\ gcd for PL/0, June 2015

var
  gcdout;

procedure gcd(xin,yin);
var x,y,newx,newy;
begin
  x:=xin;
  y:=yin;
  while x#y do
  begin
    if x>y then
    begin
      newx:=x-y;
      newy:=y
    end;
    if x<y then
    begin
      newx:=x;
      newy:=y-x
    end;
    x:=newx;
    y:=newy
  end;
  gcdout:=x
end;

procedure exteuclid(xin,yin);
var
  t{4},uarray{4},varray{4},q;
begin
  uarray{1}:=1;
  uarray{2}:=0;
  uarray{3}:=xin;

  varray{1}:=0;
  varray{2}:=1;
  varray{3}:=yin;

  while varray{3}#0 do
  begin
    q:=uarray{3} / varray{3};

    t{1}:=uarray{1}-varray{1}*q;
    t{2}:=uarray{2}-varray{2}*q;
    t{3}:=uarray{3}-varray{3}*q;

    uarray{1}:=varray{1};
    uarray{2}:=varray{2};
    uarray{3}:=varray{3};

    varray{1}:=t{1};
    varray{2}:=t{2};
    varray{3}:=t{3}
  end;
  out:=uarray{1};
  out:=uarray{2};
  gcdout:=uarray{3}
end;

procedure main;
var xin,yin;
begin
  xin:=in;
  yin:=in;
  call gcd(xin,yin);
  out:=gcdout;
  call exteuclid(xin,yin);
  out:=gcdout
end;

begin
  call main
end.