/*
    fsm_traffic
*/
module fsm_traffic (
    input clk,
    input rst,
    output red_led,
    output green_led,
    output yellow_led
);
    localparam RED      = 2'b00;
    localparam GREEN    = 2'b01;
    localparam YELLOW   = 2'b10;

    reg[1:0] state, next_state;
    reg[7:0] timer;

    always @(posedge clk or posedge rst) begin
       if (rst) begin
        state <= RED;
        timer <= 3;
       end
       else if (timer == 0) begin
            state <= next_state;

            case (next_state)
                RED:     timer <= 3;
                YELLOW:  timer <= 1;
                GREEN:   timer <= 2;
                default: timer <= 3;
            endcase
       end
       else timer <= timer - 1; 
    end

    always @(*) begin
        case (state)
            RED:     next_state = GREEN;
            YELLOW:  next_state = RED;
            GREEN:   next_state = YELLOW;
            default: next_state = RED;
        endcase
    end

    assign red_led = state == RED;
    assign green_led = state == GREEN;
    assign yellow_led = state == YELLOW;
endmodule