```/*****************************************************************************
* Project: RooFit                                                           *
* Package: RooFitCore                                                       *
*    File: \$Id: RooEllipse.cc,v 1.11 2005/06/20 15:44:51 wverkerke Exp \$
* Authors:                                                                  *
*   WV, Wouter Verkerke, UC Santa Barbara, verkerke@slac.stanford.edu       *
*   DK, David Kirkby,    UC Irvine,         dkirkby@uci.edu                 *
*                                                                           *
* Copyright (c) 2000-2005, Regents of the University of California          *
*                                                                           *
* Redistribution and use in source and binary forms,                        *
* with or without modification, are permitted according to the terms        *
*****************************************************************************/

// -- CLASS DESCRIPTION [PLOT] --
// A RooEllipse is a two-dimensional ellipse that can be used to represent
// an error contour.

#include "RooFit.h"

#include "RooEllipse.h"
#include "RooEllipse.h"
#include "TMath.h"

#include "Riostream.h"
#include <math.h>
#include <assert.h>

ClassImp(RooEllipse)

RooEllipse::RooEllipse() { }

RooEllipse::~RooEllipse()
{
}

RooEllipse::RooEllipse(const char *name, Double_t x1, Double_t x2, Double_t s1, Double_t s2, Double_t rho, Int_t points) {
// Create a 2-dimensional ellipse centered at (x1,x2) that represents the confidence
// level contour for a measurement with errors (s1,s2) and correlation coefficient rho.
// The resulting curve is defined as the unique ellipse that passes through these points:
//
//   (x1+rho*s1,x2+s2) , (x1-rho*s1,x2-s2) , (x1+s1,x2+rho*s2) , (x1-s1,x2-rho*s2)
//
// and is described by the implicit equation:
//
//   x*x      2*rho*x*y      y*y
//  -----  -  ---------  +  -----  =  1 - rho*rho
//  s1*s1       s1*s2       s2*s2
//
// The input parameters s1,s2 must be > 0 and also |rho| <= 1.
// The degenerate case |rho|=1 corresponds to a straight line and
// is handled as a special case.

SetName(name);
SetTitle(name);

if(s1 <= 0 || s2 <= 0) {
cout << "RooEllipse::RooEllipse: bad parameter s1 or s2 < 0" << endl;
return;
}
Double_t tmp= 1-rho*rho;
if(tmp < 0) {
cout << "RooEllipse::RooEllipse: bad parameter |rho| > 1" << endl;
return;
}

if(tmp == 0) {
// handle the degenerate case of |rho|=1
SetPoint(0,x1-s1,x2-s2);
SetPoint(1,x1+s1,x2+s2);
setYAxisLimits(x2-s2,x2+s2);
}
else {
Double_t r,psi,phi,u1,u2,xx1,xx2,dphi(2*TMath::Pi()/points);
for(Int_t index= 0; index < points; index++) {
phi= index*dphi;
// adjust the angular spacing of the points for the aspect ratio
psi= atan2(s2*sin(phi),s1*cos(phi));
u1= cos(psi)/s1;
u2= sin(psi)/s2;
r= sqrt(tmp/(u1*u1 - 2*rho*u1*u2 + u2*u2));
xx1= x1 + r*u1*s1;
xx2= x2 + r*u2*s2;
SetPoint(index, xx1, xx2);
if(index == 0) {
setYAxisLimits(xx2,xx2);
// add an extra segment to close the curve
SetPoint(points, xx1, xx2);
}
else {
updateYAxisLimits(xx2);
}
}
}
}

void RooEllipse::printToStream(ostream& os, PrintOption opt, TString indent) const {
//
//   Standard: number of entries
//    Verbose: print points on curve

oneLinePrint(os,*this);
RooPlotable::printToStream(os,opt,indent);
if(opt == Verbose) {
for(Int_t index=0; index < fNpoints; index++) {
cout << "Point [" << index << "] is at (" << fX[index] << "," << fY[index] << ")" << endl;
}
}
}
```

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