deltaR.h

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#ifndef DataFormats_Math_deltaR_h
#define DataFormats_Math_deltaR_h
/* functions to compute deltaR
 *
 * Ported from original code in RecoJets 
 * by Fedor Ratnikov, FNAL
 */
#include "DataFormats/Math/interface/deltaPhi.h"
#include <cmath>

namespace reco {

  // assumption is that eta and phi are cached AND phi is computed using std::atan2
  // type is the type of T1::eta();
  template <typename T1, typename T2>
  constexpr auto deltaR2(const T1& t1, const T2& t2) -> decltype(t1.eta()) {
    typedef decltype(t1.eta()) Float;
    Float p1 = t1.phi();
    Float p2 = t2.phi();
    Float e1 = t1.eta();
    Float e2 = t2.eta();
    auto dp = std::abs(p1 - p2);
    if (dp > Float(M_PI))
      dp -= Float(2 * M_PI);
    return (e1 - e2) * (e1 - e2) + dp * dp;
  }

  // do not use it: always cut in deltaR2!
  template <typename T1, typename T2>
  constexpr auto deltaR(const T1& t1, const T2& t2) -> decltype(t1.eta()) {
    return std::sqrt(deltaR2(t1, t2));
  }

  //prefer the above...
  template <class T1, class T2, class T3, class T4>
  constexpr T1 deltaR2(T1 eta1, T2 phi1, T3 eta2, T4 phi2) {
    T1 deta = eta1 - eta2;
    T1 dphi = std::abs(phi1 - phi2);
    if (dphi > T1(M_PI))
      dphi -= T1(2 * M_PI);
    return deta * deta + dphi * dphi;
  }

  // to be avoided
  template <class T1, class T2, class T3, class T4>
  constexpr T1 deltaR(T1 eta1, T2 phi1, T3 eta2, T4 phi2) {
    return std::sqrt(deltaR2(eta1, phi1, eta2, phi2));
  }

}  // namespace reco

// woderful!  VI
using reco::deltaR;
using reco::deltaR2;

// obsolete use lambdas (and cut in deltaR2!)
template <typename T1, typename T2 = T1>
struct DeltaR {
  constexpr double operator()(const T1& t1, const T2& t2) const { return reco::deltaR(t1, t2); }
};
#endif