module top_module(
    input clk,
    input areset,    // Freshly brainwashed Lemmings walk left.
    input bump_left,
    input bump_right,
    input ground,
    output walk_left,
    output walk_right,
    output aaah );
​
    localparam left = 0;
    localparam right = 1;
    localparam fall_l = 2;
    localparam fall_r = 3;
    
    reg[1:0] state, next_state;
    reg[2:0] out;
    
    //state
    always@(posedge clk or posedge areset) begin
        if(areset)
            state <= left;
        else
            state <= next_state;
    end
    
    //transition
    always@(*)begin
        case(state)
            left:next_state=ground?(bump_left?right:left):fall_l;
            right:next_state=ground?(bump_right?left:right):fall_r;
            fall_l:next_state=ground?left:fall_l;
            fall_r:next_state=ground?right:fall_r;
        endcase
    end
    
    //out
    always@(posedge clk or posedge areset) begin
        if(areset)
            out <= 3'b100;
        else
            case(next_state)
                left: out <= 3'b100;
                fall_l: out <= 3'b001;
                right: out <= 3'b010;
                fall_r: out <= 3'b001;
            endcase
    end
    
    assign {walk_left, walk_right, aaah} = out;
    
endmodule

反思：应该把状态想完整 只想到了三个状态 left right fall，用三个寄存器next state pre ，参考答案有四个状态 left right fall-l fall-r，用四个状态可以描述全部状态，并且代码整洁。 我的代码错误，里可能是逻辑不通，只有一开始的状态是正确的，到了后面状态就不会改变了。