Function tangent_outer

code/geometry/circles.cpp:25–34

  r1 = A + rotate(v, alpha), r2 = A + rotate(v, -alpha);
  return 1 + (abs(v) > EPS); }
void tangent_outer(C(A,rA), C(B,rB), PP(P), PP(Q)) {
  // if (rA - rB > EPS) { swap(rA, rB); swap(A, B); }
  double theta = asin((rB - rA)/abs(A - B));
  point v = rotate(B - A, theta + pi/2),
        u = rotate(B - A, -(theta + pi/2));
  u = normalize(u, rA);
  P.first = A + normalize(v, rA);
  P.second = B + normalize(v, rB);
  Q.first = A + normalize(u, rA);
  Q.second = B + normalize(u, rB); }
void tangent_inner(C(A,rA), C(B,rB), PP(P), PP(Q)) {
  point ip = (rA*B + rB*A)/(rA+rB);
  assert(tangent(ip, A, rA, P.first, Q.first) == 2);