## main296-python

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    # main296.py is a part of the PYTHIA event generator.
    # Copyright (C) 2025 Torbjorn Sjostrand.
    # PYTHIA is licenced under the GNU GPL v2 or later, see COPYING for details.
    # Please respect the MCnet Guidelines, see GUIDELINES for details.

    # Keywords:
    #            Python
    #            Total cross section
    #            Partial cross sections

    # Authors:
    #            Christian Bierlich

    # This code is an example of how Pythia can be used as a shared library.
    # The physics case is a study of total cross sections, which cannot be
    # exposed through the normal Python interface. Instead a class encapsulating
    # the desired functionality (PythiaCrossSection) is implemented in
    # main296.cc, and exposed to calls below using ctypes, which comes
    # standard with Python.
    # To use this example, main296Lib.cc must first be compiled as a shared
    # library with make libmain296Lib.so, and then the Python code below run.

    # Import ctypes and load the library.
    import ctypes
    lib = ctypes.cdll.LoadLibrary('./libmain296Lib.so')

    # The Python class, PythiaCrossSection, is the Python interface to the
    # PythiaCrossSection C++ class defined in main296.cc.
    # The class is written as a resource class, with the actual interface only
    # accesible inside the class as PythiaInterfacer, to ensure proper garbage
    # collection.
    # The PythiaCrossSection class should therefore only be instantiated using
    # a with statement, which will give direct access to the interface, as seen
    # in the example use case below.
    class PythiaCrossSection:
        def __init__(self, modeSigma):
            self.modeSigma = modeSigma
        def __enter__(self):
            # The inner class, which provides the interface.
            class PythiaInterfacer:
                # Argument types and return types across the interface must be
                # declared as follows.
                def __init__(self, modeSigma):
                    lib.PythiaCrossSection_new.argtypes = [ctypes.c_int]
                    lib.PythiaCrossSection_new.restype = ctypes.c_void_p

                    lib.PythiaCrossSection_del.argtypes = [ctypes.c_void_p]

                    lib.PythiaCrossSection_calc.argtypes = [ctypes.c_void_p,
                    ctypes.c_int, ctypes.c_int, ctypes.c_double]

                    lib.PythiaCrossSection_sigmaTot.argtypes = [ctypes.c_void_p]
                    lib.PythiaCrossSection_sigmaTot.restype = ctypes.c_double

                    lib.PythiaCrossSection_sigmaEl.argtypes = [ctypes.c_void_p]
                    lib.PythiaCrossSection_sigmaEl.restype = ctypes.c_double

                    self.obj = lib.PythiaCrossSection_new(modeSigma)

                # Calculate cross sections (done in the C++ class).
                def calc(self, idA, idB, sqrtS):
                    lib.PythiaCrossSection_calc(self.obj, idA, idB, sqrtS)

                # Return the total cross section in mb.
                def sigmaTot(self):
                    return lib.PythiaCrossSection_sigmaTot(self.obj)

                # Return the elastic cross section in mb.
                def sigmaEl(self):
                    return lib.PythiaCrossSection_sigmaEl(self.obj)

                # Clean up.
                def cleanup(self):
                    lib.PythiaCrossSection_del(self.obj)

            self.interfacer = PythiaInterfacer(self.modeSigma)
            return self.interfacer
        # Clean up the interface by deleting the Pythia object.
        def __exit__(self, exc_type, exc_value, traceback):
            self.interfacer.cleanup()

    # The following is the example code illustrating a use case of the above.
    # Calculate cross section in the range 10-10000 GeV.
    sqrts = [10.*i for i in range(1,1000)]
    ppTotal = []
    ppElastic = []
    # mode = 1 is The Schuler and Sjostrand model.
    with PythiaCrossSection(1) as pythiaCross:
        for sqs in sqrts:
            pythiaCross.calc(2212,2212,sqs)
            ppTotal.append(pythiaCross.sigmaTot())
            ppElastic.append(pythiaCross.sigmaEl())

    # Use matplotlib to directly plot the results.
    import matplotlib.pyplot as plt

    plt.plot(sqrts, ppTotal,color='red',label=r'$\sigma_{tot}$ (SaS)')
    plt.plot(sqrts, ppElastic,color='blue',label=r'$\sigma_{el}$ (SaS)')

    plt.xlabel(r'$\sqrt{s}$ [GeV]')
    plt.ylabel(r'$\sigma$ [mb]')
    plt.xscale('log')
    plt.legend()
    plt.show()