/* X - Y = k */

static int fd_minus_eq_filter(fd_constraint this)
{
  fd_int x, y;
  int k;

  x = VAR(this, 0);
  y = VAR(this, 1);
  k = this->constants[0];

  // XXX: just do bounds consistency, for now

  if (_fd_var_del_lt(_fd_var_min(x) - k, y) |
      _fd_var_del_gt(_fd_var_max(x) - k, y))
    {
      if (fd_domain_empty(y))
	return FD_NOSOLUTION;

      _fd_revise_connected(NULL, y);
    }

  if (_fd_var_del_lt(_fd_var_min(y) + k, x) |
      _fd_var_del_gt(_fd_var_max(y) + k, x))
    {
      if (fd_domain_empty(x))
	return FD_NOSOLUTION;

      _fd_revise_connected(NULL, x);
    }

  return FD_OK;
}

int fd_minus_eq_propagate2(fd_constraint this, fd_int culprit)
{
  fd_int revise;
  int changed;

  // XXX: just do bounds consistency, for now

  if (culprit == VAR(this, 0))
    {
      revise = VAR(this, 1);

      changed =
	_fd_var_del_lt(_fd_var_min(culprit) - this->constants[0], revise);
      changed |=
	_fd_var_del_gt(_fd_var_max(culprit) - this->constants[0], revise);
    }
  else
    {
      revise = VAR(this, 0);      

      changed =
	_fd_var_del_lt(_fd_var_min(culprit) + this->constants[0], revise);
      changed |=
	_fd_var_del_gt(_fd_var_max(culprit) + this->constants[0], revise);
    }

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

  if (changed)
    _fd_revise_connected(NULL, revise);

  return FD_OK;
}

int fd_minus_eq_propagate(fd_constraint this, fd_int revise)
{
  fd_int other;
  int idx;
  int changed;

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

  // XXX: just do bounds consistency, for now

  if (idx == 0)
    {
      changed =
	_fd_var_del_lt(_fd_var_min(other) + this->constants[0], revise);
      changed |=
	_fd_var_del_gt(_fd_var_max(other) + this->constants[0], revise);
    }
  else
    {
      changed =
	_fd_var_del_lt(_fd_var_min(other) - this->constants[0], revise);
      changed |=
	_fd_var_del_gt(_fd_var_max(other) - this->constants[0], revise);
    }

  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_minus_eq(fd_int x, fd_int y, int k)
{
  fd_constraint c = _fd_constraint_new(2, 1);

  if (c)
    {
      c->variables[0] = FD_INT2C_VAR(x); c->variables[1] = FD_INT2C_VAR(y);
      c->constants[0] = k;
#ifdef CONSTRAINT_CLASS
      c->kind = FD_CONSTR_MINUS_EQ;
#else /* CONSTRAINT_CLASS */
      c->propagator2 = fd_minus_eq_propagate2;
      c->propagator = fd_minus_eq_propagate;
#endif /* CONSTRAINT_CLASS */

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

      _fd_add_constraint(c);
    }

  return c;
}