//
// Determine Pi by integration of a circle with random numbers
// 
int calc_pi(){

  gStyle->SetOptStat(0);

  float	  pi=0.0;
  int	  Nac=0,Ntot=0;
  TCanvas *c1,*c2;
  TPad    *pad,*padtx;

  int     nevents=1000000;

//  create canvasses for
  c2 = new TCanvas("stat","Calculate Pi",200,10,800,600);
  TH1F* h1rnd = new TH1F("h1rnd","test",1000,0.,1.);
  TProfile* h1stat = new TProfile("h1stat","",(nevents-100.)/10.,100.,nevents,2.9,3.3,"s");
  TF1 *f1 = new TF1("f1","3.1415-3.1415/sqrt(x)",0.,nevents)  ;
  TF1 *f2 = new TF1("f2","3.1415+3.1415/sqrt(x)",0.,nevents)  ;

  c1 = new TCanvas("cpi","Calculate Pi",200,10,700,800);

// in c1 make pads for text and plot
  padtx = new TPad("pad2","Pad for value of Pi",0.02,0.92,0.98,0.98,21);
  padtx->Draw();
  char text[50];
  sprintf(text,"For N = %i   pi = %f",Ntot,pi);
  TText* Pitxt=new TText(0.1,0.1,text);

  pad = new TPad("pad1","Pad for 2D Plot",0.02,0.02,0.98,0.9,21);
  pad->Draw();
  TH2F* h2fac = new TH2F("h2fPiac","Circle accepted random numbers",1000,0.0,1.0,1000,0.,1.);
  h2fac->SetMarkerStyle(21);
  h2fac->SetMarkerSize(0.05);
  h2fac->SetMarkerColor(2);
  h2fac->SetFillColor(2);
  TH2F* h2frej = new TH2F("h2fPirej","Rejected random numbers",1000,0.0,1.0,1000,0.,1.);
  h2frej->SetMarkerStyle(21);
  h2frej->SetMarkerSize(0.05);
  h2frej->SetMarkerColor(3);
  h2frej->SetFillColor(3);

// create random object (Argument 0 gives random seed)
  TRandom3 *myRnd=new TRandom3(0);

  float x1,x2;
  int j=10;
  for(int i=1;i<=nevents;++i) {
    x1 = myRnd->Rndm();
    x2 = myRnd->Rndm();
    Ntot++;
    if(sqrt(x1*x1+x2*x2) <= 1.0) {
	  Nac++;
	  h2fac->Fill(x1,x2);
    }
    else {
	  h2frej->Fill(x1,x2);
    }
	  h1rnd->Fill(x1);
	  h1rnd->Fill(x2);

    pi = (float)Nac/(float)Ntot*4.0;
    
    sprintf(text,"N(events) = %i :   pi(MC) = %f",Ntot,pi);
	h1stat->Fill(i,pi);
	h1stat->Fill(i,pi);

// Draw only every 1000 events
   if(i%j == 0) {
  	  cout << j << " " << i << " " << pi << endl;
	  if (j<=1000) j=j*10;
	 
      c1->cd();
      pad->cd();
      h2fac->Draw();
      h2frej->Draw("same");
      if(Ntot > 0) pi = (float)Nac/(float)Ntot*4.0;

      padtx->cd();
	  Pitxt->SetTextSize(0.6);
      Pitxt->SetText(0.1,0.1,text);
      Pitxt->Draw();
      gPad->Update();

      c2->cd();
	  c2->SetLogx();
	  h1stat->SetMinimum(2.9);
	  h1stat->SetMaximum(3.3);
	  h1stat->SetMarkerStyle(20);
	  h1stat->SetMarkerSize(0.1);
	  h1stat->Draw("e1");
	  f1->Draw("same");
	  f2->Draw("same");
	  c2->Update();

      gSystem->ProcessEvents();

    }  

  }

  return 0;
  
}