module clean(input logic clk, b, output logic bo);
   logic clr, done, b1, b2, b3;
   localparam WID = 22;
   logic [WID:0] cnt;
  
  typedef enum logic[1:0] {A, B, C, D} sttype;  
  sttype ns, cs;

   always_ff @(posedge clk)
     if (clr)
       cnt = 0;
     else cnt = cnt + 1;
   assign done = cnt[WID];

   always_ff @(posedge clk)
   begin
      cs <= ns;
      b1 <= b;
      b2 <= (ns == C || ns == D);
      b3 <= b2;
      bo <= b2 & ~b3;
   end

   always_comb begin
      clr = 0;
      ns = cs;
      case (cs)
        A: if (b1) 
          begin
             ns = B;
             clr = 1;
          end
        B: if (done)
          ns = C;
        C: if (~b1)
           begin
              ns = D;
              clr = 1;
           end
        D: begin
           if (done)
             ns = A;
           end
        default: ns = A;
      endcase // case (cs)
   end // always_comb
   
endmodule