def calculate_iou(lhs_bboxes, rhs_bboxes):
    """Calculate iou of two bboxes
        bbox: (x1, y1, x2, y2)

    Arguments:
        lhs_bboxes {np.ndarray} -- (m, 4)
        rhs_bboxes {np.ndarray} -- (n, 4)

    Returns:
        np.ndarray -- (m, n)
    """
    lhs_bboxes = np.expand_dims(lhs_bboxes, 1)
    x_y_min = np.maximum(lhs_bboxes[:, :, :2], rhs_bboxes[:, :2])
    x_y_max = np.minimum(lhs_bboxes[:, :, 2:], rhs_bboxes[:, 2:])
    w_h = x_y_max - x_y_min
    w_h = np.clip(w_h, a_min=0, a_max=None)
    intersect_area = w_h[:, :, 0] * w_h[:, :, 1]
    lhs_area = ((lhs_bboxes[:, :, 2] - lhs_bboxes[:, :, 0]) *
                (lhs_bboxes[:, :, 3] - lhs_bboxes[:, :, 1]))
    rhs_area = ((rhs_bboxes[:, 2] - rhs_bboxes[:, 0]) *
                (rhs_bboxes[:, 3] - rhs_bboxes[:, 1]))
    union_area = lhs_area + rhs_area - intersect_area
    iou = intersect_area / union_area
    return iou