7.5. External PDF

To use an external PDF (not included in LHAPDF) in Sherpa, you need to provide an interface to your PDF in an external dynamic library. This library is then loaded at runtime and it is possible within Sherpa to access all PDFs included.

The simplest C++ code to implement your interface looks as follows

#include "PDF/Main/PDF_Base.H"

using namespace PDF;

class Example_PDF: public PDF_Base {
public:
  void Calculate(double x,double Q2)
  {
    // calculate values x f_a(x,Q2) for all a
  }
  double GetXPDF(const ATOOLS::Flavour a)
  {
    // return x f_a(x,Q2)
  }
  virtual PDF_Base *GetCopy()
  {
    return new Example_PDF();
  }
};// end of class Example_PDF

// this makes Example_PDF loadable in Sherpa
DECLARE_PDF_GETTER(Example_PDF_Getter);
PDF_Base *Example_PDF_Getter::operator()(const Parameter_Type &args) const
{ return new Example_PDF(); }
// this eventually prints a help message
void Example_PDF_Getter::PrintInfo
(std::ostream &str,const size_t width) const
{ str<<"example PDF"; }
// this lets Sherpa initialize and unload the library
Example_PDF_Getter *p_get=NULL;
extern "C" void InitPDFLib()
{ p_get = new Example_PDF_Getter("ExamplePDF"); }
extern "C" void ExitPDFLib() { delete p_get; }

If the code is compiled into a library called libExamplePDFSherpa.so, then this library is loaded dynamically in Sherpa using PDF_LIBRARY: ExamplePDFSherpa either on the command line, or in the Sherpa.yaml. If the library is bound at compile time, like e.g. in cmt, you may skip this step. It is now possible to list all accessible PDF sets by specifying SHOW_PDF_SETS: 1 on the command line.

Finally Sherpa is instructed to retrieve the external PDF by specifying PDF_SET: ExamplePDF on the command line or in the Sherpa.yaml.