// These are the symbolic names for states
// 定义状态的符号名称
parameter  [1:0]
  S0 = 2'h0,
  S1 = 2'h1,
  S2 = 2'h2,
  S3 = 2'h3;
 
parameter  [3:0] 
  s0 = 4'h1,
  s1 = 4'h2,
  s2 = 4'h4,
  s3 = 4'h8;
  
// These are the current state and next state variables
// 定义当前状态和下一状态变量
reg [3:0] state;
reg [3:0] next_state;
 
 
// state_vector state
// 状态向量的转移关系
always @ (state or y or x)
begin 
  next_state = state;
  case (1)                 
    state[S0]: begin 
      if (x) begin 
        next_state = 1 << S1;
      end
      else begin 
        next_state = 1 << S2;
      end
    end
    state[S1]: begin 
      if (y) begin 
        next_state = 1 << S2;
      end
      else begin 
        next_state = 1 << S0;
      end
    end
    state[S2]: begin 
      if (x & y) begin 
        next_state = 1 << S3;
      end
      else begin 
        next_state = 1 << S0;
      end
    end
    state[S3]: begin 
      next_state = 1 << S0;
    end
  endcase
end


always @ (posedge clk or posedge reset)
begin 
  if (reset) begin 
    state <= 1 << S0;
  end
  else begin 
    state <= next_state;
  end
end