Source code in the pari/gp programming language

bc()={
  my(u,v,bulls,cows);
  while(#vecsort(v=vector(4,i,random(9)+1),,8)<4,);
  while(bulls<4,
    u=input();
    if(type(u)!="t_VEC"|#u!=4,next);
    bulls=sum(i=1,4,u[i]==v[i]);
    cows=sum(i=1,4,sum(j=1,4,i!=j&v[i]==u[j]));
    print("You have "bulls" bulls and "cows" cows")
  )
};