Get the size of an area defined by 2 overlapping circles

This link helped me out in this situation. The MathWorld version is very descriptive, but I think the above link explains what is going on more clearly. I created a javascript function for this purpose, given below in case it is useful to anyone else.

function areaOfIntersection(x0, y0, r0, x1, y1, r1)
{
  var rr0 = r0*r0;
  var rr1 = r1*r1;
  var c = Math.sqrt((x1-x0)*(x1-x0) + (y1-y0)*(y1-y0));
  var phi = (Math.acos((rr0+(c*c)-rr1) / (2*r0*c)))*2;
  var theta = (Math.acos((rr1+(c*c)-rr0) / (2*r1*c)))*2;
  var area1 = 0.5*theta*rr1 - 0.5*rr1*Math.sin(theta);
  var area2 = 0.5*phi*rr0 - 0.5*rr0*Math.sin(phi);
  return area1 + area2;
}

A formula for the area is worked out in Circle-Circle Intersection at Wolfram MathWorld: $$ A = r^{2}\cos^{-1}\left(\frac{d^{2}+r^{2}-R^{2}}{2dr}\right) + R^{2}\cos^{-1}\left(\frac{d^{2}+R^{2}-r^{2}}{2dR}\right) - \frac12 \sqrt{(-d+r+R)(d+r-R)(d-r+R)(d+r+R)} $$ where $r$ and $R$ are the radii and $d$ is the distance between the centres.