gistfile1.h

static int 
genericCallback(cpArbiter *arb, cpSpace * space, 
  void * data, ID func) 
{  
  int arity = 3;
  /* int arity = rb_obj_method_arity((VALUE) data, func); */
  /* XXX: this doesn't work. */
  VALUE arbiter   = Qnil;
  cpShape *a      = NULL;
  cpShape *b      = NULL;
  VALUE toCall;

  if(isBlock((VALUE) data)) {
    toCall = (VALUE)data;
  } else  {
    toCall = callbackToMethod((VALUE) data, func);
  }

  arity         = NUM2INT(callbackArity(toCall));
   
  switch (arity) { 
    case 1:
      arbiter = ARBWRAP(arb);
      return rb_funcall(toCall, rb_intern("call"), 1, arbiter);      
    case 2:
      cpArbiterGetShapes(arb, &a, &b);
      return rb_funcall(toCall, rb_intern("call"), 2, (VALUE)a->data, (VALUE)b->data);
    case 3:
      cpArbiterGetShapes(arb, &a, &b);
      arbiter = ARBWRAP(arb);
      return rb_funcall(toCall, rb_intern("call"), 3, (VALUE)a->data, (VALUE)b->data, arbiter);
    default: 
      rb_raise(rb_eArgError, "Arity of callback must be 1, 2, or 3.");
  }
    
}