Resolution.cc

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//
//
// File: src/Resolution.cc
// Purpose: Calculate resolutions for a quantity.
// Created: Jul, 2000, sss, based on run 1 mass analysis code.
//
// CMSSW File      : src/Resolution.cc
// Original Author : Scott Stuart Snyder <snyder@bnl.gov> for D0
// Imported to CMSSW by Haryo Sumowidagdo <Suharyo.Sumowidagdo@cern.ch>
//

#include "TopQuarkAnalysis/TopHitFit/interface/Resolution.h"
#include "CLHEP/Random/RandGauss.h"
#include <cmath>
#include <iostream>
#include <cctype>
#include <cstdlib>

using std::isdigit;
using std::isspace;
using std::ostream;
using std::sqrt;
using std::string;
#ifndef __GNUC__
using std::atof;
#endif

namespace {

  bool get_field(string s, string::size_type i, double& x)
  //
  // Purpose: Scan string S starting at position I.
  //          Find the value of the first floating-point number we
  //          find there.
  //
  // Inputs:
  //   s -           The string to scan.
  //   i -           Starting character position in the string.
  //
  // Outputs:
  //   x -           The value of the number we found.
  //
  // Returns:
  //   True if we found something that looks like a number, false otherwise.
  //
  {
    string::size_type j = i;
    while (j < s.size() && s[j] != ',' && !isdigit(s[j]) && s[j] != '.')
      ++j;
    if (j < s.size() && (isdigit(s[j]) || s[j] == '.')) {
      x = atof(s.c_str() + j);
      return true;
    }
    return false;
  }

}  // unnamed namespace

namespace hitfit {

  Resolution::Resolution(std::string s /*= ""*/)
      //
      // Purpose: Constructor.
      //
      // Inputs:
      //   s -           String encoding the resolution parameters, as described
      //                 in the comments in the header.
      //
      : _resolution_exponent(0) {
    _inverse = false;
    _constant_sigma = 0;
    _resolution_sigma = 0;
    _noise_sigma = 0;

    // Skip spaces.
    double x;
    string::size_type i = 0;
    while (i < s.size() && isspace(s[i]))
      ++i;

    // Check for the inverse flag.
    if (s[i] == '-') {
      _inverse = true;
      ++i;
    } else if (s[i] == '+') {
      ++i;
    }

    // Get the constant term.
    if (get_field(s, i, x))
      _constant_sigma = x;
    i = s.find(',', i);

    // Look for a resolution term.
    if (i != string::npos) {
      ++i;
      if (get_field(s, i, x))
        _resolution_sigma = x;

      // Look for a noise term.
      i = s.find(',', i);
      if (i != string::npos) {
        if (get_field(s, i + 1, x))
          _noise_sigma = x;
      }
    }
  }

  Resolution::Resolution(double C, double R, double m, double N, bool inverse /*= false*/)
      : _constant_sigma(C), _resolution_sigma(R), _resolution_exponent(m), _noise_sigma(N), _inverse(inverse) {}

  Resolution::Resolution(double res, bool inverse /*= false*/)
      //
      // Purpose: Constructor, to give a constant resolution.
      //          I.e., sigma() will always return RES.
      //
      // Inputs:
      //   res -         The resolution value.
      //   inverse -     The inverse flag.
      //
      : _constant_sigma(0), _resolution_sigma(0), _resolution_exponent(0), _noise_sigma(res), _inverse(inverse) {}

  bool Resolution::inverse() const
  //
  // Purpose: Return the inverse flag.
  //
  // Returns:
  //   The inverse flag.
  //
  {
    return _inverse;
  }

  double Resolution::C() const { return _constant_sigma; }

  double Resolution::R() const { return _resolution_sigma; }

  double Resolution::m() const { return _resolution_exponent; }

  double Resolution::N() const { return _noise_sigma; }

  double Resolution::sigma(double p) const
  //
  // Purpose: Return the uncertainty for a momentum P.
  //
  // Inputs:
  //    p -          The momentum
  //
  // Returns:
  //   The uncertainty for a momentum P.
  //
  {
    if (_inverse)
      p = 1 / p;

    return sqrt((_constant_sigma * _constant_sigma * p + _resolution_sigma * _resolution_sigma) * p +
                _noise_sigma * _noise_sigma);
  }

  double Resolution::pick(double x, double p, CLHEP::HepRandomEngine& engine) const
  //
  // Purpose: 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).
  //
  // Inputs:
  //    x -          The quantity value (distribution mean).
  //    p -          The momentum, for calculating the width.
  //    engine -     The underlying RNG.
  //
  // Returns:
  //   A smeared value of X.
  //
  {
    CLHEP::RandGauss gen(engine);
    if (_inverse)
      return 1 / gen.fire(1 / x, sigma(p));
    else
      return gen.fire(x, sigma(p));
  }

  std::ostream& operator<<(std::ostream& s, const Resolution& r)
  //
  // Purpose: Dump this object to S.
  //
  // Inputs:
  //    s -          The stream to which to write.
  //    r -          The object to dump.
  //
  // Returns:
  //   The stream S.
  //
  {
    if (r._inverse)
      s << "-";
    s << r._constant_sigma << "," << r._resolution_sigma << "," << r._noise_sigma;
    return s;
  }

}  // namespace hitfit