87. 4-bit Bidirectional Shift Register

How do you plan to solve it?

module shift_reg_bidirectional (
    input        CLK,
    input        RST,
    input        DIR,
    input        serial_in,
    output reg [3:0] Q
);
always @(posedge CLK or posedge RST) begin
    if (RST) Q <= 4'd0;
    else if (DIR) Q <= {Q[2:0],serial_in};
    else Q <= {serial_in,Q[3:1]};
end
endmodule