RKCurvilinearDistance.h

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#ifndef RKCurvilinearDistance_H
#define RKCurvilinearDistance_H

#include "DataFormats/GeometryVector/interface/Basic3DVector.h"
#include "FWCore/Utilities/interface/Visibility.h"
#include "RKDistance.h"
#include "RKSmallVector.h"

template <typename T, int N>
class dso_internal RKCurvilinearDistance : public RKDistance<T, N> {
public:
  typedef T Scalar;
  typedef RKSmallVector<T, N> Vector;

  ~RKCurvilinearDistance() override {}

  Scalar operator()(const Vector& a, const Vector& b, const Scalar& s) const override {
    Basic3DVector<Scalar> amom = momentum(a);
    Basic3DVector<Scalar> bmom = momentum(b);

    return sqrt(sqr(a(0) - b(0)) + sqr(a(1) - b(1))) + (amom - bmom).mag() / bmom.mag();
  }

  Basic3DVector<Scalar> momentum(const Vector& v) const {
    Scalar k = sqrt(1 + sqr(v(2)) + sqr(v(3)));
    Scalar p = std::abs(1 / v(4));
    Scalar pz = p / k;
    return Basic3DVector<Scalar>(v(2) * pz, v(3) * pz, pz);
  }

  T sqr(const T& t) const { return t * t; }
};

#endif