void genr8_mix_eta(void)
{
// 
//  Generate combinatorical background events
// Input the ascii file produced by genr8 and generate ascii output with photons from different events
//

//#include <TMath.h>
#include <TRandom.h>
#include <TTree.h>

gROOT->Reset();
//TTree *Bfield = (TTree *) gROOT->FindObject("Bfield");
//gROOT->LoadMacro("$ROOTSYS/test/libEvent.so"); 
gStyle->SetPalette(1,0);
gStyle->SetOptStat(kFALSE);
gStyle->SetOptFit(kFALSE);
// gStyle->SetOptFit(1111);
gStyle->SetPadRightMargin(0.15);
gStyle->SetPadLeftMargin(0.15);
gStyle->SetPadBottomMargin(0.15);
gStyle->SetFillColor(0);
//

   char string[256];
   char filename[132];
   char name[10];
   Double_t pi=3.14159;
   Int_t j,jj;
   
   #define npts 8;
   Int_t maxevents=100000;

   Double_t xmin=2;
   Double_t xmax=4;
   Double_t ymin=0.5;
   Double_t ymax=2.5;
   Double_t Ebeam=11;

   // open output files

   Int_t runno, eventno, nparticles;
   Int_t partid, q;
   Float_t mass,px,py,pz,E;

   sprintf(filename,"eta_%dgev.list",Ebeam);
   printf ("Input filename=%s\n",filename);
   FILE *in1;
   in1 = fopen(filename,"r");
   sprintf(filename,"eta_mixed_%dgev.list",Ebeam);
   printf ("Output filename=%s\n",filename);
   FILE *out1;
   out1 = fopen(filename,"w+");

    Float_t g1_px=0;
    Float_t  g1_py=0;
    Float_t  g1_pz=0;
    Float_t  g1_E=0;
    Float_t px1;
    Float_t  py1;
    Float_t  pz1;
    Float_t  E1;

    // get information from first event do not output

    fgets(string, 256, in1);
    // fprintf (out1,"%s",string);
    sscanf (string,"%d %d %d",&runno,&eventno,&nparticles);
    // printf ("runno=%d, eventno=%d, nparticles=%d\n",runno,eventno,nparticles);

      for (j=0;j<nparticles;j++) {

          // input data

         fgets(string, 256,in1);
         // fprintf (out1,"%s",string);
         sscanf (string,"%d%s%f",&partid,name,&mass);
         // printf ("partid=%d, name=%s, mass=%f\n",partid,name,mass);
         fgets(string, 256,in1);
         sscanf (string,"%d%f%f%f%f",&q,&px,&py,&pz,&E);
         // printf ("runno=%d, eventno=%d, nparticles=%d, partid=%d, name=%s, mass=%f, q=%d, px=%f, py=%f, pz=%f, E=%f\n",runno,eventno,nparticles,partid,name,mass,q,px,py,pz,E);
         if (partid == 1) {

           // use g1 from previous event

           px1 = g1_px;
           py1 = g1_py;
           pz1 = g1_pz;
           E1 = g1_E;
           g1_px = px;
           g1_py = py;
           g1_pz = pz;
           g1_E = E;
           px = px1;
           py = py1;
           pz = pz1;
           E = E1;
         }
         sprintf (string,"%d %g %g %g %g",q,px,py,pz,E);
         // fprintf (out1,"%s\n",string);

      }

   // now do all the rest of the events.

   jj=1;
   while(fgets(string, 256, in1)!=NULL && jj < maxevents) {
      jj++;
      fprintf (out1,"%s",string);
      sscanf (string,"%d %d %d",&runno,&eventno,&nparticles);
      // printf ("runno=%d, eventno=%d, nparticles=%d\n",runno,eventno,nparticles);

      for (j=0;j<nparticles;j++) {

          // input data

         fgets(string, 256,in1);
         fprintf (out1,"%s",string);
         sscanf (string,"%d%s%f",&partid,name,&mass);
         // printf ("partid=%d, name=%s, mass=%f\n",partid,name,mass);
         fgets(string, 256,in1);
         sscanf (string,"%d%f%f%f%f",&q,&px,&py,&pz,&E);
         // printf ("runno=%d, eventno=%d, nparticles=%d, partid=%d, name=%s, mass=%f, q=%d, px=%f, py=%f, pz=%f, E=%f\n",runno,eventno,nparticles,partid,name,mass,q,px,py,pz,E);
         if (partid == 1) {

           // use g1 from previous event

           px1 = g1_px;
           py1 = g1_py;
           pz1 = g1_pz;
           E1 = g1_E;
           g1_px = px;
           g1_py = py;
           g1_pz = pz;
           g1_E = E;
           px = px1;
           py = py1;
           pz = pz1;
           E = E1;
         }
         sprintf (string,"%d %g %g %g %g",q,px,py,pz,E);
         fprintf (out1,"%s\n",string);

      }

   }


   fclose (in1);
   fclose(out1);

}