Resolution.h

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//
// $Id: Resolution.h,v 1.2 2012/03/19 18:15:04 vadler Exp $
//
// File: hitfit/Resolution.h
// Purpose: Calculate resolutions for a quantity.
// Created: Jul, 2000, sss, based on run 1 mass analysis code.
//
// This object will calculate resolutions for some quantity.
// We have three parameters:
//
//   C - constant term
//   R - resolution term
//   N - noise term
//
// Given a `momentum' p, we calculate the uncertainty in a quantity x as
//
//   sigma(x) = sqrt (C^2 p^2 + R^2 p + N^2)
//
// In addition, we have an `inverse' flag.  If that is set,
// we take the inverse of p before doing the above calculation
// (and for p, `sigma(p)' is regarded as actually sigma(1/p)).
//
// We encode the resolution parameters into a string, from which these
// objects get initialized.  The format is
//
//    [-]C[,R[,N]]
//
// If a leading minus sign is present, that turns on the invert flag.
// Omitted parameters are set to 0.
//
// CMSSW File      : interface/Resolution.h
// Original Author : Scott Stuart Snyder <snyder@bnl.gov> for D0
// Imported to CMSSW by Haryo Sumowidagdo <Suharyo.Sumowidagdo@cern.ch>
//


#ifndef HITFIT_RESOLUTION_H
#define HITFIT_RESOLUTION_H


#include <string>
#include <iosfwd>
#include "CLHEP/Random/Random.h"


namespace hitfit {


class Resolution
//
// Purpose: Calculate resolutions for a quantity.
//
{
public:
  // Initialize from a string S.  The format is as described above.
  Resolution (std::string s = "");

  Resolution (double C,
              double R,
              double m,
              double N,
              bool inverse = false);

  // Initialize to a constant resolution RES.  I.e., sigma() will
  // always return RES.  If INVERSE is true, set the inverse flag.
  Resolution (double res, bool inverse = false);

  // Return the setting of the inverse flag.
  bool inverse () const;

  double C() const;

  double R() const;

  double m() const;

  double N() const;

  // Return the uncertainty for a momentum P.
  double sigma (double p) const;

  // Given a value X, measured for an object with momentum P,
  // pick a new value from a Gaussian distribution
  // described by this resolution --- with mean X and width sigma(P).
  double pick (double x, double p, CLHEP::HepRandomEngine& engine) const;

  // Dump, for debugging.
  friend std::ostream& operator<< (std::ostream& s, const Resolution& r);


private:
  // The resolution parameters.

  double _constant_sigma;

  double _resolution_sigma;

  double _resolution_exponent;

  double _noise_sigma;

  bool _inverse;
};


} // namespace hitfit


#endif // not HITFIT_RESOLUTION_H