  It is often envisaged that dense filament material lies in the dips of
magnetic field lines belonging to either a sheared arcade or a magnetic flux
rope. But it is also debated which configuration correctly depicts filaments'
magnetic structure, due to our incapacity to measure the coronal magnetic
field. In this paper, we address this issue by employing mass motions in an
active-region filament to diagnose its magnetic structure. The disturbance in
the filament was driven by a surge initiated at the filament's eastern end in
the NOAA active region 12685, which was observed by the 1-m New Vacuum Solar
Telescope (NVST) in the H$\alpha$ line center and line wing ($\pm0.4$~{\AA}).
Filament material predominately exhibits two kinds of motions, namely, rotation
about the spine and longitudinal oscillation along the spine. The former is
evidenced by antisymmetric Doppler shifts about the spine; the latter features
a dynamic barb with mass extending away from the H$\alpha$ spine until the
transversal edge of the EUV filament channel. The longitudinal oscillation in
the eastern section of the filament is distinct from that in the west, implying
that the underlying field lines have different lengths and curvature radii. The
composite motions of filament material suggest a double-decker host structure
with mixed signs of helicity, comprising a flux rope atop a sheared-arcade
system.
