采用读热码编写方式：

module top_module(
    input clk,
    input in,
    input areset,
    output out); //
   //reg [3:0]A = 4'd0001;
   // reg [3:0]B = 4'd0010;
   //reg [3:0]C = 4'd0100;
   // reg [3:0]D = 4'd1000;
//1、首先用读热码定义四个状态变量
 	parameter A = 4'd0001 ,B = 4'd0010, C = 4'd0100,D = 4'd1000;
//2、定义两个状态（因为4个状态变量为4为所以这两个状态也应该定义为4位）
    reg [3:0]state,next_state;
//同步时序下的状态复位
    always@(posedge clk or posedge areset)begin
        if(areset)
            state <= A;
        else 
            state <= next_state;
    end
 // 组合逻辑描述状态转移
    assign next_state[0] = (state[0]&& ~in) || (state[2]&& ~in);
    assign next_state[1] = (state[0]&& in) || (state[3]&& in )|| (state[1]&& in);
    assign next_state[2] = (state[1]&& ~in) || (state[3]&& ~in);                   
    assign next_state[3] = (state[2]&& in);
    //输出逻辑 
 assign out = (state[3] == 1);
endmodule

 二进制码 编写方式 ：

module top_module(
    input clk,
    input in,
    input areset,
    output out); //
 //1、定义两个状态
    reg [1:0] state,next_state;
//2、二进制码定义4个状态变量
    parameter A=0,B=1,C=2,D=3;

     // State flip-flops with asynchronous reset
    always@(posedge clk or posedge areset)begin
        if(areset)
            state <= A;
        else
            state <= next_state;
     end
       // State transition logic
    always@(*)begin
        case(state)
        A:next_state = in? B:A;
        B:next_state = in? B:C;
        C:next_state = in? D:A;
        D:next_state = in? B:C;
        endcase
    end
	
    // Output logic
    //assign out = (state == D);
    always@(posedge clk or posedge areset)begin
         if(areset)
             out<= 0;
         else begin
             if(next_state == D)
                out <= 1;
         	else
                out <= 0;
         end
    end

endmodule

注意定义A B C D为 parameter