Line Segment & Rotated Rectangle Intersection Detection

// Returns the distance of the intersection point from the ray's origin.
// Negative results mean no intersection.
// Fails when the origin is contained in the rectangle, test for that separately, if needed.
float intersect(Ray ray, Box box) {
 Vector2D verts[5]; // array to store the vertices of the box. verts[5] = verts[1] to make things easier
 box.getVerts(verts);
 float sin_phi = sinf(-ray.dir);
 float cos_phi = cosf(-ray.dir);

 // Rotate everything so the ray is aligned with the y-axis, its x-coord is zero
 ray.orig.rotateM(sin_phi,cos_phi);
 float xshift = -ray.orig.r[0]; // we'll skip shifting the ray itself, x is zero, y is unaffected anyway
 for (int i=0; i<4; ++i) {
  verts[i].rotateM(sin_phi,cos_phi);
  verts[i].r[0] += xshift;
 }
 verts[4] = verts[0]; // makes the next loop simpler
 for (int i=0; i<4; ++i) {
  if ((verts[i].x() < 0) && (verts[i+1].x() >= 0)) { // intersection
   Vector2D diff = verts[i+1] - verts[i];
   return verts[i].y() - diff.y()/diff.x() * verts[i].x() - ray.orig.y();
  }
 }
 // Why the above if() works:
 // - the ray's direction vector is (0,1), so the sides facing the ray have a normal vector with n.y < 0
 // - the vertices are in CCW order, so the normal of an edge will be (d.y,-d.x), where d = v[i+1] - v[i]
 // - substituting into n.y < 0 we get v[i].x - v[i+1].x < 0, which is v[i].x < v[i+1].x
 // - thus, we are only interested in the edges going left to right, so we needn't test for the opposite case
 return -1;
}