Competitive Programming

/**
 * Author: Ulf Lundstrom
 * Date: 2009-04-06
 * License: CC0
 * Source:
 * Description:Given 2 points on the circle (p1 and p2) and radius r of the corresponding circle, we
 can determine the location of the centers (c1 and c2) of the two possible circles
 */
bool circle2PtsRad(point p1, point p2, double r, point &c) {
  double d2 = (p1.x - p2.x) * (p1.x - p2.x) +
              (p1.y - p2.y) * (p1.y - p2.y);
  double det = r * r / d2 - 0.25;
  if (det < 0.0) return false;
  double h = sqrt(det);
  c.x = (p1.x + p2.x) * 0.5 + (p1.y - p2.y) * h;
  c.y = (p1.y + p2.y) * 0.5 + (p2.x - p1.x) * h;
  return true; }