線分の交差判定

intersects.cpp

bool ntersects(double ax, double ay, double bx, double by, double cx,
                double cy, double dx, double dy) {
  double ta = (cx - dx) * (ay - cy) + (cy - dy) * (cx - ax);
  double tb = (cx - dx) * (by - cy) + (cy - dy) * (cx - bx);
  double tc = (ax - bx) * (cy - ay) + (ay - by) * (ax - cx);
  double td = (ax - bx) * (dy - ay) + (ay - by) * (ax - dx);

  return tc * td < 0 && ta * tb < 0;
};