source: svn/trunk/Utilities/HepMC/src/GenEvent.cc@ 760

//////////////////////////////////////////////////////////////////////////
// Matt.Dobbs@Cern.CH, September 1999
// Updated: 7.1.2000 iterators complete and working!
// Updated: 10.1.2000 GenEvent::vertex, particle iterators are made 
//                    constant WRT this event ... note that 
//                    GenVertex::***_iterator is not const, since it must
//                    be able to return a mutable pointer to itself.
// Updated: 08.2.2000 the event now holds a set of all attached vertices
//                    rather than just the roots of the graph
// Event record for MC generators (for use at any stage of generation)
//////////////////////////////////////////////////////////////////////////

#include <iomanip>

#include "GenEvent.h"
#include "GenCrossSection.h"
#include "Version.h"
#include "StreamHelpers.h"

namespace HepMC {

    GenEvent::GenEvent( int signal_process_id, 
                        int event_number,
                        GenVertex* signal_vertex,
                        const WeightContainer& weights,
                        const std::vector<long>& random_states,
                        Units::MomentumUnit mom, 
                        Units::LengthUnit len ) :
        m_signal_process_id(signal_process_id), 
        m_event_number(event_number),
        m_mpi(-1),
        m_event_scale(-1), 
        m_alphaQCD(-1), 
        m_alphaQED(-1),
        m_signal_process_vertex(signal_vertex), 
        m_beam_particle_1(0),
        m_beam_particle_2(0),
        m_weights(weights),
        m_random_states(random_states),
        m_vertex_barcodes(),
        m_particle_barcodes(),
        m_cross_section(0), 
        m_heavy_ion(0), 
        m_pdf_info(0),
        m_momentum_unit(mom),
        m_position_unit(len)
    {
        /// This constructor only allows null pointers to HeavyIon and PdfInfo
        ///
        /// note: default values for m_event_scale, m_alphaQCD, m_alphaQED
        ///       are as suggested in hep-ph/0109068, "Generic Interface..."
        ///
    }

    GenEvent::GenEvent( int signal_process_id, int event_number,
                        GenVertex* signal_vertex,
                        const WeightContainer& weights,
                        const std::vector<long>& random_states,
                        const HeavyIon& ion, 
                        const PdfInfo& pdf,
                        Units::MomentumUnit mom, 
                        Units::LengthUnit len ) :
        m_signal_process_id(signal_process_id), 
        m_event_number(event_number),
        m_mpi(-1),
        m_event_scale(-1), 
        m_alphaQCD(-1), 
        m_alphaQED(-1),
        m_signal_process_vertex(signal_vertex), 
        m_beam_particle_1(0),
        m_beam_particle_2(0),
        m_weights(weights),
        m_random_states(random_states), 
        m_vertex_barcodes(),
        m_particle_barcodes(),
        m_cross_section(0), 
        m_heavy_ion( new HeavyIon(ion) ), 
        m_pdf_info( new PdfInfo(pdf) ),
        m_momentum_unit(mom),
        m_position_unit(len)
    {
        /// GenEvent makes its own copy of HeavyIon and PdfInfo
        ///
        /// note: default values for m_event_scale, m_alphaQCD, m_alphaQED
        ///       are as suggested in hep-ph/0109068, "Generic Interface..."
    }

    GenEvent::GenEvent( Units::MomentumUnit mom, 
                        Units::LengthUnit len, 
                        int signal_process_id, 
                        int event_number,
                        GenVertex* signal_vertex,
                        const WeightContainer& weights,
                        const std::vector<long>& random_states ) :
        m_signal_process_id(signal_process_id), 
        m_event_number(event_number),
        m_mpi(-1),
        m_event_scale(-1), 
        m_alphaQCD(-1), 
        m_alphaQED(-1),
        m_signal_process_vertex(signal_vertex), 
        m_beam_particle_1(0),
        m_beam_particle_2(0),
        m_weights(weights),
        m_random_states(random_states),
        m_vertex_barcodes(),
        m_particle_barcodes(),
        m_cross_section(0), 
        m_heavy_ion(0), 
        m_pdf_info(0),
        m_momentum_unit(mom),
        m_position_unit(len)
    {
        /// constructor requiring units - all else is default
        /// This constructor only allows null pointers to HeavyIon and PdfInfo
        ///
        /// note: default values for m_event_scale, m_alphaQCD, m_alphaQED
        ///       are as suggested in hep-ph/0109068, "Generic Interface..."
        ///
    }

    GenEvent::GenEvent( Units::MomentumUnit mom, 
                        Units::LengthUnit len,
                        int signal_process_id, int event_number,
                        GenVertex* signal_vertex,
                        const WeightContainer& weights,
                        const std::vector<long>& random_states,
                        const HeavyIon& ion, 
                        const PdfInfo& pdf ) :
        m_signal_process_id(signal_process_id), 
        m_event_number(event_number),
        m_mpi(-1),
        m_event_scale(-1), 
        m_alphaQCD(-1), 
        m_alphaQED(-1),
        m_signal_process_vertex(signal_vertex), 
        m_beam_particle_1(0),
        m_beam_particle_2(0),
        m_weights(weights),
        m_random_states(random_states), 
        m_vertex_barcodes(),
        m_particle_barcodes(),
        m_cross_section(0), 
        m_heavy_ion( new HeavyIon(ion) ), 
        m_pdf_info( new PdfInfo(pdf) ),
        m_momentum_unit(mom),
        m_position_unit(len)
    {
        /// explicit constructor with units first that takes HeavyIon and PdfInfo
        /// GenEvent makes its own copy of HeavyIon and PdfInfo
        ///
        /// note: default values for m_event_scale, m_alphaQCD, m_alphaQED
        ///       are as suggested in hep-ph/0109068, "Generic Interface..."
    }

    GenEvent::GenEvent( const GenEvent& inevent ) 
      : m_signal_process_id    ( inevent.signal_process_id() ),
        m_event_number         ( inevent.event_number() ),
        m_mpi                  ( inevent.mpi() ),
        m_event_scale          ( inevent.event_scale() ),
        m_alphaQCD             ( inevent.alphaQCD() ),
        m_alphaQED             ( inevent.alphaQED() ),
        m_signal_process_vertex( /* inevent.m_signal_process_vertex */ ),
        m_beam_particle_1      ( /* inevent.m_beam_particle_1 */ ),
        m_beam_particle_2      ( /* inevent.m_beam_particle_2 */ ),
        m_weights              ( /* inevent.m_weights */ ),
        m_random_states        ( /* inevent.m_random_states */ ),
        m_vertex_barcodes      ( /* inevent.m_vertex_barcodes */ ),
        m_particle_barcodes    ( /* inevent.m_particle_barcodes */ ),
        m_cross_section        ( inevent.cross_section() ? new GenCrossSection(*inevent.cross_section()) : 0 ),
        m_heavy_ion            ( inevent.heavy_ion() ? new HeavyIon(*inevent.heavy_ion()) : 0 ),
        m_pdf_info             ( inevent.pdf_info() ? new PdfInfo(*inevent.pdf_info()) : 0 ),
        m_momentum_unit        ( inevent.momentum_unit() ),
        m_position_unit        ( inevent.length_unit() )
    {
        /// deep copy - makes a copy of all vertices!
        //

        // 1. create a NEW copy of all vertices from inevent
        //    taking care to map new vertices onto the vertices being copied
        //    and add these new vertices to this event.
        //    We do not use GenVertex::operator= because that would copy
        //    the attached particles as well.
        std::map<const GenVertex*,GenVertex*> map_in_to_new;
        for ( GenEvent::vertex_const_iterator v = inevent.vertices_begin();
              v != inevent.vertices_end(); ++v ) {
            GenVertex* newvertex = new GenVertex(
                (*v)->position(), (*v)->id(), (*v)->weights() );
            newvertex->suggest_barcode( (*v)->barcode() );
            map_in_to_new[*v] = newvertex;
            add_vertex( newvertex );
        }
        // 2. copy the signal process vertex info.
        if ( inevent.signal_process_vertex() ) {
            set_signal_process_vertex( 
                map_in_to_new[inevent.signal_process_vertex()] );
        } else set_signal_process_vertex( 0 );
        //
        // 3. create a NEW copy of all particles from inevent
        //    taking care to attach them to the appropriate vertex
        GenParticle* beam1(0);
        GenParticle* beam2(0);
        for ( GenEvent::particle_const_iterator p = inevent.particles_begin();
              p != inevent.particles_end(); ++p ) 
        {
            GenParticle* oldparticle = *p;
            GenParticle* newparticle = new GenParticle(*oldparticle);
            if ( oldparticle->end_vertex() ) {
                map_in_to_new[ oldparticle->end_vertex() ]->
                                         add_particle_in(newparticle);
            }
            if ( oldparticle->production_vertex() ) {
                map_in_to_new[ oldparticle->production_vertex() ]->
                                         add_particle_out(newparticle);
            }
            if ( oldparticle == inevent.beam_particles().first ) beam1 = newparticle;
            if ( oldparticle == inevent.beam_particles().second ) beam2 = newparticle;
        }
        set_beam_particles( beam1, beam2 );
        //
        // 4. now that vtx/particles are copied, copy weights and random states
        set_random_states( inevent.random_states() );
        weights() = inevent.weights();
    }

    void GenEvent::swap( GenEvent & other )
    {
        // if a container has a swap method, use that for improved performance
        std::swap(m_signal_process_id    , other.m_signal_process_id    );
        std::swap(m_event_number         , other.m_event_number         );
        std::swap(m_mpi                  , other.m_mpi                  );
        std::swap(m_event_scale          , other.m_event_scale          );
        std::swap(m_alphaQCD             , other.m_alphaQCD             );
        std::swap(m_alphaQED             , other.m_alphaQED             );
        std::swap(m_signal_process_vertex, other.m_signal_process_vertex);
        std::swap(m_beam_particle_1      , other.m_beam_particle_1      );
        std::swap(m_beam_particle_2      , other.m_beam_particle_2      );
        m_weights.swap(           other.m_weights  );
        m_random_states.swap(     other.m_random_states  );
        m_vertex_barcodes.swap(   other.m_vertex_barcodes );
        m_particle_barcodes.swap( other.m_particle_barcodes );
        std::swap(m_cross_section        , other.m_cross_section        );
        std::swap(m_heavy_ion            , other.m_heavy_ion            );
        std::swap(m_pdf_info             , other.m_pdf_info             );
        std::swap(m_momentum_unit       , other.m_momentum_unit       );
        std::swap(m_position_unit       , other.m_position_unit       );
        // must now adjust GenVertex back pointers
        for ( GenEvent::vertex_const_iterator vthis = vertices_begin();
              vthis != vertices_end(); ++vthis ) {
            (*vthis)->change_parent_event_( this );
        }
        for ( GenEvent::vertex_const_iterator voth = other.vertices_begin();
              voth != other.vertices_end(); ++voth ) {
            (*voth)->change_parent_event_( &other );
        }
    }

    GenEvent::~GenEvent() 
    {
        /// Deep destructor.
        /// deletes all vertices/particles in this GenEvent
        /// deletes the associated HeavyIon and PdfInfo
        delete_all_vertices();
        delete m_cross_section;
        delete m_heavy_ion;
        delete m_pdf_info;
    }

    GenEvent& GenEvent::operator=( const GenEvent& inevent ) 
    {
        /// best practices implementation
        GenEvent tmp( inevent );
        swap( tmp );
        return *this;
    }

    void GenEvent::print( std::ostream& ostr ) const {
        /// dumps the content of this event to ostr
        ///   to dump to cout use: event.print();
        ///   if you want to write this event to file outfile.txt you could use:
        ///      std::ofstream outfile("outfile.txt"); event.print( outfile );
        ostr << "________________________________________"
             << "________________________________________\n";
        ostr << "GenEvent: #" << event_number() 
             << " ID=" << signal_process_id() 
             << " SignalProcessGenVertex Barcode: " 
             << ( signal_process_vertex() ? signal_process_vertex()->barcode()
                  : 0 )
             << "\n";
        write_units( ostr );
        write_cross_section(ostr);
        ostr << " Entries this event: " << vertices_size() << " vertices, "
             << particles_size() << " particles.\n"; 
        if( m_beam_particle_1 && m_beam_particle_2 ) {
          ostr << " Beam Particle barcodes: " << beam_particles().first->barcode() << " "
               << beam_particles().second->barcode() << " \n";
        } else {
          ostr << " Beam Particles are not defined.\n";
        }
        // Random State
        ostr << " RndmState(" << m_random_states.size() << ")=";
        for ( std::vector<long>::const_iterator rs 
                  = m_random_states.begin();
              rs != m_random_states.end(); ++rs ) { ostr << *rs << " "; }
        ostr << "\n";
        // Weights
        ostr << " Wgts(" << weights().size() << ")=";
        weights().print(ostr);
        ostr << " EventScale " << event_scale() 
             << " [energy] \t alphaQCD=" << alphaQCD() 
             << "\t alphaQED=" << alphaQED() << std::endl;
        // print a legend to describe the particle info
        ostr << "                                    GenParticle Legend\n";
        ostr  << "        Barcode   PDG ID      "
              << "( Px,       Py,       Pz,     E )"
              << " Stat  DecayVtx\n";
        ostr << "________________________________________"
             << "________________________________________\n";
        // Print all Vertices
        for ( GenEvent::vertex_const_iterator vtx = this->vertices_begin();
              vtx != this->vertices_end(); ++vtx ) {
            (*vtx)->print(ostr); 
        }
        ostr << "________________________________________"
             << "________________________________________" << std::endl;
    }

    void GenEvent::print_version( std::ostream& ostr ) const {
        ostr << "---------------------------------------------" << std::endl;
        writeVersion( ostr );
        ostr << "---------------------------------------------" << std::endl;
    }

    bool GenEvent::add_vertex( GenVertex* vtx ) {
        /// returns true if successful - generally will only return false
        /// if the inserted vertex is already included in the event.
        if ( !vtx ) return false;
        // if vtx previously pointed to another GenEvent, remove it from that
        // GenEvent's list
        if ( vtx->parent_event() && vtx->parent_event() != this ) {
            bool remove_status = vtx->parent_event()->remove_vertex( vtx );
            if ( !remove_status ) {            
                std::cerr << "GenEvent::add_vertex ERROR "
                          << "GenVertex::parent_event points to \n"
                          << "an event that does not point back to the "
                          << "GenVertex. \n This probably indicates a deeper "
                          << "problem. " << std::endl;
            }
        }
        //
        // setting the vertex parent also inserts the vertex into this
        // event
        vtx->set_parent_event_( this );
        return ( m_vertex_barcodes.count(vtx->barcode()) ? true : false );
    }

    bool GenEvent::remove_vertex( GenVertex* vtx ) {
        /// this removes vtx from the event but does NOT delete it.
        /// returns True if an entry vtx existed in the table and was erased
        if ( m_signal_process_vertex == vtx ) m_signal_process_vertex = 0;
        if ( vtx->parent_event() == this ) vtx->set_parent_event_( 0 );
        return ( m_vertex_barcodes.count(vtx->barcode()) ? false : true );
    }

    void GenEvent::clear() 
    {
        /// remove all information from the event
        /// deletes all vertices/particles in this evt
        ///
        delete_all_vertices();
        // remove existing objects and set pointers to null
        delete m_cross_section;
        m_cross_section = 0;
        delete m_heavy_ion;
        m_heavy_ion = 0;
        delete m_pdf_info;
        m_pdf_info = 0;
        m_signal_process_id = 0;
        m_beam_particle_1 = 0;
        m_beam_particle_2 = 0;
        m_event_number = 0;
        m_mpi = -1;
        m_event_scale = -1;
        m_alphaQCD = -1;
        m_alphaQED = -1;
        m_weights = std::vector<double>();
        m_random_states = std::vector<long>();
        // resetting unit information
        m_momentum_unit = Units::default_momentum_unit();
        m_position_unit = Units::default_length_unit();
        // error check just to be safe
        if ( m_vertex_barcodes.size() != 0 
             || m_particle_barcodes.size() != 0 ) {
            std::cerr << "GenEvent::clear() strange result ... \n"
                      << "either the particle and/or the vertex map isn't empty" << std::endl;
            std::cerr << "Number vtx,particle the event after deleting = "
                      << m_vertex_barcodes.size() << "  " 
                      << m_particle_barcodes.size() << std::endl;
        }
        return;
    }
    
    void GenEvent::delete_all_vertices() {
        /// deletes all vertices in the vertex container
        /// (i.e. all vertices owned by this event)
        /// The vertices are the "owners" of the particles, so as we delete
        ///   the vertices, the vertex desctructors are automatically
        ///   deleting their particles.

        // delete each vertex individually (this deletes particles as well)
        while ( !vertices_empty() ) {
            GenVertex* vtx = ( m_vertex_barcodes.begin() )->second;
            m_vertex_barcodes.erase( m_vertex_barcodes.begin() );
            delete vtx;
        }
        //
        // Error checking:
        if ( !vertices_empty() || ! particles_empty() ) {
            std::cerr << "GenEvent::delete_all_vertices strange result ... "
                      << "after deleting all vertices, \nthe particle and "
                      << "vertex maps aren't empty.\n  This probably " 
                      << "indicates deeper problems or memory leak in the "
                      << "code." << std::endl;
            std::cerr << "Number vtx,particle the event after deleting = "
                      << m_vertex_barcodes.size() << "  " 
                      << m_particle_barcodes.size() << std::endl;
        }
    }
    
    bool GenEvent::set_barcode( GenParticle* p, int suggested_barcode )
    {
        if ( p->parent_event() != this ) {
            std::cerr << "GenEvent::set_barcode attempted, but the argument's"
                      << "\n parent_event is not this ... request rejected."
                      << std::endl;
            return false;
        }
        // M.Dobbs  Nov 4, 2002
        // First we must check to see if the particle already has a
        // barcode which is different from the suggestion. If yes, we
        // remove it from the particle map.
        if ( p->barcode() != 0 && p->barcode() != suggested_barcode ) {
            if ( m_particle_barcodes.count(p->barcode()) &&
                 m_particle_barcodes[p->barcode()] == p ) {
                m_particle_barcodes.erase( p->barcode() );
            }
            // At this point either the particle is NOT in
            // m_particle_barcodes, or else it is in the map, but
            // already with the suggested barcode.
        }
        //
        // First case --- a valid barcode has been suggested
        //     (valid barcodes are numbers greater than zero)
        bool insert_success = true;
        if ( suggested_barcode > 0 ) {
            if ( m_particle_barcodes.count(suggested_barcode) ) {
                // the suggested_barcode is already used.
                if ( m_particle_barcodes[suggested_barcode] == p ) {
                    // but it was used for this particle ... so everythings ok
                    p->set_barcode_( suggested_barcode );
                    return true;
                }
                insert_success = false;
                suggested_barcode = 0;
            } else { // suggested barcode is OK, proceed to insert
                m_particle_barcodes[suggested_barcode] = p;
                p->set_barcode_( suggested_barcode );
                return true;
            }
        }
        //
        // Other possibility -- a valid barcode has not been suggested,
        //    so one is automatically generated
        if ( suggested_barcode < 0 ) insert_success = false;
        if ( suggested_barcode <= 0 ) {
            if ( !m_particle_barcodes.empty() ) {
                // in this case we find the highest barcode that was used,
                // and increment it by 1
                suggested_barcode = m_particle_barcodes.rbegin()->first;
                ++suggested_barcode;
            }
            // For the automatically assigned barcodes, the first one
            //   we use is 10001 ... this is just because when we read 
            //   events from HEPEVT, we will suggest their index as the
            //   barcode, and that index has maximum value 10000.
            //  This way we will immediately be able to recognize the hepevt
            //   particles from the auto-assigned ones.
            if ( suggested_barcode <= 10000 ) suggested_barcode = 10001;
        }
        // At this point we should have a valid barcode
        if ( m_particle_barcodes.count(suggested_barcode) ) {
            std::cerr << "GenEvent::set_barcode ERROR, this should never "
                      << "happen \n report bug to matt.dobbs@cern.ch" 
                      << std::endl;
        }
        m_particle_barcodes[suggested_barcode] = p;
        p->set_barcode_( suggested_barcode );
        return insert_success;
    }

    bool GenEvent::set_barcode( GenVertex* v, int suggested_barcode )
    {
        if ( v->parent_event() != this ) {
            std::cerr << "GenEvent::set_barcode attempted, but the argument's"
                      << "\n parent_event is not this ... request rejected."
                      << std::endl;
            return false;
        }
        // M.Dobbs Nov 4, 2002
        // First we must check to see if the vertex already has a
        // barcode which is different from the suggestion. If yes, we
        // remove it from the vertex map.
        if ( v->barcode() != 0 && v->barcode() != suggested_barcode ) {
            if ( m_vertex_barcodes.count(v->barcode()) &&
                 m_vertex_barcodes[v->barcode()] == v ) {
                m_vertex_barcodes.erase( v->barcode() );
            }
            // At this point either the vertex is NOT in
            // m_vertex_barcodes, or else it is in the map, but
            // already with the suggested barcode.
        }
        
        //
        // First case --- a valid barcode has been suggested
        //     (valid barcodes are numbers greater than zero)
        bool insert_success = true;
        if ( suggested_barcode < 0 ) {
            if ( m_vertex_barcodes.count(suggested_barcode) ) {
                // the suggested_barcode is already used.
                if ( m_vertex_barcodes[suggested_barcode] == v ) {
                    // but it was used for this vertex ... so everythings ok
                    v->set_barcode_( suggested_barcode );
                    return true;
                }
                insert_success = false;
                suggested_barcode = 0;
            } else { // suggested barcode is OK, proceed to insert
                m_vertex_barcodes[suggested_barcode] = v;
                v->set_barcode_( suggested_barcode );
                return true;
            }
        }
        //
        // Other possibility -- a valid barcode has not been suggested,
        //    so one is automatically generated
        if ( suggested_barcode > 0 ) insert_success = false;
        if ( suggested_barcode >= 0 ) {
            if ( !m_vertex_barcodes.empty() ) {
                // in this case we find the highest barcode that was used,
                // and increment it by 1, (vertex barcodes are negative)
                suggested_barcode = m_vertex_barcodes.rbegin()->first;
                --suggested_barcode;
            }
            if ( suggested_barcode >= 0 ) suggested_barcode = -1;
        }
        // At this point we should have a valid barcode
        if ( m_vertex_barcodes.count(suggested_barcode) ) {
            std::cerr << "GenEvent::set_barcode ERROR, this should never "
                      << "happen \n report bug to matt.dobbs@cern.ch" 
                      << std::endl;
        }
        m_vertex_barcodes[suggested_barcode] = v;
        v->set_barcode_( suggested_barcode );
        return insert_success;
    }

    /// test to see if we have two valid beam particles
    bool  GenEvent::valid_beam_particles() const {
        bool have1 = false;
        bool have2 = false;
        // first check that both are defined
        if(m_beam_particle_1 && m_beam_particle_2) {
            // now look for a match with the particle "list"
            for ( particle_const_iterator p = particles_begin();
                  p != particles_end(); ++p ) {
                if( m_beam_particle_1 == *p ) have1 = true;
                if( m_beam_particle_2 == *p ) have2 = true;
            }
        }
        if( have1 && have2 ) return true;
        return false;
    }
    
    /// construct the beam particle information using pointers to GenParticle
    /// returns false if either GenParticle* is null
    bool  GenEvent::set_beam_particles(GenParticle* bp1, GenParticle* bp2) {
        m_beam_particle_1 = bp1;
        m_beam_particle_2 = bp2;
        if( m_beam_particle_1 && m_beam_particle_2 ) return true;
        return false;
    }

    /// construct the beam particle information using a std::pair of pointers to GenParticle
    /// returns false if either GenParticle* is null
    bool  GenEvent::set_beam_particles(std::pair<HepMC::GenParticle*, HepMC::GenParticle*> const & bp) {
        return set_beam_particles(bp.first,bp.second);
    }

    void GenEvent::write_units( std::ostream & os ) const {
        os << " Momenutm units:" << std::setw(8) << name(momentum_unit());
        os << "     Position units:" << std::setw(8) << name(length_unit());
        os << std::endl;
    }

    void GenEvent::write_cross_section( std::ostream& os ) const
    {
        // write the GenCrossSection information if the cross section was set
        if( !cross_section() ) return;
        if( cross_section()->is_set() ) {
            os << " Cross Section: " << cross_section()->cross_section() ;
            os << " +/- " << cross_section()->cross_section_error() ;
            os << std::endl;
        }
    }

   bool GenEvent::use_momentum_unit( Units::MomentumUnit newunit ) { 
        // currently not exception-safe. 
        // Easy to fix, though, if needed.
        if ( m_momentum_unit != newunit ) { 
            const double factor = Units::conversion_factor( m_momentum_unit, newunit );
            // multiply all momenta by 'factor',  
            // loop is entered only if particle list is not empty
            for ( GenEvent::particle_iterator p = particles_begin();
                                              p != particles_end(); ++p ) 
            {
                (*p)->convert_momentum(factor);
            }
            // ... 
            m_momentum_unit = newunit; 
        }
        return true; 
    }
    
    bool GenEvent::use_length_unit( Units::LengthUnit newunit ) { 
        // currently not exception-safe. 
        // Easy to fix, though, if needed.
        if ( m_position_unit != newunit ) { 
            const double factor = Units::conversion_factor( m_position_unit, newunit );
            // multiply all lengths by 'factor', 
            // loop is entered only if vertex list is not empty
            for ( GenEvent::vertex_iterator vtx = vertices_begin();
                                            vtx != vertices_end(); ++vtx ) {
                (*vtx)->convert_position(factor);
            }
            // ... 
            m_position_unit = newunit; 
        } 
        return true; 
    }  

    bool GenEvent::use_momentum_unit( std::string& newunit ) { 
        if     ( newunit == "MEV" ) return use_momentum_unit( Units::MEV );
        else if( newunit == "GEV" ) return use_momentum_unit( Units::GEV );
        else std::cerr << "GenEvent::use_momentum_unit ERROR: use either MEV or GEV\n";
        return false;
    }
    
    bool GenEvent::use_length_unit( std::string& newunit ) { 
        if     ( newunit == "MM" ) return use_length_unit( Units::MM );
        else if( newunit == "CM" ) return use_length_unit( Units::CM );
        else std::cerr << "GenEvent::use_length_unit ERROR: use either MEV or GEV\n";
        return false;
    }  

    bool GenEvent::is_valid() const {
        /// A GenEvent is presumed valid if it has both associated
        /// particles and vertices.   No other information is checked.
        if ( vertices_empty() ) return false;
        if ( particles_empty() ) return false;
        return true;
    }

    std::ostream & GenEvent::write_beam_particles(std::ostream & os, 
                         std::pair<HepMC::GenParticle *,HepMC::GenParticle *> pr )
    {
        GenParticle* p = pr.first;
        if(!p) {
           detail::output( os, 0 );
        } else {
           detail::output( os, p->barcode() );
        }
        p = pr.second;
        if(!p) {
           detail::output( os, 0 );
        } else {
           detail::output( os, p->barcode() );
        }

        return os;
    }

    std::ostream & GenEvent::write_vertex(std::ostream & os, GenVertex const * v)
    {
        if ( !v || !os ) {
            std::cerr << "GenEvent::write_vertex !v||!os, "
                      << "v="<< v << " setting badbit" << std::endl;
            os.clear(std::ios::badbit); 
            return os;
        }
        // First collect info we need
        // count the number of orphan particles going into v
        int num_orphans_in = 0;
        for ( GenVertex::particles_in_const_iterator p1
                  = v->particles_in_const_begin();
              p1 != v->particles_in_const_end(); ++p1 ) {
            if ( !(*p1)->production_vertex() ) ++num_orphans_in;
        }
        //
        os << 'V';
        detail::output( os, v->barcode() ); // v's unique identifier
        detail::output( os, v->id() );
        detail::output( os, v->position().x() );
        detail::output( os, v->position().y() );
        detail::output( os, v->position().z() );
        detail::output( os, v->position().t() );
        detail::output( os, num_orphans_in );
        detail::output( os, (int)v->particles_out_size() );
        detail::output( os, (int)v->weights().size() );
        for ( WeightContainer::const_iterator w = v->weights().begin(); 
              w != v->weights().end(); ++w ) {
            detail::output( os, *w );
        }
        detail::output( os,'\n');
        // incoming particles
        for ( GenVertex::particles_in_const_iterator p2 
                  = v->particles_in_const_begin();
              p2 != v->particles_in_const_end(); ++p2 ) {
            if ( !(*p2)->production_vertex() ) {
                write_particle( os, *p2 );
            }
        }
        // outgoing particles
        for ( GenVertex::particles_out_const_iterator p3 
                  = v->particles_out_const_begin();
              p3 != v->particles_out_const_end(); ++p3 ) {
            write_particle( os, *p3 );
        }
        return os;
    }

    std::ostream & GenEvent::write_particle( std::ostream & os, GenParticle const * p )
    {
        if ( !p || !os ) {
            std::cerr << "GenEvent::write_particle !p||!os, "
                      << "p="<< p << " setting badbit" << std::endl;
            os.clear(std::ios::badbit); 
            return os;
        }
        os << 'P';
        detail::output( os, p->barcode() );
        detail::output( os, p->pdg_id() );
        detail::output( os, p->momentum().px() );
        detail::output( os, p->momentum().py() );
        detail::output( os, p->momentum().pz() );
        detail::output( os, p->momentum().e() );
        detail::output( os, p->generated_mass() );
        detail::output( os, p->status() );
        detail::output( os, p->polarization().theta() );
        detail::output( os, p->polarization().phi() );
        // since end_vertex is oftentimes null, this CREATES a null vertex
        // in the map
        detail::output( os,   ( p->end_vertex() ? p->end_vertex()->barcode() : 0 )  );
        os << ' ' << p->flow() << "\n";

        return os;
    }

} // HepMC