RooPowerSum.cxx

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/*
 * Project: RooFit
 *
 * Copyright (c) 2022, CERN
 *
 * Redistribution and use in source and binary forms,
 * with or without modification, are permitted according to the terms
 * listed in LICENSE (http://roofit.sourceforge.net/license.txt)
 */
 
/** \class RooPowerSum
    \ingroup Roofit
 
RooPowerSum implements a power law PDF of the form
\f[ f(x) = \mathcal{N} \cdot \sum_{i} a_{i} * x^{b_i} \f]
 
\image html RooPowerSum.png
**/
 
#include <RooPowerSum.h>
 
#include <RooAbsReal.h>
#include <RooArgList.h>
#include <RooMsgService.h>
#include "RooBatchCompute.h"
 
#include <TError.h>
 
#include <array>
#include <cassert>
#include <cmath>
#include <sstream>
 
 
////////////////////////////////////////////////////////////////////////////////
/// Create a power law in the variable `x`.
/// \param[in] name Name of the PDF
/// \param[in] title Title for plotting the PDF
/// \param[in] x The variable of the polynomial
/// \param[in] coefList The coefficients \f$ a_i \f$
/// \param[in] expList The exponentials \f$ b_i \f$
/// \f[
///     \sum_{i=0}^{n} a_{i} * x^{b_{i}}
/// \f]
///
/// This means that
/// \code{.cpp}
/// RooPowerSum powl("pow", "pow", x, RooArgList(a1, a2), RooArgList(b1,b2))
/// \endcode
/// computes
/// \f[
///   \mathrm{pol}(x) = a1 * x^b1 + a2 * x^b2
/// \f]
 
RooPowerSum::RooPowerSum(const char *name, const char *title, RooAbsReal &x, const RooArgList &coefList,
                   const RooArgList &expList)
   : RooAbsPdf(name, title),
     _x("x", "Dependent", this, x),
     _coefList("coefList", "List of coefficients", this),
     _expList("expList", "List of exponents", this)
{
   if (coefList.size() != expList.size()) {
      coutE(InputArguments) << "RooPowerSum::ctor(" << GetName()
                            << ") ERROR: coefficient list and exponent list must be of same length" << std::endl;
      return;
   }
   _coefList.addTyped<RooAbsReal>(coefList);
   _expList.addTyped<RooAbsReal>(expList);
}
 
////////////////////////////////////////////////////////////////////////////////
/// Copy constructor
 
RooPowerSum::RooPowerSum(const RooPowerSum &other, const char *name)
   : RooAbsPdf(other, name),
     _x("x", this, other._x),
     _coefList("coefList", this, other._coefList),
     _expList("expList", this, other._expList)
{
}
 
////////////////////////////////////////////////////////////////////////////////
 
/// Compute multiple values of Power distribution.
void RooPowerSum::doEval(RooFit::EvalContext &ctx) const
{
    std::vector<std::span<const double>> vars;
    vars.reserve(2 *  _coefList.size() + 1);
    vars.push_back(ctx.at(_x));
 
    assert(_coefList.size() == _expList.size());
 
   for (std::size_t i = 0; i < _coefList.size(); ++i) {
     vars.push_back(ctx.at(&_coefList[i]));
     vars.push_back(ctx.at(&_expList[i]));
   }
 
   std::array<double, 1> args{static_cast<double>(_coefList.size())};
 
   RooBatchCompute::compute(ctx.config(this), RooBatchCompute::Power, ctx.output(), vars, args);
}
 
////////////////////////////////////////////////////////////////////////////////
 
double RooPowerSum::evaluate() const
{
   // Calculate and return value of polynomial
 
   const unsigned sz = _coefList.size();
   if (!sz) {
      return 0.;
   }
 
   std::vector<double> coefs;
   std::vector<double> exps;
   coefs.reserve(sz);
   exps.reserve(sz);
   const RooArgSet *nset = _coefList.nset();
   for (auto c : _coefList) {
      coefs.push_back(static_cast<RooAbsReal *>(c)->getVal(nset));
   }
   for (auto c : _expList) {
      exps.push_back(static_cast<RooAbsReal *>(c)->getVal(nset));
   }
   double x = this->_x;
   double retval = 0;
   for (unsigned int i = 0; i < sz; ++i) {
      retval += coefs[i] * std::pow(x, exps[i]);
   }
   return retval;
}
 
////////////////////////////////////////////////////////////////////////////////
/// Advertise to RooFit that this function can be analytically integrated.
int RooPowerSum::getAnalyticalIntegral(RooArgSet &allVars, RooArgSet &analVars, const char * /*rangeName*/) const
{
   if (matchArgs(allVars, analVars, _x))
      return 1;
   return 0;
}
 
////////////////////////////////////////////////////////////////////////////////
/// Do the analytical integral according to the code that was returned by getAnalyticalIntegral().
double RooPowerSum::analyticalIntegral(int /*code*/, const char *rangeName) const
{
   const double xmin = _x.min(rangeName);
   const double xmax = _x.max(rangeName);
   const unsigned sz = _coefList.size();
   if (!sz) {
      return xmax - xmin;
   }
 
   std::vector<double> coefs;
   std::vector<double> exps;
   coefs.reserve(sz);
   exps.reserve(sz);
   const RooArgSet *nset = _coefList.nset();
   for (auto c : _coefList) {
      coefs.push_back(static_cast<RooAbsReal *>(c)->getVal(nset));
   }
   for (auto c : _expList) {
      exps.push_back(static_cast<RooAbsReal *>(c)->getVal(nset));
   }
 
   double retval = 0;
   for (unsigned int i = 0; i < sz; ++i) {
      if (exps[i] == -1) {
         retval += coefs[i] * (log(xmax) - log(xmin));
      } else {
         retval += coefs[i] / (exps[i] + 1) * (pow(xmax, (exps[i] + 1)) - pow(xmin, (exps[i] + 1)));
      }
   }
   return retval;
}
 
std::string RooPowerSum::getFormulaExpression(bool expand) const
{
   std::stringstream ss;
   for (std::size_t i = 0; i < _coefList.size(); ++i) {
      if (i != 0)
         ss << "+";
      if (expand) {
         ss << static_cast<RooAbsReal *>(_coefList.at(i))->getVal();
      } else {
         ss << _coefList.at(i)->GetName();
      }
      ss << "*pow(" << _x.GetName() << ",";
      if (expand) {
         ss << static_cast<RooAbsReal *>(_expList.at(i))->getVal();
      } else {
         ss << _expList.at(i)->GetName();
      }
      ss << ")";
   }
   return ss.str();
}