/* X eq Y */

static int fd_eq_filter(fd_constraint this)
{
  int i;

  // only propagate if culprit's domain is a singleton
  for (i = 0; i < 2; ++i)
    {
      fd_int revise = VAR(this, 1 - i);

      if (_fd_var_intersect(revise, VAR(this, i)))
	{
	  if (fd_domain_empty(revise))
	    return FD_NOSOLUTION;

	  _fd_revise_connected(this, revise);
	}
    }

  return FD_OK;
}

int fd_eq_propagate2(fd_constraint this, fd_int culprit)
{
  fd_int revise;
  int changed = 0;

  revise = VAR(this, culprit == VAR(this, 0) ? 1 : 0);

  changed = _fd_var_intersect(revise, culprit);

  if (!changed)
    return FD_OK;

  if (fd_domain_empty(revise))
    return FD_NOSOLUTION;

  _fd_revise_connected(this, revise);

  return FD_OK;
}

int fd_eq_propagate(fd_constraint this, fd_int revise)
{
  fd_int other;
  int changed = 0;

  other = VAR(this, revise == VAR(this, 0) ? 1 : 0);

  changed = _fd_var_intersect(revise, other);

  if (changed && fd_domain_empty(revise))
    return FD_NOSOLUTION;

  // XXX: enqueue further updates here?
  if (changed)
    _fd_revise_connected(this, revise);

  return FD_OK;
}

fd_constraint fd_eq(fd_int x, fd_int y)
{
  fd_constraint c = _fd_constraint_new(2, 0);

  if (c)
    {
      c->variables[0] = FD_INT2C_VAR(x); c->variables[1] = FD_INT2C_VAR(y);
#ifdef CONSTRAINT_CLASS
      c->kind = FD_CONSTR_EQ;
#else /* CONSTRAINT_CLASS */
      c->propagator2 = fd_eq_propagate2;
      c->propagator = fd_eq_propagate;
#endif /* CONSTRAINT_CLASS */

      _fd_var_add_constraint(x, c);
      _fd_var_add_constraint(y, c);

      _fd_add_constraint(c);
    }

  return c;
}