Get the downslope directions of an dinf direction value
        Args:
            a: Dinf value

        Returns:
            downslope directions

def dinf_downslope_direction(a):
        """Get the downslope directions of an dinf direction value
        Args:
            a: Dinf value

        Returns:
            downslope directions
        """
        taud, d = DinfUtil.check_orthogonal(a)
        if d != -1:
            down = [d]
            return down
        else:
            if a < FlowModelConst.ne:  # 129 = 1+128
                down = [1, 2]
            elif a < FlowModelConst.n:  # 192 = 128+64
                down = [2, 3]
            elif a < FlowModelConst.nw:  # 96 = 64+32
                down = [3, 4]
            elif a < FlowModelConst.w:  # 48 = 32+16
                down = [4, 5]
            elif a < FlowModelConst.sw:  # 24 = 16+8
                down = [5, 6]
            elif a < FlowModelConst.s:  # 12 = 8+4
                down = [6, 7]
            elif a < FlowModelConst.se:  # 6 = 4+2
                down = [7, 8]
            else:  # 3 = 2+1
                down = [8, 1]
            return down