Code

POINT rotatePointAt(POINT point, POINT pivot, float angle)
{
	if (angle > -0.00001f && angle < 0.00001f)
		return point;

	point.x -= pivot.x;
	point.y -= pivot.y;

	float rad = angle * M_PI / 180;
	POINT res = {
		point.x * cos(rad) - point.y * sin(rad),
		point.x * sin(rad) + point.y * cos(rad)
	};

	res.x += pivot.x;
	res.y += pivot.y;
	return res;
}


void rotatePointsAt(POINT* points, int count, POINT pivot, float angle)
{
	if (angle > -0.00001f && angle < 0.00001f)
		return;

	for (int i = 0; i < count; ++i)
		points[i] = rotatePointAt(points[i], pivot, angle);
}


RECT rotateRectAt(const RECT& rc, POINT pivot, float angle)
{
	if (angle > -0.00001f && angle < 0.00001f)
		return rc;

	POINT p[] = {
		{ rc.left, rc.top },
		{ rc.right, rc.top },
		{ rc.right, rc.bottom },
		{ rc.left, rc.bottom }
	};
	rotatePointsAt(p, 4, pivot, angle);

	float minX = min(p[0].x, min(p[1].x, min(p[2].x, p[3].x)));
	float minY = min(p[0].y, min(p[1].y, min(p[2].y, p[3].y)));
	float maxX = max(p[0].x, max(p[1].x, max(p[2].x, p[3].x)));
	float maxY = max(p[0].y, max(p[1].y, max(p[2].y, p[3].y)));

	RECT res = { minX, minY, maxX, maxY };
	return res;
}