 
   iret |= testVectorUtil();
-
+  iret |= testLorentzVector();
 
   if (iret !=0) std::cout << "\nTest GenVector FAILED!!!!!!!!!\n";
   return iret;