def METHOD_NAME(self,
          X: ndarray,
          find: Optional[ndarray] = None) -> ndarray:
    if X.shape[0] == 1:
        segs = self.G(np.array([[0., 1.]]), side=0)
        return np.sqrt(np.diff(segs[0], axis=1) ** 2 +
                       np.diff(segs[1], axis=1) ** 2)
    raise NotImplementedError