122. Simple state transitions 3.v

module top_module(
    input in,
    input [1:0] state,
    output [1:0] next_state,
    output out); //

    parameter A=0, B=1, C=2, D=3;

    // State transition logic: next_state = f(state, in)
    always @(*)
    begin
        case(state)
            A : begin
                if(in == 1'b0)
                    next_state <= A;
                else
                    next_state <= B;
            end

            B : begin
                if(in == 1'b0)
                    next_state <= C;
                else
                    next_state <= B;
            end

            C : begin
                if(in == 1'b0)
                    next_state <= A;
                else
                    next_state <= D;
            end

            D : begin
                if(in == 1'b0)
                    next_state <= C;
                else
                    next_state <= B;
            end
        endcase
    end

    // Output logic:  out = f(state) for a Moore state machine

    assign out = (state == D )?1:0;
endmodule