typedef  UInt#(51) GCDInt;

interface ArithIO#(parameter type a);
    method Action setInput(a x1, a x2);
    method a getOutput;
endinterface: ArithIO

module mkGCD(ArithIO#(GCDInt));
  Reg#(GCDInt) x();
  mkRegU the_x(x);
  Reg#(GCDInt) y();
  mkReg#(0) the_y(y);
  rule flip (x > y &&  y != 0);
	x <= y; y <= x;
  endrule
  rule sub (x <= y &&  y != 0);
      y <= y - x;
  endrule

  method setInput(a, b) if (y == 0);
	     action
	       x <= a; y <= b;
	     endaction
  endmethod: setInput
  method getOutput() if (y == 0); getOutput = x;
  endmethod: getOutput

endmodule: mkGCD